@article{bhatia_saggi_kais_2025, title={Application of quantum-inspired tensor networks to optimize federated learning systems}, volume={7}, ISSN={["2524-4914"]}, DOI={10.1007/s42484-025-00243-x}, number={1}, journal={QUANTUM MACHINE INTELLIGENCE}, author={Bhatia, Amandeep Singh and Saggi, Mandeep Kaur and Kais, Sabre}, year={2025}, month={Jun} }
 @article{shivpuje_sajjan_wang_hu_kais_2025, title={Designing Variational Ansatz for Quantum‐Enabled Simulation of Non‐Unitary Dynamical Evolution ‐ An Excursion into Dicke Supperradiance}, url={https://doi.org/10.1002/qute.202400088}, DOI={10.1002/qute.202400088}, abstractNote={Abstract Adaptive Variational Quantum Dynamics (AVQD) algorithms offer a promising approach to providing quantum‐enabled solutions for systems treated within the purview of open quantum dynamical evolution. In this study, the unrestricted‐vectorization variant of AVQD is employed to simulate and benchmark various non‐unitarily evolving systems. Exemplification of how construction of an expressible ansatz unitary and the associated operator pool can be implemented to analyze examples such as the Fenna–Matthews–Olson complex (FMO) and even the permutational invariant Dicke model of quantum optics. Furthermore, an efficient decomposition scheme is shown for the ansatz used, which can extend its applications to a wide range of other open quantum system scenarios in near future. In all cases the results obtained are in excellent agreement with exact numerical computations that bolsters the effectiveness of this technique. The successful demonstrations pave the way for utilizing this adaptive variational technique to study complex systems in chemistry and physics, like light‐harvesting devices, thermal, and opto‐mechanical switches, to name a few.}, journal={Advanced Quantum Technologies}, author={Shivpuje, Saurabh and Sajjan, Manas and Wang, Yuchen and Hu, Zixuan and Kais, Sabre}, year={2025}, month={Feb} }
 @article{delgado-granados_krogmeier_sager-smith_avdic_hu_sajjan_abbasi_smart_narang_kais_et al._2025, title={Quantum Algorithms and Applications for Open Quantum Systems}, volume={2}, ISSN={["1520-6890"]}, url={https://doi.org/10.1021/acs.chemrev.4c00428}, DOI={10.1021/acs.chemrev.4c00428}, abstractNote={Accurate models for open quantum systems─quantum states that have nontrivial interactions with their environment─may aid in the advancement of a diverse array of fields, including quantum computation, informatics, and the prediction of static and dynamic molecular properties. In recent years, quantum algorithms have been leveraged for the computation of open quantum systems as the predicted quantum advantage of quantum devices over classical ones may allow previously inaccessible applications. Accomplishing this goal will require input and expertise from different research perspectives, as well as the training of a diverse quantum workforce, making a compilation of current quantum methods for treating open quantum systems both useful and timely. In this Review, we first provide a succinct summary of the fundamental theory of open quantum systems and then delve into a discussion on recent quantum algorithms. We conclude with a discussion of pertinent applications, demonstrating the applicability of this field to realistic chemical, biological, and material systems.}, journal={CHEMICAL REVIEWS}, author={Delgado-Granados, Luis H. and Krogmeier, Timothy J. and Sager-Smith, LeeAnn M. and Avdic, Irma and Hu, Zixuan and Sajjan, Manas and Abbasi, Maryam and Smart, Scott E. and Narang, Prineha and Kais, Sabre and et al.}, year={2025}, month={Feb} }
 @article{mousa_wehefritz-kaufmann_kais_cui_kaufmann_2025, title={Refined phase diagram for a spin-1 system exhibiting a Haldane phase}, url={https://doi.org/10.1103/PhysRevB.111.085303}, DOI={10.1103/PhysRevB.111.085303}, journal={Physical Review B}, author={Mousa, Mohamad and Wehefritz-Kaufmann, Birgit and Kais, Sabre and Cui, Shawn and Kaufmann, Ralph}, year={2025}, month={Feb} }
 @article{muruganandam_sajjan_kais_2024, title={Defect-induced localization of information scrambling in 1D Kitaev model}, volume={99}, ISSN={["1402-4896"]}, url={https://doi.org/10.1088/1402-4896/ad7911}, DOI={10.1088/1402-4896/ad7911}, abstractNote={Abstract We discuss one-dimensional(1D) spin compass model or 1D Kitaev model in the presence of local bond defects. Three types of local disorders concerning both bond-nature and bond-strength that occur on kitaev materials have been investigated. Using exact diagonalization, two-point spin-spin structural correlations and four-point Out-of-Time-Order Correlators(OTOC) have been computed for the defective spin chains. The proposed quantities give signatures of these defects in terms of their responses to location and strength of defects. A key observation is that the information scrambling in the OTOC space gets trapped at the defect site giving rise to the phenomena of localization of information scrambling thus making these correlators a suitable diagnostic tool to detect and characterize these defects.}, number={10}, journal={PHYSICA SCRIPTA}, author={Muruganandam, Varadharajan and Sajjan, Manas and Kais, Sabre}, year={2024}, month={Oct} }
 @article{gupta_drzazga-szczesniak_kais_szczesniak_2024, title={Entropy corrected geometric Brownian motion}, volume={14}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-024-79714-3}, abstractNote={The geometric Brownian motion (GBM) is widely used for modeling stochastic processes, particularly in finance. However, its solutions are constrained by the assumption that the underlying distribution of returns follows a log-normal distribution. This assumption limits the predictive power of GBM, especially in capturing the complexities of real-world data, where deviations from log-normality are common. In this work, we introduce entropy corrections to the GBM framework to relax the log-normality constraint and better account for the inherent structures in real data. We demonstrate that as the deterministic components within the data increase, entropy decreases, leading to refinements in GBM's predictive accuracy. Our approach shows significant improvements over conventional GBM in handling distributions that deviate from log-normal behavior, as demonstrated through both a simple dice roll experiment and real-world financial data. Beyond just financial modeling, this research also opens up new avenues for generating synthetic data that better captures real-world complexity, enhancing applications in fields like machine learning and statistical modeling.}, number={1}, journal={SCIENTIFIC REPORTS}, author={Gupta, Rishabh and Drzazga-Szczesniak, Ewa A. and Kais, Sabre and Szczesniak, Dominik}, year={2024}, month={Nov} }
 @article{wang_xie_ming_wang_wang_meng_liu_xu_tang_ye_et al._2024, title={Experimental test of generalized multipartite entropic uncertainty relations}, volume={110}, ISSN={["2469-9934"]}, DOI={10.1103/PhysRevA.110.062220}, number={6}, journal={PHYSICAL REVIEW A}, author={Wang, Zhao-An and Xie, Bo-Fu and Ming, Fei and Wang, Yi-Tao and Wang, Dong and Meng, Yu and Liu, Zheng-Hao and Xu, Kai and Tang, Jian-Shun and Ye, Liu and et al.}, year={2024}, month={Dec} }
 @article{kumaran_sajjan_oh_kais_2024, title={Random projection using random quantum circuits}, url={https://doi.org/10.1103/PhysRevResearch.6.013010}, DOI={10.1103/PhysRevResearch.6.013010}, abstractNote={The random sampling task performed by Google's Sycamore processor gave us a glimpse of the “quantum supremacy era.” This has definitely shed some light on the power of random quantum circuits in this abstract task of sampling outputs from the (pseudo)random circuits. In this paper, we explore a practical near-term use of local random quantum circuits in dimensional reduction of large low-rank data sets. We make use of the well-studied dimensionality reduction technique called the random projection method. This method has been extensively used in various applications such as image processing, logistic regression, entropy computation of low-rank matrices, etc. We prove that the matrix representations of local random quantum circuits with sufficiently shorter depths <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>[</a:mo><a:mo>∼</a:mo><a:mi>O</a:mi><a:mo>(</a:mo><a:mi>n</a:mi><a:mo>)</a:mo><a:mo>]</a:mo></a:mrow></a:math> serve as good candidates for random projection. We demonstrate numerically that their projection abilities are not far off from the computationally expensive classical principal components analysis on MNIST and CIFAR-100 image datasets. We also benchmark the performance of quantum random projection against the commonly used classical random projection in the tasks of dimensionality reduction of image data sets and computing von Neumann entropies of large low-rank density matrices. And finally, using variational quantum singular value decomposition, we demonstrate a near-term implementation of extracting the singular vectors with dominant singular values after quantum random projecting a large low-rank matrix to lower dimensions. All such numerical experiments unequivocally demonstrate the ability of local random circuits to randomize a large Hilbert space at sufficiently shorter depths with robust retention of properties of large data sets in reduced dimensions. Published by the American Physical Society 2024}, journal={Physical Review Research}, author={Kumaran, Keerthi and Sajjan, Manas and Oh, Sangchul and Kais, Sabre}, year={2024}, month={Jan} }
 @article{kale_kais_2024, title={Simulation of Chemical Reactions on a Quantum Computer}, url={https://doi.org/10.1021/acs.jpclett.4c01100}, DOI={10.1021/acs.jpclett.4c01100}, abstractNote={Studying chemical reactions, particularly in the gas phase, relies heavily on computing scattering matrix elements. These elements are essential for characterizing molecular reactions and accurately determining reaction probabilities. However, the intricate nature of quantum interactions poses challenges, necessitating the use of advanced mathematical models and computational approaches to tackle the inherent complexities. In this study, we develop and apply a quantum computing algorithm for the calculation of scattering matrix elements. In our approach, we employ the time-dependent method based on the Møller operator formulation where the S-matrix element between the respective reactant and product channels is determined through the time correlation function of the reactant and product Møller wavepackets. We successfully apply our quantum algorithm to calculate scattering matrix elements for 1D semi-infinite square well potential and on the colinear hydrogen exchange reaction. As we navigate the complexities of quantum interactions, this quantum algorithm is general and emerges as a promising avenue, shedding light on new possibilities for simulating chemical reactions on quantum computers.}, journal={The Journal of Physical Chemistry Letters}, author={Kale, Sumit Suresh and Kais, Sabre}, year={2024}, month={May} }
 @article{makhija_gupta_neville_schuurman_francisco_kais_2024, title={Time Resolved Quantum Tomography in Molecular Spectroscopy by the Maximal Entropy Approach}, volume={9}, ISSN={["1948-7185"]}, url={https://doi.org/10.1021/acs.jpclett.4c02368}, DOI={10.1021/acs.jpclett.4c02368}, abstractNote={Attosecond science offers unprecedented precision in probing the initial moments of chemical reactions, revealing the dynamics of molecular electrons that shape reaction pathways. A fundamental question emerges: what role, if any, do quantum coherences between molecular electron states play in photochemical reactions? Answering this question necessitates quantum tomography─the determination of the electronic density matrix from experimental data, where the off-diagonal elements represent these coherences. The Maximal Entropy (MaxEnt) based Quantum State Tomography (QST) approach offers unique advantages in studying molecular dynamics, particularly with partial tomographic data. Here, we explore the application of MaxEnt-based QST on photoexcited ammonia, necessitating the operator form of observables specific to the performed measurements. We present two methodologies for constructing these operators: one leveraging Molecular Angular Distribution Moments (MADMs) which accurately capture the orientation-dependent vibronic dynamics of molecules and another utilizing Angular Momentum Coherence Operators to construct measurement operators for the full rovibronic density matrix in the symmetric top basis. A key revelation of our study is the direct link between Lagrange multipliers in the MaxEnt formalism and the unique set of MADMs. Additionally, we visualize the electron density within the molecular frame, demonstrating charge migration across the molecule. Furthermore, we achieve a groundbreaking milestone by constructing, for the first time, the entanglement entropy of the electronic subsystem─a metric that was previously inaccessible. The entropy vividly reveals and quantifies the effects of coupling between the excited electron and nuclear degrees of freedom. Consequently, our findings open new avenues for research in ultrafast molecular spectroscopy within the broader domain of quantum information science, offering profound implications for the study of molecular systems under excitation using quantum tomographic schemes.}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Makhija, Varun and Gupta, Rishabh and Neville, Simon and Schuurman, Michael and Francisco, Joseph and Kais, Sabre}, year={2024}, month={Sep} }
 @article{kais_2024, title={Walking with the Atoms in a Chemical Bond: A Perspective Using Quantum Phase Transition}, url={https://www.mdpi.com/1099-4300/26/3/230}, DOI={10.3390/e26030230}, abstractNote={Phase transitions happen at critical values of the controlling parameters, such as the critical temperature in classical phase transitions, and system critical parameters in the quantum case. However, true criticality happens only at the thermodynamic limit, when the number of particles goes to infinity with constant density. To perform the calculations for the critical parameters, a finite-size scaling approach was developed to extrapolate information from a finite system to the thermodynamic limit. With the advancement in the experimental and theoretical work in the field of ultra-cold systems, particularly trapping and controlling single atomic and molecular systems, one can ask: do finite systems exhibit quantum phase transition? To address this question, finite-size scaling for finite systems was developed to calculate the quantum critical parameters. The recent observation of a quantum phase transition in a single trapped}, journal={Entropy}, author={Kais, Sabre}, year={2024}, month={Mar} }
 @article{iyengar_kais_2023, title={Analogy between Boltzmann Machines and Feynman Path Integrals}, url={https://doi.org/10.1021/acs.jctc.3c00187}, DOI={10.1021/acs.jctc.3c00187}, abstractNote={Machine learning has had a significant impact on multiple areas of science, technology, health, and computer and information sciences. Through the advent of quantum computing, quantum machine learning has developed as a new and important avenue for the study of complex learning problems. Yet there is substantial debate and uncertainty in regard to the foundations of machine learning. Here, we provide a detailed exposition of the mathematical connections between a general machine learning approach called Boltzmann machines and Feynman's description of quantum and statistical mechanics. In Feynman's description, quantum phenomena arise from an elegant, weighted sum over (or superposition of) paths. Our analysis shows that Boltzmann machines and neural networks have a similar mathematical structure. This allows the interpretation that the hidden layers in Boltzmann machines and neural networks are discrete versions of path elements and allows a path integral interpretation of machine learning similar to that in quantum and statistical mechanics. Since Feynman paths are a natural and elegant depiction of interference phenomena and the superposition principle germane to quantum mechanics, this analysis allows us to interpret the goal in machine learning as finding an appropriate combination of paths, and accumulated path-weights, through a network, that cumulatively captures the correct properties of an x-to-y map for a given mathematical problem. We are forced to conclude that neural networks are naturally related to Feynman path-integrals and hence may present one avenue to be considered as quantum problems. Consequently, we provide general quantum circuit models applicable to both Boltzmann machines and Feynman path integrals.}, journal={Journal of Chemical Theory and Computation}, author={Iyengar, Srinivasan S. and Kais, Sabre}, year={2023}, month={May} }
 @article{oh_kais_2023, title={Comparison of quantum advantage experiments using random circuit sampling}, url={https://doi.org/10.1103/PhysRevA.107.022610}, DOI={10.1103/PhysRevA.107.022610}, abstractNote={Random circuit sampling, the task of sampling bit strings from a random unitary operator, has been implemented to demonstrate quantum advantage on the Sycamore quantum processor with 53 qubits and on the Zuchongzhi quantum processor with 56 and 61 qubits. Recently, it was claimed that classical computers using tensor network simulation could catch on to current noisy quantum processors for random circuit sampling. While the linear cross-entropy benchmark fidelity was used to certify all these claims, it may not capture statistical properties of outputs in detail. Here, we compare the bit strings sampled from classical computers using tensor network simulation by Pan et al. [F. Pan, K. Chen, and P. Zhang, Phys. Rev. Lett. 129, 090502 (2022)] and by Kalachev et al. [G. Kalachev, P. Panteleev, P. Zhou, and M.-H. Yung, arXiv:2112.15083] with the bit strings from the Sycamore quantum processor. It is shown that all of Kalachev et al.'s samples passed the NIST random number tests. The heat maps of bit strings show that Pan et al.'s and Kalachev et al.'s samples are quite different from the Sycamore or Zuchongzhi samples. The analysis with the Marchenko-Pastur distribution and the Wasssertein distances demonstrates that Kalachev et al.'s samples are statistically closer to the Sycamore samples than Pan et al.'s while the three datasets have similar values for the linear cross-entropy fidelity. Our finding implies that further study is needed to certify or beat the claims of quantum advantage using random circuit sampling.}, journal={Physical Review A}, author={Oh, Sangchul and Kais, Sabre}, year={2023}, month={Feb} }
 @article{oh_kais_2023, title={Cutoff phenomenon and entropic uncertainty for random quantum circuits}, url={https://doi.org/10.1088/2516-1075/acf2d3}, DOI={10.1088/2516-1075/acf2d3}, abstractNote={Abstract How fast a state of a system converges to a stationary state is one of the fundamental questions in science. Some Markov chains and random walks on finite groups are known to exhibit the non-asymptotic convergence to a stationary distribution, called the cutoff phenomenon. Here, we examine how quickly a random quantum circuit could transform a quantum state to a Haar-measure random quantum state. We find that random quantum states, as stationary states of random walks on a unitary group, are invariant under the quantum Fourier transform (QFT). Thus the entropic uncertainty of random quantum states has balanced Shannon entropies for the computational basis and the QFT basis. By calculating the Shannon entropy for random quantum states and the Wasserstein distances for the eigenvalues of random quantum circuits, we show that the cutoff phenomenon occurs for the random quantum circuit. It is also demonstrated that the Dyson-Brownian motion for the eigenvalues of a random unitary matrix as a continuous random walk exhibits the cutoff phenomenon. The results here imply that random quantum states could be generated with shallow random circuits.}, journal={Electronic Structure}, author={Oh, Sangchul and Kais, Sabre}, year={2023}, month={Sep} }
 @article{daskin_gupta_kais_2023, title={Dimension Reduction and Redundancy Removal through Successive Schmidt Decompositions}, url={https://doi.org/10.3390/app13053172}, DOI={10.3390/app13053172}, abstractNote={Quantum computers are believed to have the ability to process huge data sizes, which can be seen in machine learning applications. In these applications, the data, in general, are classical. Therefore, to process them on a quantum computer, there is a need for efficient methods that can be used to map classical data on quantum states in a concise manner. On the other hand, to verify the results of quantum computers and study quantum algorithms, we need to be able to approximate quantum operations into forms that are easier to simulate on classical computers with some errors. Motivated by these needs, in this paper, we study the approximation of matrices and vectors by using their tensor products obtained through successive Schmidt decompositions. We show that data with distributions such as uniform, Poisson, exponential, or similar to these distributions can be approximated by using only a few terms, which can be easily mapped onto quantum circuits. The examples include random data with different distributions, the Gram matrices of iris flower, handwritten digits, 20newsgroup, and labeled faces in the wild. Similarly, some quantum operations, such as quantum Fourier transform and variational quantum circuits with a small depth, may also be approximated with a few terms that are easier to simulate on classical computers. Furthermore, we show how the method can be used to simplify quantum Hamiltonians: In particular, we show the application to randomly generated transverse field Ising model Hamiltonians. The reduced Hamiltonians can be mapped into quantum circuits easily and, therefore, can be simulated more efficiently.}, journal={Applied Sciences}, author={Daskin, Ammar and Gupta, Rishabh and Kais, Sabre}, year={2023}, month={Mar} }
 @article{selvarajan_sajjan_humble_kais_2023, title={Dimensionality Reduction with Variational Encoders Based on Subsystem Purification}, url={https://doi.org/10.3390/math11224678}, DOI={10.3390/math11224678}, abstractNote={Efficient methods for encoding and compression are likely to pave the way toward the problem of efficient trainability on higher-dimensional Hilbert spaces, overcoming issues of barren plateaus. Here, we propose an alternative approach to variational autoencoders to reduce the dimensionality of states represented in higher dimensional Hilbert spaces. To this end, we build a variational algorithm-based autoencoder circuit that takes as input a dataset and optimizes the parameters of a Parameterized Quantum Circuit (PQC) ansatz to produce an output state that can be represented as a tensor product of two subsystems by minimizing Tr(ρ2). The output of this circuit is passed through a series of controlled swap gates and measurements to output a state with half the number of qubits while retaining the features of the starting state in the same spirit as any dimension-reduction technique used in classical algorithms. The output obtained is used for supervised learning to guarantee the working of the encoding procedure thus developed. We make use of the Bars and Stripes (BAS) dataset for an 8 × 8 grid to create efficient encoding states and report a classification accuracy of 95% on the same. Thus, the demonstrated example provides proof for the working of the method in reducing states represented in large Hilbert spaces while maintaining the features required for any further machine learning algorithm that follows.}, journal={Mathematics}, author={Selvarajan, Raja and Sajjan, Manas and Humble, Travis S. and Kais, Sabre}, year={2023}, month={Nov} }
 @article{bhatia_kais_alam_2023, title={Federated quanvolutional neural network: a new paradigm for collaborative quantum learning}, url={https://doi.org/10.1088/2058-9565/acfc61}, DOI={10.1088/2058-9565/acfc61}, abstractNote={Abstract In recent years, the concept of federated machine learning has been actively driven by scientists to ease the privacy concerns of data owners. Currently, the combination of machine learning and quantum computing technologies is a hot industry topic and is positioned to be a major disruptor. It has become an effective new tool for reshaping several industries ranging from healthcare to finance. Data sharing poses a significant hurdle for large-scale machine learning in numerous industries. It is a natural goal to study the advanced quantum computing ecosystem, which will be comprised of heterogeneous federated resources. In this work, the problem of data governance and privacy is handled by developing a quantum federated learning approach, that can be efficiently executed on quantum hardware in the noisy intermediate-scale quantum era. We present the federated hybrid quantum–classical algorithm called a quanvolutional neural network with distributed training on different sites without exchanging data. The hybrid algorithm requires small quantum circuits to produce meaningful features for image classification tasks, which makes it ideal for near-term quantum computing. The primary goal of this work is to evaluate the potential benefits of hybrid quantum–classical and classical-quantum convolutional neural networks on non-independently and non-identically partitioned (Non-IID) and real-world data partitioned datasets among several healthcare institutions/clients. We investigated the performance of a collaborative quanvolutional neural network on two medical machine learning datasets, COVID-19 and MedNIST. Extensive experiments are carried out to validate the robustness and feasibility of the proposed quantum federated learning framework. Our findings demonstrate a decrease of 2%–39% times in necessary communication rounds compared to the federated stochastic gradient descent approach. The hybrid federated framework maintained a high classification testing accuracy and generalizability, even in scenarios where the medical data is unevenly distributed among clients.}, journal={Quantum Science and Technology}, author={Bhatia, Amandeep Singh and Kais, Sabre and Alam, Muhammad Ashraful}, year={2023}, month={Oct} }
 @article{muruganandam_sajjan_kais_2023, title={Foray into the topology of poly-bi-[8]-annulenylene}, url={https://doi.org/10.22541/au.168630127.74843559/v1}, DOI={10.22541/au.168630127.74843559/v1}, abstractNote={Analyzing phase transitions using the inherent geometrical attributes of a system has garnered enormous interest over the past few decades. The usual candidate often used for investigation is graphene- the most celebrated material among the family of tri co-ordinated graphed lattices. We show in this report that other inhabitants of the family demonstrate equally admirable structural and functional properties that at its core are controlled by their topology. Two interesting members of the family are Cylooctatrene(COT) and COT-based polymer: poly-bi-[8]-annulenylene both in one and two dimensions that have been investigated by polymer chemists over a period of 50 years for its possible application in batteries exploiting its conducting properties. A single COT unit is demonstrated herein to exhibit topological solitons at sites of a broken bond similar to an open one-dimensional Su-Schrieffer-Heeger (SSH) chain. We observe that Poly-bi-[8]-annulenylene in 1D mimics two coupled SSH chains in the weak coupling limit thereby showing the presence of topological edge modes. In the strong coupling limit, we investigate the different parameter values of our system for which we observe zero energy modes. Further, the application of an external magnetic field and its effects on the band-flattening of the energy bands has also been studied. In 2D, poly-bi-[8]-annulenylene forms a square-octagon lattice which upon breaking time-reversal symmetry goes into a topological phase forming noise-resilient edge modes. We hope our analysis would pave the way for synthesizing such topological materials and exploiting their properties for promising applications in optoelectronics, photovoltaics, and renewable energy sources.}, author={Muruganandam, Varadharajan and Sajjan, Manas and Kais, Sabre}, year={2023}, month={Jun} }
 @article{muruganandam_sajjan_kais_2023, title={Foray into the topology of poly‐bi‐[8]‐annulenylene}, url={https://doi.org/10.1002/ntls.20230015}, DOI={10.1002/ntls.20230015}, abstractNote={Abstract Analyzing phase transitions using the inherent geometrical attributes of a system has garnered enormous interest over the past few decades. The usual candidate often used for investigation is graphene—the most celebrated material among the family of tri‐coordinated graphed lattices. We show in this report that other inhabitants of the family demonstrate equally admirable structural and functional properties that at its core are controlled by their topology. Two interesting members of the family are cyclooctatrene (COT) and COT‐based polymer: poly‐bi‐[8]‐annulenylene, both in one and two dimensions that have been investigated by polymer chemists over a period of 50 years for its possible application in batteries exploiting its conducting properties. A single COT unit is demonstrated herein to exhibit topological solitons at sites of a broken bond similar to an open one‐dimensional Su–Schrieffer–Heeger (SSH) chain. We observe that poly‐bi‐[8]‐annulenylene in one dimension mimics two coupled SSH chains in the weak coupling limit, thereby showing the presence of topological edge modes. In the strong coupling limit, we investigate the different parameter values of our system for which we observe zero‐energy modes. Further, the application of an external magnetic field and its effects on the band flattening of the energy bands has also been studied. In two dimensions, poly‐bi‐[8]‐annulenylene forms a square‐octagon lattice which upon breaking time‐reversal symmetry goes into a topological phase forming noise‐resilient edge modes. We hope our analysis would pave the way for synthesizing such topological materials and exploiting their properties for promising applications in optoelectronics, photovoltaics, and renewable energy sources. Key Points We show in this paper tri‐coordinated lattice systems: cylooctatrene (COT) and COT‐based polymer: poly‐bi‐[8]‐annulenylene exhibit exotic topological properties. Flat bands are generated upon application of tailored magnetic flux for poly‐bi‐[8]‐annulenylene in one dimension. Insights from this paper open the possibility of using these polymers as an experimental ground to observe many flat‐band and topology‐related phenomena.}, journal={Natural Sciences}, author={Muruganandam, Varadharajan and Sajjan, Manas and Kais, Sabre}, year={2023}, month={Oct} }
 @article{gupta_selvarajan_sajjan_levine_kais_2023, title={Hamiltonian Learning from Time Dynamics Using Variational Algorithms}, url={https://doi.org/10.1021/acs.jpca.2c08993}, DOI={10.1021/acs.jpca.2c08993}, abstractNote={The Hamiltonian of a quantum system governs the dynamics of the system via the Schrodinger equation. In this paper, the Hamiltonian is reconstructed in the Pauli basis using measurables on random states forming a time series dataset. The time propagation is implemented through Trotterization and optimized variationally with gradients computed on the quantum circuit. We validate our output by reproducing the dynamics of unseen observables on a randomly chosen state not used for the optimization. Unlike the existing techniques that try and exploit the structure/properties of the Hamiltonian, our scheme is general and provides freedom with regard to what observables or initial states can be used while still remaining efficient with regard to implementation. We extend our protocol to doing quantum state learning where we solve the reverse problem of doing state learning given time series data of observables generated against several Hamiltonian dynamics. We show results on Hamiltonians involving XX, ZZ couplings along with transverse field Ising Hamiltonians and propose an analytical method for the learning of Hamiltonians consisting of generators of the SU(3) group. This paper is likely to pave the way toward using Hamiltonian learning for time series prediction within the context of quantum machine learning algorithms.}, journal={The Journal of Physical Chemistry A}, author={Gupta, Rishabh and Selvarajan, Raja and Sajjan, Manas and Levine, Raphael D. and Kais, Sabre}, year={2023}, month={Apr} }
 @article{sajjan_singh_selvarajan_kais_2023, title={Imaginary components of out-of-time-order correlator and information scrambling for navigating the learning landscape of a quantum machine learning model}, url={https://doi.org/10.1103/PhysRevResearch.5.013146}, DOI={10.1103/PhysRevResearch.5.013146}, abstractNote={We introduce and analytically illustrate that hitherto unexplored imaginary components of out-of-time correlators can provide unprecedented insight into the information scrambling capacity of a graph neural network. Furthermore, we demonstrate that it can be related to conventional measures of correlation like quantum mutual information and rigorously establish the inherent mathematical bounds (both upper and lower bound) jointly shared by such seemingly disparate quantities. To consolidate the geometrical ramifications of such bounds during the dynamical evolution of training we thereafter construct an emergent convex space. This newly designed space offers much surprising information including the saturation of lower bound by the trained network even for physical systems of large sizes, transference, and quantitative mirroring of spin correlation from the simulated physical system across phase boundaries as desirable features within the latent sub-units of the network (even though the latent units are directly oblivious to the simulated physical system) and the ability of the network to distinguish exotic spin connectivity(volume-law vs area law). Such an analysis demystifies the training of quantum machine learning models by unraveling how quantum information is scrambled through such a network introducing correlation surreptitiously among its constituent sub-systems and open a window into the underlying physical mechanism behind the emulative ability of the model.}, journal={Physical Review Research}, author={Sajjan, Manas and Singh, Vinit and Selvarajan, Raja and Kais, Sabre}, year={2023}, month={Feb} }
 @article{lyu_miano_tsioutsios_cortiñas_jung_wang_hu_geva_kais_batista_2023, title={Mapping Molecular Hamiltonians into Hamiltonians of Modular cQED Processors}, url={https://doi.org/10.1021/acs.jctc.3c00620}, DOI={10.1021/acs.jctc.3c00620}, abstractNote={We introduce a general method based on the operators of the Dyson-Masleev transformation to map the Hamiltonian of an arbitrary model system into the Hamiltonian of a circuit Quantum Electrodynamics (cQED) processor. Furthermore, we introduce a modular approach to programming a cQED processor with components corresponding to the mapping Hamiltonian. The method is illustrated as applied to quantum dynamics simulations of the Fenna-Matthews-Olson (FMO) complex and the spin-boson model of charge transfer. Beyond applications to molecular Hamiltonians, the mapping provides a general approach to implement any unitary operator in terms of a sequence of unitary transformations corresponding to powers of creation and annihilation operators of a single bosonic mode in a cQED processor.}, journal={Journal of Chemical Theory and Computation}, author={Lyu, Ningyi and Miano, Alessandro and Tsioutsios, Ioannis and Cortiñas, Rodrigo G. and Jung, Kenneth and Wang, Yuchen and Hu, Zixuan and Geva, Eitan and Kais, Sabre and Batista, Victor S.}, year={2023}, month={Oct} }
 @article{kaczmarek_szczȩśniak_kais_2023, title={Measurement-Induced Nonlocality for Observers Near a Black Hole}, url={https://doi.org/10.3390/universe9040199}, DOI={10.3390/universe9040199}, abstractNote={We present a systematic and complementary study of quantum correlations near a black hole by considering measurement-induced nonlocality (MIN). The quantum measure of interest is discussed for the fermionic, bosonic and mixed fermion–boson modes on equal footing with respect to the Hawking radiation. The obtained results show that in the infinite Hawking temperature limit, the physically accessible correlations do not vanish only in the fermionic case. However, the higher frequency modes can sustain correlations for the finite Hawking temperature, with mixed systems being more sensitive towards the increase in the fermionic frequencies than the bosonic ones. Since the MIN for the latter modes quickly diminishes, the increased frequency may be a way to maintain nonlocal correlations for the scenarios at the finite Hawking temperature.}, journal={Universe}, author={Kaczmarek, Adam Z. and Szczȩśniak, Dominik and Kais, Sabre}, year={2023}, month={Apr} }
 @article{sajjan_gupta_kale_singh_kumaran_kais_2023, title={Physics-Inspired Quantum Simulation of Resonating Valence Bond States─A Prototypical Template for a Spin-Liquid Ground State}, url={https://doi.org/10.1021/acs.jpca.3c05172}, DOI={10.1021/acs.jpca.3c05172}, abstractNote={Spin liquids─an emergent, exotic collective phase of matter─have garnered enormous attention in recent years. While experimentally many prospective candidates have been proposed and realized, theoretically modeling real materials that display such behavior may pose serious challenges due to the inherently high correlation content of such phases. Over the last few decades, the second-quantum revolution has been the harbinger of a novel computational paradigm capable of initiating a foundational evolution in computational physics. In this report, we strive to use the power of the latter to study a prototypical model, a spin-1/2-unit cell of a Kagome antiferromagnet. Extended lattices of such unit cells are known to possess a magnetically disordered spin-liquid ground state. We employ robust classical numerical techniques such as the density-matrix renormalization group (DMRG) to identify the nature of the ground state through a matrix-product state (MPS) formulation. We subsequently use the gained insight to construct an auxiliary Hamiltonian with reduced measurables and also design an ansatz that is modular and gate-efficient. With robust error-mitigation strategies, we are able to demonstrate that the said ansatz is capable of accurately representing the target ground state even on a real IBMQ backend within 1% accuracy in energy. Since the protocol is linearly scaling O(n) in the number of unit cells, gate requirements, and the number of measurements, it is straightforwardly extendable to larger Kagome lattices that can pave the way for efficient construction of spin-liquid ground states on a quantum device.}, journal={The Journal of Physical Chemistry A}, author={Sajjan, Manas and Gupta, Rishabh and Kale, Sumit Suresh and Singh, Vinit and Kumaran, Keerthi and Kais, Sabre}, year={2023}, month={Oct} }
 @article{bhatia_saggi_kais_2023, title={Quantum Machine Learning Predicting ADME-Tox Properties in Drug Discovery}, url={https://doi.org/10.1021/acs.jcim.3c01079}, DOI={10.1021/acs.jcim.3c01079}, abstractNote={In the drug discovery paradigm, the evaluation of absorption, distribution, metabolism, and excretion (ADME) and toxicity properties of new chemical entities is one of the most critical issues, which is a time-consuming process, immensely expensive, and poses formidable challenges in pharmaceutical R&D. In recent years, emerging technologies like artificial intelligence (AI), big data, and cloud technologies have garnered great attention to predict the ADME and toxicity of molecules. Currently, the blend of quantum computation and machine learning has attracted considerable attention in almost every field ranging from chemistry to biomedicine and several engineering disciplines as well. Quantum computers have the potential to bring advances in high-throughput experimental techniques and in screening billions of molecules by reducing development costs and time associated with the drug discovery process. Motivated by the efficiency of quantum kernel methods, we proposed a quantum machine learning (QML) framework consisting of a classical support vector classifier algorithm with a kernel-based quantum classifier. To demonstrate the feasibility of the proposed QML framework, the simplified molecular input line entry system (SMILES) notation-based string kernel, combined with a quantum support vector classifier, is used for the evaluation of chemical/drug ADME-Tox properties. The proposed quantum machine learning framework is validated and assessed via large-scale simulations. Based on our results from numerical simulations, the quantum model achieved the best performance as compared to classical counterparts in terms of the area under the curve of the receiver operating characteristic curve (AUC ROC; 0.80-0.95) for predicting outcomes on ADME-Tox data sets for small molecules, with a different number of features. The deployment of the proposed framework in the pharmaceutical industry would be extremely valuable in making the best decisions possible.}, journal={Journal of Chemical Information and Modeling}, author={Bhatia, Amandeep Singh and Saggi, Mandeep Kaur and Kais, Sabre}, year={2023}, month={Nov} }
 @article{zhang_hu_wang_kais_2023, title={Quantum Simulation of the Radical Pair Dynamics of the Avian Compass}, url={https://doi.org/10.1021/acs.jpclett.2c03617}, DOI={10.1021/acs.jpclett.2c03617}, abstractNote={The simulation of open quantum dynamics on quantum circuits has attracted wide interests recently with a variety of quantum algorithms developed and demonstrated. Among these, one particular design of a unitary-dilation-based quantum algorithm is capable of simulating general and complex physical systems. In this paper, we apply this quantum algorithm to simulating the dynamics of the radical pair mechanism in the avian compass. This application is demonstrated on the IBM QASM quantum simulator. This work is the first application of any quantum algorithm to simulating the radical pair mechanism in the avian compass, which not only demonstrates the generality of the quantum algorithm but also opens new opportunities for studying the avian compass with quantum computing devices.}, journal={The Journal of Physical Chemistry Letters}, author={Zhang, Yiteng and Hu, Zixuan and Wang, Yuchen and Kais, Sabre}, year={2023}, month={Jan} }
 @article{wang_mulvihill_hu_lyu_shivpuje_liu_soley_geva_batista_kais_2023, title={Simulating Open Quantum System Dynamics on NISQ Computers with Generalized Quantum Master Equations}, url={https://doi.org/10.1021/acs.jctc.3c00316}, DOI={10.1021/acs.jctc.3c00316}, abstractNote={We present a quantum algorithm based on the generalized quantum master equation (GQME) approach to simulate open quantum system dynamics on noisy intermediate-scale quantum (NISQ) computers. This approach overcomes the limitations of the Lindblad equation, which assumes weak system-bath coupling and Markovity, by providing a rigorous derivation of the equations of motion for any subset of elements of the reduced density matrix. The memory kernel resulting from the effect of the remaining degrees of freedom is used as input to calculate the corresponding non-unitary propagator. We demonstrate how the Sz.-Nagy dilation theorem can be employed to transform the non-unitary propagator into a unitary one in a higher-dimensional Hilbert space, which can then be implemented on quantum circuits of NISQ computers. We validate our quantum algorithm as applied to the spin-boson benchmark model by analyzing the impact of the quantum circuit depth on the accuracy of the results when the subset is limited to the diagonal elements of the reduced density matrix. Our findings demonstrate that our approach yields reliable results on NISQ IBM computers.}, journal={Journal of Chemical Theory and Computation}, author={Wang, Yuchen and Mulvihill, Ellen and Hu, Zixuan and Lyu, Ningyi and Shivpuje, Saurabh and Liu, Yudan and Soley, Micheline B. and Geva, Eitan and Batista, Victor S. and Kais, Sabre}, year={2023}, month={Aug} }
 @article{hu_kais_2023, title={The unitary dependence theory for characterizing quantum circuits and states}, url={https://doi.org/10.1038/s42005-023-01188-y}, DOI={10.1038/s42005-023-01188-y}, abstractNote={Abstract Most existing quantum algorithms are discovered accidentally or adapted from classical algorithms, and there is the need for a systematic theory to understand and design quantum circuits. Here we develop a unitary dependence theory to characterize the behaviors of quantum circuits and states in terms of how quantum gates manipulate qubits and determine their measurement probabilities. Compared to the conventional entanglement description of quantum circuits and states, the unitary dependence picture offers more practical information on the measurement and manipulation of qubits, easier generalization to many-qubit systems, and better robustness upon partitioning of the system. The unitary dependence theory can be applied to systematically understand existing quantum circuits and design new quantum algorithms.}, journal={Communications Physics}, author={Hu, Zixuan and Kais, Sabre}, year={2023}, month={Apr} }
 @article{li_jones_kais_2023, title={Toward perturbation theory methods on a quantum computer}, url={https://doi.org/10.1126/sciadv.adg4576}, DOI={10.1126/sciadv.adg4576}, abstractNote={Perturbation theory, used in a wide range of fields, is a powerful tool for approximate solutions to complex problems, starting from the exact solution of a related, simpler problem. Advances in quantum computing, especially over the past several years, provide opportunities for alternatives to classical methods. Here, we present a general quantum circuit estimating both the energy and eigenstates corrections that is far superior to the classical version when estimating second-order energy corrections. We demonstrate our approach as applied to the two-site extended Hubbard model. In addition to numerical simulations based on qiskit, results on IBM's quantum hardware are also presented. Our work offers a general approach to studying complex systems with quantum devices, with no training or optimization process needed to obtain the perturbative terms, which can be generalized to other Hamiltonian systems both in chemistry and physics.}, journal={Science Advances}, author={Li, Junxu and Jones, Barbara A. and Kais, Sabre}, year={2023}, month={May} }
 @article{hu_head-marsden_mazziotti_narang_kais_2022, title={A general quantum algorithm for open quantum dynamics demonstrated with the Fenna-Matthews-Olson complex}, url={https://doi.org/10.22331/q-2022-05-30-726}, DOI={10.22331/q-2022-05-30-726}, abstractNote={Using quantum algorithms to simulate complex physical processes and correlations in quantum matter has been a major direction of quantum computing research, towards the promise of a quantum advantage over classical approaches. In this work we develop a generalized quantum algorithm to simulate any dynamical process represented by either the operator sum representation or the Lindblad master equation. We then demonstrate the quantum algorithm by simulating the dynamics of the Fenna-Matthews-Olson (FMO) complex on the IBM QASM quantum simulator. This work represents a first demonstration of a quantum algorithm for open quantum dynamics with a moderately sophisticated dynamical process involving a realistic biological structure. We discuss the complexity of the quantum algorithm relative to the classical method for the same purpose, presenting a decisive query complexity advantage of the quantum approach based on the unique property of quantum measurement.}, journal={Quantum}, author={Hu, Zixuan and Head-Marsden, Kade and Mazziotti, David A. and Narang, Prineha and Kais, Sabre}, year={2022}, month={May} }
 @article{ghosh_kais_herschbach_2022, title={Geometrical picture of the electron–electron correlation at the large-D limit}, volume={24}, url={https://doi.org/10.1039/D2CP00438K}, DOI={10.1039/D2CP00438K}, abstractNote={In electronic structure calculations, the correlation energy is defined as the difference between the mean field and the exact solution of the non relativistic Schrödinger equation. Such an error in the different calculations is not directly observable as there is no simple quantum mechanical operator, apart from correlation functions, that correspond to such quantity. Here, we use the dimensional scaling approach, in which the electrons are localized at the large-dimensional scaled space, to describe a geometric picture of the electronic correlation. Both, the mean field, and the exact solutions at the large-D limit have distinct geometries. Thus, the difference might be used to describe the correlation effect. Moreover, correlations can be also described and quantified by the entanglement between the electrons, which is a strong correlation without a classical analog. Entanglement is directly observable and it is one of the most striking properties of quantum mechanics and bounded by the area law for local gapped Hamiltonians of interacting many-body systems. This study opens up the possibility of presenting a geometrical picture of the electron-electron correlations and might give a bound on the correlation energy. The results at the large-D limit and at D = 3 indicate the feasibility of using the geometrical picture to get a bound on the electron-electron correlations.}, number={16}, journal={Physical Chemistry Chemical Physics}, publisher={Royal Society of Chemistry (RSC)}, author={Ghosh, Kumar J. B. and Kais, Sabre and Herschbach, Dudley R.}, year={2022}, pages={9298–9307} }
 @article{sajjan_alaeian_kais_2022, title={Magnetic phases of spatially modulated spin-1 chains in Rydberg excitons: Classical and quantum simulations}, url={https://doi.org/10.1063/5.0128283}, DOI={10.1063/5.0128283}, abstractNote={In this work, we study the magnetic phases of a spatially-modulated chain of spin-1 Rydberg excitons. Using the Density Matrix Renormalization Group (DMRG) technique we study various magnetic and topologically nontrivial phases using both single-particle properties like local magnetization and quantum entropy as well as many-body ones like pair-wise N\'eel and long-range string correlations. In particular, we investigate the emergence and robustness of Haldane phase, a topological phase of anti-ferromagnetic spin-1 chains. Further, we devise a hybrid quantum algorithm employing Restricted Boltzmann Machine to simulate the ground state of such a system which shows very good agreement with the results of exact diagonalization (ED) and DMRG.}, journal={The Journal of Chemical Physics}, author={Sajjan, Manas and Alaeian, Hadiseh and Kais, Sabre}, year={2022}, month={Dec} }
 @article{sajjan_li_selvarajan_sureshbabu_kale_gupta_singh_kais_2022, title={Quantum machine learning for chemistry and physics}, url={https://doi.org/10.1039/D2CS00203E}, DOI={10.1039/D2CS00203E}, abstractNote={Machine learning (ML) has emerged into formidable force for identifying hidden but pertinent patterns within a given data set with the objective of subsequent generation of automated predictive behavior. In the recent years, it is safe to conclude that ML and its close cousin deep learning (DL) have ushered unprecedented developments in all areas of physical sciences especially chemistry. Not only the classical variants of ML , even those trainable on near-term quantum hardwares have been developed with promising outcomes. Such algorithms have revolutionzed material design and performance of photo-voltaics, electronic structure calculations of ground and excited states of correlated matter, computation of force-fields and potential energy surfaces informing chemical reaction dynamics, reactivity inspired rational strategies of drug designing and even classification of phases of matter with accurate identification of emergent criticality. In this review we shall explicate a subset of such topics and delineate the contributions made by both classical and quantum computing enhanced machine learning algorithms over the past few years. We shall not only present a brief overview of the well-known techniques but also highlight their learning strategies using statistical physical insight. The objective of the review is to not only to foster exposition to the aforesaid techniques but also to empower and promote cross-pollination among future-research in all areas of chemistry which can benefit from ML and in turn can potentially accelerate the growth of such algorithms.}, journal={Chemical Society Reviews}, author={Sajjan, Manas and Li, Junxu and Selvarajan, Raja and Sureshbabu, Shree Hari and Kale, Sumit Suresh and Gupta, Rishabh and Singh, Vinit and Kais, Sabre}, year={2022} }
 @article{yue_wei_kais_friedrich_herschbach_2022, title={Realization of Heisenberg models of spin systems with polar molecules in pendular states}, url={https://doi.org/10.1039/D2CP00380E}, DOI={10.1039/D2CP00380E}, abstractNote={We show that ultra-cold polar diatomic or linear molecules, oriented in an external electric field and mutually coupled by dipole-dipole interactions, can be used to realize the exact Heisenberg XYZ, XXZ and XY models without invoking any approximation. The two lowest lying excited pendular states coupled by microwave or radio-frequency fields are used to encode the pseudo-spin. We map out the general features of the models by evaluating the models constants as functions of the molecular dipole moment, rotational constant, strength and direction of the external field as well as the distance between molecules. We calculate the phase diagram for a linear chain of polar molecules based on the Heisenberg models and discuss their drawbacks, advantages, and potential applications.}, journal={Physical Chemistry Chemical Physics}, author={Yue, Wenjing and Wei, Qi and Kais, Sabre and Friedrich, Bretislav and Herschbach, Dudley}, year={2022} }
 @article{oh_kais_2022, title={Statistical Properties of Bit Strings Sampled from Sycamore Random Quantum Circuits}, url={https://doi.org/10.1021/acs.jpclett.2c02045}, DOI={10.1021/acs.jpclett.2c02045}, abstractNote={Quantum supremacy has been recently reported for random circuit sampling on the Sycamore processor with 53 qubits. Here, we analyze the statistical properties of bit strings sampled from random quantum circuits. In contrast to classical random bit strings, bit strings sampled from Sycamore random circuits give rise to heat maps with stripe patterns at specific qubits, have more bit 1 than 0, and do not pass the NIST random number tests. The difference between the Sycamore bit strings and classical random bit strings is also demonstrated by the Marchenko-Pastur distribution and the Girko circular law of random matrices. The calculation of Wasserstein distances shows that the Sycamore bit strings are farther from bit strings sampled from Haar-measure random quantum circuits than classical random bit strings. Our results show that random matrices and Wasserstein distances could be used to analyze the performance of quantum computers.}, journal={The Journal of Physical Chemistry Letters}, author={Oh, Sangchul and Kais, Sabre}, year={2022}, month={Aug} }
 @article{oh_kais_2022, title={Statistical analysis on random quantum circuit sampling by Sycamore and Zuchongzhi quantum processors}, url={https://doi.org/10.1103/PhysRevA.106.032433}, DOI={10.1103/PhysRevA.106.032433}, abstractNote={Random quantum sampling, a task to sample bit-strings from a random quantum circuit, is considered one of suitable benchmark tasks to demonstrate the outperformance of quantum computers even with noisy qubits. Recently, random quantum sampling was performed on the Sycamore quantum processor with 53 qubits [Nature 574, 505 (2019)] and on the Zuchongzhi quantum processor with 56 qubits [Phys. Rev. Lett. 127, 180501 (2021)]. Here, we analyze and compare statistical properties of the outputs of random quantum sampling by Sycamore and Zuchongzhi. Using the Marchenko-Pastur law and the Wasssertein distances, we find that quantum random sampling of Zuchongzhi is more closer to classical uniform random sampling than those of Sycamore. Some Zuchongzhi's bit-strings pass the random number tests while both Sycamore and Zuchongzhi show similar patterns in heatmaps of bit-strings. It is shown that statistical properties of both random quantum samples change little as the depth of random quantum circuits increases. Our findings raise a question about computational reliability of noisy quantum processors that could produce statistically different outputs for the same random quantum sampling task.}, journal={Physical Review A}, author={Oh, Sangchul and Kais, Sabre}, year={2022}, month={Sep} }
 @article{dixit_selvarajan_aldwairi_koshka_novotny_humble_alam_kais_2022, title={Training a Quantum Annealing Based Restricted Boltzmann Machine on Cybersecurity Data}, volume={6}, url={https://doi.org/10.1109/TETCI.2021.3074916}, DOI={10.1109/TETCI.2021.3074916}, abstractNote={We present a real-world application that uses a quantum computer. Specifically, we train a RBM using QA for cybersecurity applications. The D-Wave 2000Q has been used to implement QA. RBMs are trained on the ISCX data, which is a benchmark dataset for cybersecurity. For comparison, RBMs are also trained using CD. CD is a commonly used method for RBM training. Our analysis of the ISCX data shows that the dataset is imbalanced. We present two different schemes to balance the training dataset before feeding it to a classifier. The first scheme is based on the undersampling of benign instances. The imbalanced training dataset is divided into five sub-datasets that are trained separately. A majority voting is then performed to get the result. Our results show the majority vote increases the classification accuracy up from 90.24% to 95.68%, in the case of CD. For the case of QA, the classification accuracy increases from 74.14% to 80.04%. In the second scheme, a RBM is used to generate synthetic data to balance the training dataset. We show that both QA and CD-trained RBM can be used to generate useful synthetic data. Balanced training data is used to evaluate several classifiers. Among the classifiers investigated, K-Nearest Neighbor (KNN) and Neural Network (NN) perform better than other classifiers. They both show an accuracy of 93%. Our results show a proof-of-concept that a QA-based RBM can be trained on a 64-bit binary dataset. The illustrative example suggests the possibility to migrate many practical classification problems to QA-based techniques. Further, we show that synthetic data generated from a RBM can be used to balance the original dataset.}, number={3}, journal={IEEE Transactions on Emerging Topics in Computational Intelligence}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Dixit, Vivek and Selvarajan, Raja and Aldwairi, Tamer and Koshka, Yaroslav and Novotny, Mark A. and Humble, Travis S. and Alam, Muhammad A. and Kais, Sabre}, year={2022}, month={Jun}, pages={417–428} }
 @article{verde_cremer_kais_mochizuki_roke_2022, title={Tribute to Dor Ben-Amotz}, volume={126}, url={https://doi.org/10.1021/acs.jpcb.2c01804}, DOI={10.1021/acs.jpcb.2c01804}, abstractNote={ADVERTISEMENT RETURN TO ISSUESpecial Issue Prefac...Special Issue PrefaceNEXTTribute to Dor Ben-AmotzAna Vila Verde*Ana Vila VerdeUniversity of Duisburg-Essen, Faculty of Physics, Lotharstrasse 1, 47057 Duisburg, Germany*Email: [email protected]More by Ana Vila Verdehttps://orcid.org/0000-0003-0337-3972, Paul Cremer*Paul CremerDepartment of Chemistry and Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, United States*Email: [email protected]More by Paul Cremerhttps://orcid.org/0000-0002-8524-0438, Sabre Kais*Sabre KaisDepartment of Chemistry, Department of Physics and Astronomy, and Purdue Quantum Science and Engineering Institute, Purdue University, West Lafayette, Indiana 47907, United States*Email: [email protected]More by Sabre Kaishttps://orcid.org/0000-0003-0574-5346, Kenji Mochizuki*Kenji MochizukiDepartment of Chemistry, Zhejiang University, Hangzhou 310028, China*Email: [email protected]More by Kenji Mochizukihttps://orcid.org/0000-0002-8947-9980, and Sylvie Roke*Sylvie RokeLaboratory for fundamental BioPhotonics (LBP), Institute of Bioengineering (IBI), and Institute of Materials Science (IMX), School of Engineering (STI), and Lausanne Center for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland*Email: [email protected]More by Sylvie Rokehttps://orcid.org/0000-0002-6062-7871Cite this: J. Phys. Chem. B 2022, 126, 16, 2943–2945Publication Date (Web):April 28, 2022Publication History Published online28 April 2022Published inissue 28 April 2022https://doi.org/10.1021/acs.jpcb.2c01804Copyright © Published 2022 by American Chemical SocietyRIGHTS & PERMISSIONSArticle Views792Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Food science,Hydration,Physical chemistry,Solution chemistry,Students Get e-Alerts}, number={16}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Verde, Ana Vila and Cremer, Paul and Kais, Sabre and Mochizuki, Kenji and Roke, Sylvie}, year={2022}, month={Apr}, pages={2943–2945} }
 @article{selvarajan_sajjan_kais_2022, title={Variational Quantum Circuits to Prepare Low Energy Symmetry States}, url={https://doi.org/10.3390/sym14030457}, DOI={10.3390/sym14030457}, abstractNote={We explore how to build quantum circuits that compute the lowest energy state corresponding to a given Hamiltonian within a symmetry subspace by explicitly encoding it into the circuit. We create an explicit unitary and a variationally trained unitary that maps any vector output by ansatz A(α→) from a defined subspace to a vector in the symmetry space. The parameters are trained varitionally to minimize the energy, thus keeping the output within the labelled symmetry value. The method was tested for a spin XXZ Hamiltonian using rotation and reflection symmetry and H2 Hamiltonian within Sz=0 subspace using S2 symmetry. We have found the variationally trained unitary gives good results with very low depth circuits and can thus be used to prepare symmetry states within near term quantum computers.}, journal={Symmetry}, author={Selvarajan, Raja and Sajjan, Manas and Kais, Sabre}, year={2022}, month={Feb} }
 @article{gupta_sajjan_levine_kais_2022, title={Variational approach to quantum state tomography based on maximal entropy formalism}, url={https://doi.org/10.1039/D2CP04493E}, DOI={10.1039/D2CP04493E}, abstractNote={Quantum state tomography is an integral part of quantum computation and offers the starting point for the validation of various quantum devices. One of the central tasks in the field of state tomography is to reconstruct with high fidelity, the quantum states of a quantum system. From an experiment on a real quantum device, one can obtain the mean measurement values of different operators. With such a data as input, in this report we employ the maximal entropy formalism to construct the least biased mixed quantum state that is consistent with the given set of expectation values. Even though in principle, the reported formalism is quite general and should work for an arbitrary set of observables, in practice we shall demonstrate the efficacy of the algorithm on an informationally complete (IC) set of Hermitian operators. Such a set possesses the advantage of uniquely specifying a single quantum state from which the experimental measurements have been sampled and hence renders the rare opportunity to not only construct a least-biased quantum state but even replicate the exact state prepared experimentally within a preset tolerance. The primary workhorse of the algorithm is re-constructing an energy function which we designate as the effective Hamiltonian of the system, and parameterizing it with Lagrange multipliers, according to the formalism of maximal entropy. These parameters are thereafter optimized variationally so that the reconstructed quantum state of the system converges to the true quantum state within an error threshold. To this end, we employ a parameterized quantum circuit and a hybrid quantum-classical variational algorithm to obtain such a target state making our recipe easily implementable on a near-term quantum device.}, journal={Physical Chemistry Chemical Physics}, author={Gupta, Rishabh and Sajjan, Manas and Levine, Raphael D. and Kais, Sabre}, year={2022} }
 @article{li_kais_2021, title={A universal quantum circuit design for periodical functions}, volume={10}, url={https://doi.org/10.1088/1367-2630/ac2cb4}, DOI={10.1088/1367-2630/ac2cb4}, abstractNote={We propose a universal quantum circuit design that can estimate any arbitrary one-dimensional periodic functions based on the corresponding Fourier expansion. The quantum circuit contains N-qubits to store the information on the different N-Fourier components and M + 2 auxiliary qubits with M = ⌈log2 N⌉ for control operations. The desired output will be measured in the last qubit qN with a time complexity of the computation of , which leads to polynomial speedup under certain circumstances. We illustrate the approach by constructing the quantum circuit for the square wave function with accurate results obtained by direct simulations using the IBM-QASM simulator. The approach is general and can be applied to any arbitrary periodic function.}, journal={New Journal of Physics}, publisher={IOP Publishing}, author={Li, Junxu and Kais, Sabre}, year={2021}, month={Oct} }
 @article{kale_chen_kais_2021, title={Constructive Quantum Interference in Photochemical Reactions}, volume={11}, url={https://doi.org/10.1021/acs.jctc.1c00826}, DOI={10.1021/acs.jctc.1c00826}, abstractNote={Interferences emerge when multiple pathways coexist together, leading toward the same result. Here, we report a theoretical study for a reaction scheme that leads to constructive quantum interference in a photoassociation (PA) reaction of a 87Rb Bose-Einstein condensate where the reactant spin state is prepared in a coherent superposition of multiple bare spin states. This is achieved by changing the reactive scattering channel in the PA reaction. As the origin of coherent control comes from the spin part of the wavefunction, we show that it is sufficient to use radio frequency (RF) coupling to achieve the superposition state. We simulate the RF coupling on a quantum processor (IBMQ Lima), and our results show that interferences can be used as a resource for the coherent control of photochemical reactions. The approach is general and can be employed to study a wide spectrum of chemical reactions in the ultracold regime.}, journal={Journal of Chemical Theory and Computation}, publisher={American Chemical Society (ACS)}, author={Kale, Sumit Suresh and Chen, Yong P. and Kais, Sabre}, year={2021}, month={Dec} }
 @article{gupta_levine_kais_2021, title={Convergence of a Reconstructed Density Matrix to a Pure State Using the Maximal Entropy Approach}, volume={8}, url={https://doi.org/10.1021/acs.jpca.1c05884}, DOI={10.1021/acs.jpca.1c05884}, abstractNote={Impressive progress has been made in the past decade in the study of technological applications of varied types of quantum systems. With industry giants like IBM laying down their roadmap for scalable quantum devices with more than 1000-qubits by the end of 2023, efficient validation techniques are also being developed for testing quantum processing on these devices. The characterization of a quantum state is done by experimental measurements through the process called quantum state tomography (QST) which scales exponentially with the size of the system. However, QST performed using incomplete measurements is aptly suited for characterizing these quantum technologies especially with the current nature of noisy intermediate-scale quantum (NISQ) devices where not all mean measurements are available with high fidelity. We, hereby, propose an alternative approach to QST for the complete reconstruction of the density matrix of a quantum system in a pure state for any number of qubits by applying the maximal entropy formalism on the pairwise combinations of the known mean measurements. This approach provides the best estimate of the target state when we know the complete set of observables which is the case of convergence of the reconstructed density matrix to a pure state. Our goal is to provide a practical inference of a quantum system in a pure state that can find its applications in the field of quantum error mitigation on a real quantum computer that we intend to investigate further.}, journal={The Journal of Physical Chemistry A}, publisher={American Chemical Society (ACS)}, author={Gupta, Rishabh and Levine, Raphael D. and Kais, Sabre}, year={2021}, month={Sep} }
 @article{ghosh_kais_herschbach_2021, title={Dimensional Interpolation for Random Walk}, volume={8}, url={https://doi.org/10.1021/acs.jpca.1c05551}, DOI={10.1021/acs.jpca.1c05551}, abstractNote={We employ a simple and accurate dimensional interpolation formula for the shapes of random walks at $D=3$ and $D=2$ based on the analytically known solutions at both limits $D=\infty$ and $D=1$. The results obtained for the radii of gyration of an arbitrary shaped object are about $2\%$ error compared with accurate numerical results at $D = 3$ and $D = 2$. We also calculated the asphericity for a three-dimensional random walk using the dimensional interpolation formula. Result agrees very well with the numerically simulated result. The method is general and can be used to estimate other properties of random walks.}, journal={The Journal of Physical Chemistry A}, publisher={American Chemical Society (ACS)}, author={Ghosh, Kumar J. B. and Kais, Sabre and Herschbach, Dudley R.}, year={2021}, month={Sep} }
 @article{ghosh_kais_herschbach_2021, title={Dimensional interpolation for metallic hydrogen}, url={https://doi.org/10.1039/D0CP05301E}, DOI={10.1039/D0CP05301E}, abstractNote={We employ a simple and mostly accurate dimensional interpolation formula using dimensional limits $D=1$ and $D=\infty$ to obtain $D=3$ ground-state energy of metallic hydrogen. We also present results describing the phase transitions for different symmetries of three-dimensional structure lattices. The interpolation formula not only predicts fairly accurate energies but also predicts a correct functional form of the energy as a function of the lattice parameters. That allows us to calculate different physical quantities such as the bulk modulus, Debye temperature, and critical transition temperature, from the gradient and the curvature of the energy curve as a function of the lattice parameters. These theoretical calculations suggest that metallic hydrogen is a likely candidate for high temperature superconductivity. The dimensional interpolation formula is robust and might be useful to obtain the energies of complex many-body systems.}, journal={Physical Chemistry Chemical Physics}, publisher={Royal Society of Chemistry (RSC)}, author={Ghosh, Kumar J. B. and Kais, Sabre and Herschbach, Dudley R.}, year={2021} }
 @article{sureshbabu_sajjan_oh_kais_2021, title={Implementation of Quantum Machine Learning for Electronic Structure Calculations of Periodic Systems on Quantum Computing Devices}, volume={61}, url={https://doi.org/10.1021/acs.jcim.1c00294}, DOI={10.1021/acs.jcim.1c00294}, abstractNote={Quantum machine learning algorithms, the extensions of machine learning to quantum regimes, are believed to be more powerful as they leverage the power of quantum properties. Quantum machine learning methods have been employed to solve quantum many-body systems and have demonstrated accurate electronic structure calculations of lattice models, molecular systems, and recently periodic systems. A hybrid approach using restricted Boltzmann machines and a quantum algorithm to obtain the probability distribution that can be optimized classically is a promising method due to its efficiency and ease of implementation. Here, we implement the benchmark test of the hybrid quantum machine learning on the IBM-Q quantum computer to calculate the electronic structure of typical two-dimensional crystal structures: hexagonal-boron nitride and graphene. The band structures of these systems calculated using the hybrid quantum machine learning approach are in good agreement with those obtained by the conventional electronic structure calculations. This benchmark result implies that the hybrid quantum machine learning method, empowered by quantum computers, could provide a new way of calculating the electronic structures of quantum many-body systems.}, number={6}, journal={Journal of Chemical Information and Modeling}, publisher={American Chemical Society (ACS)}, author={Sureshbabu, Shree Hari and Sajjan, Manas and Oh, Sangchul and Kais, Sabre}, year={2021}, month={Jun}, pages={2667–2674} }
 @article{wilson_kudyshev_kildishev_kais_shalaev_boltasseva_2021, title={Machine learning framework for quantum sampling of highly constrained, continuous optimization problems}, url={https://doi.org/10.1063/5.0060481}, DOI={10.1063/5.0060481}, abstractNote={In recent years, there is growing interest in using quantum computers for solving combinatorial optimization problems. In this work, we developed a generic, machine learning-based framework for mapping continuous-space inverse design problems into surrogate quadratic unconstrained binary optimization (QUBO) problems by employing a binary variational autoencoder and a factorization machine. The factorization machine is trained as a low-dimensional, binary surrogate model for the continuous design space and sampled using various QUBO samplers. Using the D-Wave Advantage hybrid sampler and simulated annealing, we demonstrate that by repeated resampling and retraining of the factorization machine, our framework finds designs that exhibit figures of merit exceeding those of its training set. We showcase the framework's performance on two inverse design problems by optimizing (i) thermal emitter topologies for thermophotovoltaic applications and (ii) diffractive meta-gratings for highly efficient beam steering. This technique can be further scaled to leverage future developments in quantum optimization to solve advanced inverse design problems for science and engineering applications.}, journal={Applied Physics Reviews}, author={Wilson, Blake A. and Kudyshev, Zhaxylyk A. and Kildishev, Alexander V. and Kais, Sabre and Shalaev, Vladimir M. and Boltasseva, Alexandra}, year={2021}, month={Dec} }
 @article{gupta_xia_levine_kais_2021, title={Maximal Entropy Approach for Quantum State Tomography}, url={https://doi.org/10.1103/PRXQuantum.2.010318}, DOI={10.1103/PRXQuantum.2.010318}, abstractNote={Quantum computation has been growing rapidly in both theory and experiments. In particular, quantum computing devices with a large number of qubits have been developed by IBM, Google, IonQ, and others. The current quantum computing devices are noisy intermediate-scale quantum devices, and so approaches to validate quantum processing on these quantum devices are needed. One of the most common ways of validation for an n-qubit quantum system is quantum tomography, which tries to reconstruct a quantum system's density matrix by a complete set of observables. However, the inherent noise in the quantum systems and the intrinsic limitations pose a critical challenge to precisely know the actual measurement operators that make quantum tomography impractical in experiments. Here, we propose an alternative approach to quantum tomography, based on the maximal information entropy, that can predict the values of unknown observables based on the available mean measurement data. This can then be used to reconstruct the density matrix with high fidelity even though the results for some observables are missing. Of additional contexts, a practical approach to the inference of the quantum mechanical state using only partial information is also needed.1 MoreReceived 31 August 2020Accepted 24 December 2020DOI:https://doi.org/10.1103/PRXQuantum.2.010318Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMeasurement-based quantum computingQuantum computationQuantum information processingQuantum simulationQuantum tomographyQuantum Information}, journal={PRX Quantum}, author={Gupta, Rishabh and Xia, Rongxin and Levine, Raphael D. and Kais, Sabre}, year={2021}, month={Feb} }
 @article{li_hu_kais_2021, title={Practical quantum encryption protocol with varying encryption configurations}, url={https://doi.org/10.1103/PhysRevResearch.3.023251}, DOI={10.1103/PhysRevResearch.3.023251}, abstractNote={Quantum communication is an important application that derives from the burgeoning field of quantum information and quantum computation. Focusing on secure communication, quantum cryptography has two major directions of development, namely quantum key distribution and quantum encryption. In this work we propose a quantum encryption protocol that utilizes a quantum algorithm to create blocks of ciphertexts based on quantum states. The main feature of our quantum encryption protocol is that the encryption configuration of each block is determined by the previous blocks, such that additional security is provided. We then demonstrate our method by an example model encrypting the English alphabet, with numerical simulation results showing the large error rate of a mock attack by a potential adversary. The safety of the encryption method is further demonstrated against several possible attack models. With the improvements against noises, our quantum encryption protocol is a capable addition to the toolbox of quantum cryptography.2 MoreReceived 24 February 2021Accepted 4 June 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023251Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum communicationQuantum cryptographyQuantum Information}, journal={Physical Review Research}, author={Li, Junxu and Hu, Zixuan and Kais, Sabre}, year={2021}, month={Jun} }
 @article{sajjan_sureshbabu_kais_2021, title={Quantum Machine-Learning for Eigenstate Filtration in Two-Dimensional Materials}, volume={10}, url={https://doi.org/10.1021/jacs.1c06246}, DOI={10.1021/jacs.1c06246}, abstractNote={Quantum machine-learning algorithms have emerged to be a promising alternative to their classical counterparts as they leverage the power of quantum computers. Such algorithms have been developed to solve problems like electronic structure calculations of molecular systems and spin models in magnetic systems. However, the discussion in all these recipes focuses specifically on targeting the ground state. Herein we demonstrate a quantum algorithm that can filter any energy eigenstate of the system based on either symmetry properties or a predefined choice of the user. The workhorse of our technique is a shallow neural network encoding the desired state of the system with the amplitude computed by sampling the Gibbs-Boltzmann distribution using a quantum circuit and the phase information obtained classically from the nonlinear activation of a separate set of neurons. We show that the resource requirements of our algorithm are strictly quadratic. To demonstrate its efficacy, we use state filtration in monolayer transition metal dichalcogenides which are hitherto unexplored in any flavor of quantum simulations. We implement our algorithm not only on quantum simulators but also on actual IBM-Q quantum devices and show good agreement with the results procured from conventional electronic structure calculations. We thus expect our protocol to provide a new alternative in exploring the band structures of exquisite materials to usual electronic structure methods or machine-learning techniques that are implementable solely on a classical computer.}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Sajjan, Manas and Sureshbabu, Shree Hari and Kais, Sabre}, year={2021}, month={Nov} }
 @article{li_kais_2021, title={Quantum cluster algorithm for data classification}, url={https://doi.org/10.1186/s41313-021-00029-1}, DOI={10.1186/s41313-021-00029-1}, abstractNote={Abstract We present a quantum algorithm for data classification based on the nearest-neighbor learning algorithm. The classification algorithm is divided into two steps: Firstly, data in the same class is divided into smaller groups with sublabels assisting building boundaries between data with different labels. Secondly we construct a quantum circuit for classification that contains multi control gates. The algorithm is easy to implement and efficient in predicting the labels of test data. To illustrate the power and efficiency of this approach, we construct the phase transition diagram for the metal-insulator transition of VO 2 , using limited trained experimental data, where VO 2 is a typical strongly correlated electron materials, and the metallic-insulating phase transition has drawn much attention in condensed matter physics. Moreover, we demonstrate our algorithm on the classification of randomly generated data and the classification of entanglement for various Werner states, where the training sets can not be divided by a single curve, instead, more than one curves are required to separate them apart perfectly. Our preliminary result shows considerable potential for various classification problems, particularly for constructing different phases in materials.}, journal={Materials Theory}, author={Li, Junxu and Kais, Sabre}, year={2021}, month={Oct} }
 @article{bian_kais_2021, title={Quantum computing for atomic and molecular resonances}, url={https://doi.org/10.1063/5.0040477}, DOI={10.1063/5.0040477}, abstractNote={The complex-scaling method can be used to calculate molecular resonances within the Born-Oppenheimer approximation, assuming the electronic coordinates are dilated independently of the nuclear coordinates. With this method, one will calculate the complex energy of a non-Hermitian Hamiltonian, whose real part is associated with the resonance position and the imaginary part is the inverse of the lifetime. In this study, we propose techniques to simulate resonances on a quantum computer. First, we transformed the scaled molecular Hamiltonian to second-quantization and then used the Jordan-Wigner transformation to transform the scaled Hamiltonian to the qubit space. To obtain the complex eigenvalues, we introduce the Direct Measurement method, which is applied to obtain the resonances of a simple one-dimensional model potential that exhibits pre-dissociating resonances analogous to those found in diatomic molecules. Finally, we applied the method to simulate the resonances of the H$_2^-$ molecule. Numerical results from the IBM Qiskit simulators and IBM quantum computers verify our techniques.}, journal={The Journal of Chemical Physics}, author={Bian, Teng and Kais, Sabre}, year={2021}, month={May} }
 @article{xia_kais_2021, title={Qubit coupled cluster singles and doubles variational quantum eigensolver ansatz for electronic structure calculations}, volume={6}, url={https://doi.org/10.1088/2058-9565/abbc74}, DOI={10.1088/2058-9565/abbc74}, abstractNote={Abstract Variational quantum eigensolver (VQE) for electronic structure calculations is believed to be one major potential application of near term quantum computing. Among all proposed VQE algorithms, the unitary coupled cluster singles and doubles excitations (UCCSD) VQE ansatz has achieved high accuracy and received a lot of research interest. However, the UCCSD VQE based on fermionic excitations needs extra terms for the parity when using Jordan–Wigner transformation. Here we introduce a new VQE ansatz based on the particle preserving exchange gate to achieve qubit excitations. The proposed VQE ansatz has gate complexity up-bounded to O ( n 4 ) for all-to-all connectivity where n is the number of qubits of the Hamiltonian. Numerical results of simple molecular systems such as BeH 2 , H 2 O, N 2 , H 4 and H 6 using the proposed VQE ansatz gives very accurate results within errors about 10 −3 Hartree.}, number={1}, journal={Quantum Science and Technology}, publisher={IOP Publishing}, author={Xia, Rongxin and Kais, Sabre}, year={2021}, month={Jan}, pages={015001} }
 @article{li_sajjan_kale_kais_2021, title={Statistical Correlation Between Quantum Entanglement and Spin–Orbit Coupling in Crossed Beam Molecular Dynamics}, url={https://doi.org/10.1002/qute.202100098}, DOI={10.1002/qute.202100098}, abstractNote={Abstract Non‐classical features like interference are already being harnessed to control the output of chemical reactions. However, quantum entanglement which is an equally enigmatic many‐body quantum correlation can also be used as a powerful resource yet has eluded explicit attention. In this report, an experimental scheme under the crossed beam molecular dynamical setup, with the F + HD reaction, is proposed aiming to study the possible influence of entanglement within reactant pairs on the angular features of the product distribution. The aforesaid reaction has garnered interest recently, as an unusual horseshoe shape pattern in the product (HF) distribution was observed, which has been attributed to the coupling of spin and orbital degrees of freedom. An experimental scheme is proposed aiming to study the possible influence of entanglement on the necessity for the inclusion of such spin–orbit characteristics, under circumstances wherein the existence of entanglement and spin–orbit interaction is simultaneously detectable. The attainable results are further numerically simulated highlighting specific patterns corresponding to various possibilities. Such studies if extended can provide unforeseen mechanistic insight into analogous reactions, too, from the lens of quantum information.}, journal={Advanced Quantum Technologies}, author={Li, Junxu and Sajjan, Manas and Kale, Sumit Suresh and Kais, Sabre}, year={2021}, month={Dec} }
 @article{moore_wang_hu_kais_weiner_2021, title={Statistical approach to quantum phase estimation}, volume={10}, url={https://doi.org/10.1088/1367-2630/ac320d}, DOI={10.1088/1367-2630/ac320d}, abstractNote={We introduce a new statistical and variational approach to the phase estimation algorithm (PEA). Unlike the traditional and iterative PEAs which return only an eigenphase estimate, the proposed method can determine any unknown eigenstate-eigenphase pair from a given unitary matrix utilizing a simplified version of the hardware intended for the Iterative PEA (IPEA). This is achieved by treating the probabilistic output of an IPEA-like circuit as an eigenstate-eigenphase proximity metric, using this metric to estimate the proximity of the input state and input phase to the nearest eigenstate-eigenphase pair and approaching this pair via a variational process on the input state and phase. This method may search over the entire computational space, or can efficiently search for eigenphases (eigenstates) within some specified range (directions), allowing those with some prior knowledge of their system to search for particular solutions. We show the simulation results of the method with the Qiskit package on the IBM Q platform and on a local computer.}, journal={New Journal of Physics}, publisher={IOP Publishing}, author={Moore, Alexandria J and Wang, Yuchen and Hu, Zixuan and Kais, Sabre and Weiner, Andrew M}, year={2021}, month={Nov} }
 @article{szczęśniak_kais_2020, title={Gap states and valley-spin filtering in transition metal dichalcogenide monolayers}, url={https://doi.org/10.1103/PhysRevB.101.115423}, DOI={10.1103/PhysRevB.101.115423}, abstractNote={The magnetically-induced valley-spin filtering in transition metal dichalcogenide monolayers ($MX_{2}$, where $M$=Mo, W and $X$=S, Se, Te) promises new paradigm in information processing. However, the detailed understanding of this effect is still limited, regarding its underlying transport processes. Herein, it is suggested that the filtering mechanism can be greately elucidated by the concept of metal-induced gap states (MIGS), appearing in the electrode-terminated $MX_{2}$ materials {\it i.e.} the referential filter setup. In particular, the gap states are predicted here to mediate valley- and spin-resolved charge transport near the ideal electrode/$MX_{2}$ interface, and therefore to initiate filtering. It is also argued that the role of MIGS increases when the channel length is diminished, as they begin to govern the overall valley-spin transport in the tunneling regime. In what follows, the presented study yields fundamental scaling trends for the valley-spin selectivity with respect to the intrinsic physics of the filter materials. As a result, it facilitates insight into the analyzed effects and provide design guidelines toward efficient valley-spin filter devices, that base on the discussed materials or other hexagonal monolayers with a broken inversion symmetry.}, journal={Physical Review B}, author={Szczęśniak, Dominik and Kais, Sabre}, year={2020}, month={Mar} }
 @article{xia_kais_2020, title={Hybrid Quantum-Classical Neural Network for Calculating Ground State Energies of Molecules}, volume={22}, url={https://doi.org/10.3390/e22080828}, DOI={10.3390/e22080828}, abstractNote={We present a hybrid quantum-classical neural network that can be trained to perform electronic structure calculation and generate potential energy curves of simple molecules. The method is based on the combination of parameterized quantum circuits and measurements. With unsupervised training, the neural network can generate electronic potential energy curves based on training at certain bond lengths. To demonstrate the power of the proposed new method, we present the results of using the quantum-classical hybrid neural network to calculate ground state potential energy curves of simple molecules such as H2, LiH, and BeH2. The results are very accurate and the approach could potentially be used to generate complex molecular potential energy surfaces.}, number={8}, journal={Entropy}, publisher={MDPI AG}, author={Xia, Rongxin and Kais, Sabre}, year={2020}, month={Jul}, pages={828} }
 @article{charles_kais_kubis_2020, title={Introducing Open Boundary Conditions in Modeling Nonperiodic Materials and Interfaces: The Impact of the Periodicity Assumption}, volume={2}, url={https://doi.org/10.1021/acsmaterialslett.9b00523}, DOI={10.1021/acsmaterialslett.9b00523}, abstractNote={Simulations are essential to accelerate the discovery of new materials and to gain full understanding of known ones. Although hard to realize experimentally, periodic boundary conditions are omnipresent in material simulations. In this work, we introduce ROBIN (recursive open boundary and interfaces), the first method allowing open boundary conditions in material and interface modeling. The computational costs are limited to solving quantum properties in a focus area that allows explicitly discretizing millions of atoms in real space and to consider virtually any type of environment (be it periodic, regular, or random). The impact of the periodicity assumption is assessed in detail with silicon dopants in graphene. Graphene was confirmed to produce a band gap with periodic substitution of 3% carbon with silicon in agreement with published periodic boundary condition calculations. Instead, 3% randomly distributed silicon in graphene only shifts the energy spectrum. The predicted shift agrees quantitatively with published experimental data. Key insight of this assessment is, assuming periodicity elevates a small perturbation of a periodic cell into a strong impact on the material property prediction. Periodic boundary conditions can be applied on truly periodic systems only. More general systems should apply an open boundary method for reliable predictions.}, number={3}, journal={ACS Materials Letters}, publisher={American Chemical Society (ACS)}, author={Charles, James and Kais, Sabre and Kubis, Tillmann}, year={2020}, month={Mar}, pages={247–253} }
 @article{lu_hu_alshaykh_moore_wang_imany_weiner_kais_2020, title={Quantum Phase Estimation with Time‐Frequency Qudits in a Single Photon}, url={https://doi.org/10.1002/qute.201900074}, DOI={10.1002/qute.201900074}, abstractNote={The Phase Estimation Algorithm (PEA) is an important quantum algorithm used independently or as a key subroutine in other quantum algorithms. Currently most implementations of the PEA are based on qubits, where the computational units in the quantum circuits are two-dimensional states. Performing quantum computing tasks with higher dimensional states -- qudits -- has been proposed, yet a qudit-based PEA has not been realized. Using qudits can reduce the resources needed for achieving a given precision or success probability. Compared to other quantum computing hardware, photonic systems have the advantage of being resilient to noise, but the probabilistic nature of photon-photon interaction makes it difficult to realize two-photon controlled gates that are necessary components in many quantum algorithms. In this work, we report an experimental realization of a qudit-based PEA on a photonic platform, utilizing the high dimensionality in time and frequency degrees of freedom (DoFs) in a single photon. The controlled-unitary gates can be realized in a deterministic fashion, as the control and target registers are now represented by two DoFs in a single photon. This first implementation of a qudit PEA, on any platform, successfully retrieves any arbitrary phase with one ternary digit of precision.}, journal={Advanced Quantum Technologies}, author={Lu, Hsuan‐Hao and Hu, Zixuan and Alshaykh, Mohammed Saleh and Moore, Alexandria Jeanine and Wang, Yuchen and Imany, Poolad and Weiner, Andrew Marc and Kais, Sabre}, year={2020}, month={Feb} }
 @article{el-mellouhi_madjet_berdiyorov_bentria_rashkeev_kais_akande_motta_sanvito_alharbi_2019, title={Enhancing the electronic dimensionality of hybrid organic–inorganic frameworks by hydrogen bonded molecular cations}, url={https://doi.org/10.1039/C8MH01436A}, DOI={10.1039/C8MH01436A}, abstractNote={Hybrid materials with cations forming hydrogen bonds with inorganic frameworks can enhance the electronic dimensionality of the system by forming new inter-channels electron and hole transport pathways.}, journal={Materials Horizons}, publisher={Royal Society of Chemistry (RSC)}, author={El-Mellouhi, Fedwa and Madjet, Mohamed E. and Berdiyorov, Golibjon R. and Bentria, El Tayeb and Rashkeev, Sergey N. and Kais, Sabre and Akande, Akinlolu and Motta, Carlo and Sanvito, Stefano and Alharbi, Fahhad H.}, year={2019} }
 @article{li_kais_2019, title={Entanglement classifier in chemical reactions}, volume={5}, url={https://doi.org/10.1126/sciadv.aax5283}, DOI={10.1126/sciadv.aax5283}, abstractNote={The Einstein, Podolsky, and Rosen (EPR) entanglement, which features the essential difference between classical and quantum physics, has received wide theoretical and experimental attentions. Recently, the desire to understand and create quantum entanglement between particles such as spins, photons, atoms, and molecules is fueled by the development of quantum teleportation, quantum communication, quantum cryptography, and quantum computation. Although most of the work has focused on showing that entanglement violates the famous Bell's inequality and its generalization for discrete measurements, few recent attempts focus on continuous measurement results. Here, we have developed a general practical inequality to test entanglement for continuous measurement results, particularly scattering of chemical reactions. After we explain how to implement this inequality to classify entanglement in scattering experiments, we propose a specific chemical reaction to test the violation of this inequality. The method is general and could be used to classify entanglement for continuous measurement results.}, number={8}, journal={Science Advances}, publisher={American Association for the Advancement of Science (AAAS)}, author={Li, Junxu and Kais, Sabre}, year={2019}, month={Aug} }
 @article{massey_ting_yeh_dahlberg_sohail_allodi_martin_kais_hunter_engel_2019, title={Orientational Dynamics of Transition Dipoles and Exciton Relaxation in LH2 from Ultrafast Two-Dimensional Anisotropy}, volume={10}, url={https://doi.org/10.1021/acs.jpclett.8b03223}, DOI={10.1021/acs.jpclett.8b03223}, abstractNote={Light-harvesting complexes in photosynthetic organisms display fast and efficient energy transfer dynamics, which depend critically on the electronic structure of the coupled chromophores within the complexes and their interactions with their environment. We present ultrafast anisotropy dynamics, resolved in both time and frequency, of the transmembrane light-harvesting complex LH2 from Rhodobacter sphaeroides in its native membrane environment using polarization-controlled two-dimensional electronic spectroscopy. Time-dependent anisotropy obtained from both experiment and modified Redfield simulation reveals an orientational preference for excited state absorption and an ultrafast equilibration within the B850 band in LH2. This ultrafast equilibration is favorable for subsequent energy transfer toward the reaction center. Our results also show a dynamic difference in excited state absorption anisotropy between the directly excited B850 population and the population that is initially excited at 800 nm, suggesting absorption from B850 states to higher-lying excited states following energy transfer from B850*. These results give insight into the ultrafast dynamics of bacterial light harvesting and the excited state energy landscape of LH2 in the native membrane environment.}, number={2}, journal={The Journal of Physical Chemistry Letters}, publisher={American Chemical Society (ACS)}, author={Massey, Sara C. and Ting, Po-Chieh and Yeh, Shu-Hao and Dahlberg, Peter D. and Sohail, Sara H. and Allodi, Marco A. and Martin, Elizabeth C. and Kais, Sabre and Hunter, C. Neil and Engel, Gregory S.}, year={2019}, month={Jan}, pages={270–277} }
 @article{xia_bian_kais_2018, title={Electronic Structure Calculations and the Ising Hamiltonian}, volume={122}, url={https://doi.org/10.1021/acs.jpcb.7b10371}, DOI={10.1021/acs.jpcb.7b10371}, abstractNote={Obtaining exact solutions to the Schrödinger equation for atoms, molecules, and extended systems continues to be a "Holy Grail" problem which the fields of theoretical chemistry and physics have been striving to solve since inception. Recent breakthroughs have been made in the development of hardware-efficient quantum optimizers and coherent Ising machines capable of simulating hundreds of interacting spins with an Ising-type Hamiltonian. One of the most vital questions pertaining to these new devices is, "Can these machines be used to perform electronic structure calculations?" Within this work, we review the general procedure used by these devices and prove that there is an exact mapping between the electronic structure Hamiltonian and the Ising Hamiltonian. Additionally, we provide simulation results of the transformed Ising Hamiltonian for H2 , He2 , HeH+, and LiH molecules, which match the exact numerical calculations. This demonstrates that one can map the molecular Hamiltonian to an Ising-type Hamiltonian which could easily be implemented on currently available quantum hardware. This is an early step in developing generalized methods on such devices for chemical physics.}, number={13}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Xia, Rongxin and Bian, Teng and Kais, Sabre}, year={2018}, month={Apr}, pages={3384–3395} }
 @article{zhong_carignano_kais_zeng_francisco_gladich_2018, title={Tuning the Stereoselectivity and Solvation Selectivity at Interfacial and Bulk Environments by Changing Solvent Polarity: Isomerization of Glyoxal in Different Solvent Environments}, volume={140}, url={https://doi.org/10.1021/jacs.8b01503}, DOI={10.1021/jacs.8b01503}, abstractNote={Conformational isomerism plays a central role in organic synthesis and biological processes; however, effective control of isomerization processes still remains challenging and elusive. Here, we propose a novel paradigm for conformational control of isomerization in the condensed phase, in which the polarity of the solvent determines the relative concentration of conformers at the interfacial and bulk regions. By the use of state-of-the-art molecular dynamics simulations of glyoxal in different solvents, we demonstrate that the isomerization process is dipole driven: the solvent favors conformational changes toward conformers having molecular dipoles that better match its polar character. Thus, the solvent polarity modulates the conformational change, stabilizing and selectively segregating in the bulk vs the interface one conformer with respect to the others. The findings in this paper have broader implications affecting systems involving compounds with conformers of different polarity. This work suggests novel mechanisms for tuning the catalytic activity of surfaces in conformationally controlled (photo)chemical reactions and for designing a new class of molecular switches that are active in different solvent environments.}, number={16}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Zhong, Jie and Carignano, Marcelo A. and Kais, Sabre and Zeng, Xiao Cheng and Francisco, Joseph S. and Gladich, Ivan}, year={2018}, month={Apr}, pages={5535–5543} }
 @article{el‐mellouhi_marzouk_bentria_rashkeev_kais_alharbi_2016, title={Hydrogen Bonding and Stability of Hybrid Organic–Inorganic Perovskites}, url={https://doi.org/10.1002/cssc.201600864}, DOI={10.1002/cssc.201600864}, abstractNote={In the past few years, the efficiency of solar cells based on hybrid organic-inorganic perovskites has exceeded the level needed for commercialization. However, existing perovskites solar cells (PSCs) suffer from several intrinsic instabilities, which prevent them from reaching industrial maturity, and stabilizing PSCs has become a critically important problem. Here we propose to stabilize PSCs chemically by strengthening the interactions between the organic cation and inorganic anion of the perovskite framework. In particular, we show that replacing the methylammonium cation with alternative protonated cations allows an increase in the stability of the perovskite by forming strong hydrogen bonds with the halide anions. This interaction also provides opportunities for tuning the electronic states near the bandgap. These mechanisms should have a universal character in different hybrid organic-inorganic framework materials that are widely used.}, journal={ChemSusChem}, author={El‐Mellouhi, Fedwa and Marzouk, Asma and Bentria, El Tayeb and Rashkeev, Sergey N. and Kais, Sabre and Alharbi, Fahhad H.}, year={2016}, month={Sep} }
 @article{wei_cao_kais_friedrich_herschbach_2016, title={Quantum Computation using Arrays of N Polar Molecules in Pendular States}, volume={17}, url={https://doi.org/10.1002/cphc.201600781}, DOI={10.1002/cphc.201600781}, abstractNote={We investigate several aspects of realizing quantum computation using entangled polar molecules in pendular states. Quantum algorithms typically start from a product state |00⋯0⟩ and we show that up to a negligible error, the ground states of polar molecule arrays can be considered as the unentangled qubit basis state |00⋯0⟩ . This state can be prepared by simply allowing the system to reach thermal equilibrium at low temperature (<1 mK). We also evaluate entanglement, characterized by concurrence of pendular state qubits in dipole arrays as governed by the external electric field, dipole-dipole coupling and number N of molecules in the array. In the parameter regime that we consider for quantum computing, we find that qubit entanglement is modest, typically no greater than 10-4 , confirming the negligible entanglement in the ground state. We discuss methods for realizing quantum computation in the gate model, measurement-based model, instantaneous quantum polynomial time circuits and the adiabatic model using polar molecules in pendular states.}, number={22}, journal={ChemPhysChem}, publisher={Wiley}, author={Wei, Qi and Cao, Yudong and Kais, Sabre and Friedrich, Bretislav and Herschbach, Dudley}, year={2016}, month={Nov}, pages={3714–3722} }
 @article{wang_hoehn_ye_kais_2015, title={Generalized Remote Preparation of Arbitrary m-qubit Entangled States via Genuine Entanglements}, url={https://doi.org/10.3390/e17041755}, DOI={10.3390/e17041755}, abstractNote={Herein, we present a feasible, general protocol for quantum communication within a network via generalized remote preparation of an arbitrary m-qubit entangled state designed with genuine tripartite Greenberger–Horne–Zeilinger-type entangled resources. During the implementations, we construct novel collective unitary operations; these operations are tasked with performing the necessary phase transfers during remote state preparations. We have distilled our implementation methods into a five-step procedure, which can be used to faithfully recover the desired state during transfer. Compared to previous existing schemes, our methodology features a greatly increased success probability. After the consumption of auxiliary qubits and the performance of collective unitary operations, the probability of successful state transfer is increased four-fold and eight-fold for arbitrary two- and three-qubit entanglements when compared to other methods within the literature, respectively. We conclude this paper with a discussion of the presented scheme for state preparation, including: success probabilities, reducibility and generalizability.}, journal={Entropy}, author={Wang, Dong and Hoehn, Ross and Ye, Liu and Kais, Sabre}, year={2015}, month={Mar} }
 @article{faccin_johnson_biamonte_kais_migdal_2014, title={Degree distribution in quantum walks on complex networks}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84893547709&partnerID=MN8TOARS}, DOI={10.1103/PhysRevX.3.041007}, abstractNote={In this theoretical study, we analyze quantum walks on complex networks, which model network-based processes ranging from quantum computing to biology and even sociology. Specifically, we analytically relate the average long time probability distribution for the location of a unitary quantum walker to that of a corresponding classical walker. The distribution of the classical walker is proportional to the distribution of degrees, which measures the connectivity of the network nodes and underlies many methods for analyzing classical networks including website ranking. The quantum distribution becomes exactly equal to the classical distribution when the walk has zero energy and at higher energies the difference, the so-called quantumness, is bounded by the energy of the initial state. We give an example for which the quantumness equals a Renyi entropy of the normalized weighted degrees, guiding us to regimes for which the classical degree-dependent result is recovered and others for which quantum effects dominate.}, number={4}, journal={Physical Review X}, author={Faccin, M. and Johnson, T. and Biamonte, J. and Kais, S. and Migdal, P.}, year={2014} }
 @book{sadiek_xu_kais_2014, title={Dynamics of entanglement in one- and two-dimensional spin systems}, volume={154}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897545575&partnerID=MN8TOARS}, number={1}, journal={Advances in Chemical Physics}, author={Sadiek, G. and Xu, Q. and Kais, S.}, year={2014}, pages={449–507} }
 @article{pan_cao_yao_li_ju_chen_peng_kais_du_2014, title={Experimental realization of quantum algorithm for solving linear systems of equations}, volume={89}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84894477623&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.89.022313}, abstractNote={Quantum computers have the potential of solving certain problems exponentially faster than classical computers. Recently, Harrow, Hassidim and Lloyd proposed a quantum algorithm for solving linear systems of equations: given an $N\times{N}$ matrix $A$ and a vector $\vec b$, find the vector $\vec x$ that satisfies $A\vec x = \vec b$. It has been shown that using the algorithm one could obtain the solution encoded in a quantum state $|x$ using $O(\log{N})$ quantum operations, while classical algorithms require at least O(N) steps. If one is not interested in the solution $\vec{x}$ itself but certain statistical feature of the solution ${x}|M|x$ ($M$ is some quantum mechanical operator), the quantum algorithm will be able to achieve exponential speedup over the best classical algorithm as $N$ grows. Here we report a proof-of-concept experimental demonstration of the quantum algorithm using a 4-qubit nuclear magnetic resonance (NMR) quantum information processor. For all the three sets of experiments with different choices of $\vec b$, we obtain the solutions with over 96% fidelity. This experiment is a first implementation of the algorithm. Because solving linear systems is a common problem in nearly all fields of science and engineering, we will also discuss the implication of our results on the potential of using quantum computers for solving practical linear systems.}, number={2}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Pan, J. and Cao, Y. and Yao, X. and Li, Z. and Ju, C. and Chen, H. and Peng, X. and Kais, S. and Du, J.}, year={2014} }
 @article{li_wang_ren_wang_zhu_li_hoehn_kais_2014, title={Influence of the intensity gradient upon HHG from free electrons scattered by an intense laser beam}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84896418496&partnerID=MN8TOARS}, DOI={10.1007/s00340-014-5806-1}, journal={Applied Physics B}, author={Li, A. and Wang, J. and Ren, N. and Wang, P. and Zhu, W. and Li, X. and Hoehn, R. and Kais, S.}, year={2014} }
 @book{kais_2014, title={Introduction to quantum information and computation for chemistry}, volume={154}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897556532&partnerID=MN8TOARS}, number={1}, journal={Advances in Chemical Physics}, author={Kais, S.}, year={2014}, pages={1–38} }
 @article{jovanovic_kais_alharbi_2014, title={Spectral Method for Solving the Nonlinear Thomas-Fermi Equation Based on Exponential Functions}, volume={2014}, DOI={10.1155/2014/168568}, abstractNote={We present an efficient spectral methods solver for the Thomas-Fermi equation for neutral atoms in a semi-infinite domain. The ordinary differential equation has been solved by applying a spectral method using an exponential basis set. One of the main advantages of this approach, when compared to other relevant applications of spectral methods, is that the underlying integrals can be solved analytically and numerical integration can be avoided. The nonlinear algebraic system of equations that is derived using this method is solved using a minimization approach. The presented method has shown robustness in the sense that it can find high precision solution for a wide range of parameters that define the basis set. In our test, we show that the new approach can achieve a very high rate of convergence using a small number of bases elements. We also present a comparison of recently published results for this problem using spectral methods based on several different basis sets. The comparison shows that our method is highly competitive and in many aspects outperforms the previous work.}, journal={Journal of Applied Mathematics}, publisher={Hindawi Publishing Corporation}, author={Jovanovic, Raka and Kais, Sabre and Alharbi, Fahhad H.}, year={2014}, pages={1–8} }
 @article{hoehn_mack_kais_2014, title={Using quantum games to teach quantum mechanics, part 1}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897760024&partnerID=MN8TOARS}, DOI={10.1021/ed400385k}, abstractNote={The learning of quantum mechanics is contingent upon an understanding of the physical significance of the mathematics that one must perform. Concepts such as normalization, superposition, interference, probability amplitude, and entanglement can prove challenging for the beginning student. Several class activities that use a nonclassical version of tic-tac-toe are described to introduce several topics in an undergraduate quantum mechanics course. Quantum tic-tac-toe (QTTT) is a quantum analogue of classical tic-tac-toe (CTTT) and can be used to demonstrate the use of superposition in movement, qualitative (and later quantitative) displays of entanglement, and state collapse due to observation. QTTT can be used to aid student understanding in several other topics with the aid of proper discussion.}, number={3}, journal={Journal of Chemical Education}, author={Hoehn, R.D. and MacK, N. and Kais, S.}, year={2014}, pages={417–422} }
 @article{hoehn_mack_kais_2014, title={Using quantum games to teach quantum mechanics, part 2}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897775432&partnerID=MN8TOARS}, DOI={10.1021/ed400432y}, abstractNote={Introductory courses in computational and quantum chemistry introduce topics such as Hilbert spaces, basis set expansions, and observable matrices. These topics are fundamental in the practice of quantum computations in chemistry as most computational methods rely on basis sets to approximate the true wave function. The mechanics of these topics can easily and intuitively be shown through the use of the game quantum tic-tac-toe (QTTT). Herein we propose a series of activities, using the mechanics of both classical tic-tac-toe (CTTT) and QTTT, intended to assist in the student's understanding of these quantum chemistry topics by exploiting their intuitive comprehension of the game. Quantum tic-tac-toe QTTT is a quantum analogue of CTTT and can be used to demonstrate the use of superposition in movement, qualitative (and later quantitative) displays of entanglement, and state collapse due to observation. QTTT can be used for the benefit of the student's comprehension in several other topics with the aid of proper discussion. This paper is the second in a series on the topic published in this Journal.}, number={3}, journal={Journal of Chemical Education}, author={Hoehn, R.D. and MacK, N. and Kais, S.}, year={2014}, pages={423–427} }
 @article{daskin_grama_kais_2013, title={A universal quantum circuit scheme for finding complex eigenvalues}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84885523704&partnerID=MN8TOARS}, DOI={10.1007/s11128-013-0654-1}, journal={Quantum Information Processing}, author={Daskin, A. and Grama, A. and Kais, S.}, year={2013}, pages={1–21} }
 @article{antillon_wehefritz-kaufmann_kais_2013, title={Avalanches in the raise and peel model in the presence of a wall}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84879336547&partnerID=MN8TOARS}, DOI={10.1088/1751-8113/46/26/265001}, abstractNote={We investigate a non-equilibrium one-dimensional model known as the raise and peel model describing a growing surface which grows locally and has non-local desorption. For specific values of adsorption (ua) and desorption (ud) rates, the model shows interesting features. At ua = ud, the model is described by a conformal field theory (with conformal charge c = 0) and its stationary probability can be mapped onto the ground state of the XXZ quantum chain. Moreover, for the regime ua ⩾ ud, the model shows a phase in which the avalanche distribution is scale-invariant. In this work, we study the surface dynamics by looking at avalanche distributions using a finite-sized scaling formalism and explore the effect of adding a wall to the model. The model shows the same universality for the cases with and without a wall for an odd number of tiles removed, but we find a new exponent in the presence of a wall for an even number of tiles released in an avalanche. New insights into the effect of parity on avalanche distributions are discussed and we provide a new conjecture for the probability distribution of avalanches with a wall obtained by using an exact diagonalization of small lattices and Monte Carlo simulations.}, number={26}, journal={Journal of Physics A: Mathematical and Theoretical}, author={Antillon, E. and Wehefritz-Kaufmann, B. and Kais, S.}, year={2013} }
 @article{zhu_kais_wei_herschbach_friedrich_2013, title={Implementation of quantum logic gates using polar molecules in pendular states}, volume={138}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84872744089&partnerID=MN8TOARS}, DOI={10.1063/1.4774058}, abstractNote={We present a systematic approach to implementation of basic quantum logic gates operating on polar molecules in pendular states as qubits for a quantum computer. A static electric field prevents quenching of the dipole moments by rotation, thereby creating the pendular states; also, the field gradient enables distinguishing among qubit sites. Multi-target optimal control theory is used as a means of optimizing the initial-to-target transition probability via a laser field. We give detailed calculations for the SrO molecule, a favorite candidate for proposed quantum computers. Our simulation results indicate that NOT, Hadamard and CNOT gates can be realized with high fidelity, as high as 0.985, for such pendular qubit states.}, number={2}, journal={Journal of Chemical Physics}, author={Zhu, J. and Kais, S. and Wei, Q. and Herschbach, D. and Friedrich, B.}, year={2013} }
 @article{lemeshko_krems_doyle_kais_2013, title={Manipulation of molecules with electromagnetic fields}, volume={111}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84885614465&partnerID=MN8TOARS}, DOI={10.1080/00268976.2013.813595}, abstractNote={The goal of the present article is to review the major developments that have led to the current understanding of molecule-field interactions and experimental methods for manipulating molecules with electromagnetic fields. Molecule-field interactions are at the core of several, seemingly distinct, areas of molecular physics. This is reflected in the organization of this article, which includes sections on Field control of molecular beams, External field traps for cold molecules, Control of molecular orientation and molecular alignment, Manipulation of molecules by non-conservative forces, Ultracold molecules and ultracold chemistry, Controlled many-body phenomena, Entanglement of molecules and dipole arrays, and Stability of molecular systems in high-frequency super-intense laser fields. The article contains 853 references.}, number={12-13}, journal={Molecular Physics}, author={Lemeshko, M. and Krems, R.V. and Doyle, J.M. and Kais, S.}, year={2013}, pages={1648–1682} }
 @article{sadiek_kais_2013, title={Persistence of entanglement in thermal states of spin systems}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84890772998&partnerID=MN8TOARS}, DOI={10.1088/0953-4075/46/24/245501}, abstractNote={We study and compare the persistence of bipartite entanglement (BE) and multipartite entanglement (ME) in one-dimensional and two-dimensional spin XY models in an external transverse magnetic field under the effect of thermal excitations. We compare the threshold temperature at which the entanglement vanishes in both types of entanglement. We use the entanglement of formation as a measure of the BE and the geometric measure to evaluate the ME of the system. We have found that in both dimensions in the anisotropic and partially anisotropic spin systems at zero temperatures, all types of entanglement decay as the magnetic field increases but are sustained with very small magnitudes at high field values. Also we found that for the same systems, the threshold temperatures of the nearest neighbour (nn) BEs are higher than both of the next-to-nearest neighbour BEs and MEs and the three of them increase monotonically with the magnetic field strength. Thus, as the temperature increases, the ME and the far parts BE of the system become more fragile to thermal excitations compared to the nn BE. For the isotropic system, all types of entanglement and threshold temperatures vanish at the same exact small value of the magnetic field. We emphasise the major role played by both the properties of the ground state of the system and the energy gap in controlling the characteristics of the entanglement and threshold temperatures. In addition, we have shown how an inserted magnetic impurity can be used to preserve all types of entanglement and enhance their threshold temperatures. Furthermore, we found that the quantum effects in the spin systems can be maintained at high temperatures, as the different types of entanglements in the spin lattices are sustained at high temperatures by applying sufficiently high magnetic fields.}, number={24}, journal={Journal of Physics B: Atomic, Molecular and Optical Physics}, author={Sadiek, G. and Kais, S.}, year={2013} }
 @article{cao_papageorgiou_petras_traub_kais_2013, title={Quantum algorithm and circuit design solving the Poisson equation}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873385410&partnerID=MN8TOARS}, DOI={10.1088/1367-2630/15/1/013021}, abstractNote={The Poisson equation occurs in many areas of science and engineering. Here we focus on its numerical solution for an equation in d dimensions. In particular we present a quantum algorithm and a scalable quantum circuit design which approximates the solution of the Poisson equation on a grid with error \varepsilon. We assume we are given a supersposition of function evaluations of the right hand side of the Poisson equation. The algorithm produces a quantum state encoding the solution. The number of quantum operations and the number of qubits used by the circuit is almost linear in d and polylog in \varepsilon^{-1}. We present quantum circuit modules together with performance guarantees which can be also used for other problems.}, journal={New Journal of Physics}, author={Cao, Y. and Papageorgiou, A. and Petras, I. and Traub, J. and Kais, S.}, year={2013} }
 @article{pauls_zhang_berman_kais_2013, title={Quantum coherence and entanglement in the avian compass}, volume={87}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84879649818&partnerID=MN8TOARS}, DOI={10.1103/PhysRevE.87.062704}, abstractNote={The radical-pair mechanism is one of two distinct mechanisms used to explain the navigation of birds in geomagnetic fields, however little research has been done to explore the role of quantum entanglement in this mechanism. In this paper we study the lifetime of radical-pair entanglement corresponding to the magnitude and direction of magnetic fields to show that the entanglement lasts long enough in birds to be used for navigation. We also find that the birds appear to not be able to orient themselves directly based on radical-pair entanglement due to a lack of orientation sensitivity of the entanglement in the geomagnetic field. To explore the entanglement mechanism further, we propose a model in which the hyperfine interactions are replaced by local magnetic fields of similar strength. The entanglement of the radical pair in this model lasts longer and displays an angular sensitivity in weak magnetic fields, both of which are not present in previous models.}, number={6}, journal={Physical Review E - Statistical, Nonlinear, and Soft Matter Physics}, author={Pauls, J.A. and Zhang, Y. and Berman, G.P. and Kais, S.}, year={2013} }
 @article{serra_carignano_alharbi_kais_2013, title={Quantum confinement and negative heat capacity}, volume={104}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887838174&partnerID=MN8TOARS}, DOI={10.1209/0295-5075/104/16004}, abstractNote={Thermodynamics dictates that the specific heat of a system is strictly non-negative. However, in finite classical systems there are well known theoretical and experimental cases where this rule is violated, in particular finite atomic clusters. Here, we show for the first time that negative heat capacity can also occur in finite quantum systems. The physical scenario on which this effect might be experimentally observed is discussed. Observing such an effect might lead to the design of new light harvesting nano devices, in particular a solar nano refrigerator.}, number={1}, journal={EPL}, author={Serra, P. and Carignano, M.A. and Alharbi, F.H. and Kais, S.}, year={2013} }
 @article{alharbi_kais_2013, title={Quantum criticality analysis by finite-size scaling and exponential basis sets}, volume={87}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84877748877&partnerID=MN8TOARS}, DOI={10.1103/PhysRevE.87.043308}, abstractNote={We combine the finite-size scaling method with the mesh-free spectral method to calculate quantum critical parameters for a given Hamiltonian. The basic idea is to expand the exact wave function in a finite exponential basis set and extrapolate the information about system criticality from a finite basis to the infinite basis set limit. The used exponential basis set, though chosen intuitively, allows handling a very wide range of exponential decay rates and calculating multiple eigenvalues simultaneously. As a benchmark system to illustrate the combined approach, we choose the Hulthen potential. The results show that the method is very accurate and converges faster when compared with other basis functions. The approach is general and can be extended to examine near-threshold phenomena for atomic and molecular systems based on even-tempered exponential and Gaussian basis functions.}, number={4}, journal={Physical Review E - Statistical, Nonlinear, and Soft Matter Physics}, author={Alharbi, F.H. and Kais, S.}, year={2013} }
 @article{li_wang_ren_wang_zhu_li_hoehn_kais_2013, title={The interference effect of laser-assisted bremsstrahlung emission in Coulomb fields of two nuclei}, volume={114}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84885397374&partnerID=MN8TOARS}, DOI={10.1063/1.4822317}, abstractNote={In this paper, the spontaneous bremsstrahlung emission from an electron scattered by two fixed nuclei in an intense laser field is investigated in detail based upon the Volkov state and the Dirac-Volkov propagator. It has been found that the fundamental harmonic spectrum from the electron radiation exhibits distinctive fringes, which is dependent not only upon the internucleus distance and orientation but also upon the initial energy of the electron and the laser intensity. By analyzing the differential cross section, we are able to explain these effects in terms of interference among the electron scattering by the nuclei. These results could have promising applications in probing the atomic or molecular dressed potentials in intense laser fields.}, number={12}, journal={Journal of Applied Physics}, author={Li, A. and Wang, J. and Ren, N. and Wang, P. and Zhu, W. and Li, X. and Hoehn, R. and Kais, S.}, year={2013} }
 @article{hoehn_wang_kais_2012, title={Dimensional scaling treatment with relativistic corrections for stable multiply charged atomic ions in high-frequency super-intense laser fields}, volume={136}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863045282&partnerID=MN8TOARS}, DOI={10.1063/1.3673317}, abstractNote={We present a theoretical framework which describes multiply-charged atomic ions, their stability within super-intense laser fields,also lay corrections to the systems due to relativistic effects. Dimensional scaling calculations with relativistic corrections for systems: H, H$^{-}$, H$^{2-}$, He, He$^{-}$, He$^{2-}$, He$^{3-}$ within super-intense laser fields were completed. Also completed were three-dimensional self consistent field calculations to verify the dimensionally scaled quantities. With the aforementioned methods the system's ability to stably bind 'additional' electrons through the development of multiple isolated regions of high potential energy leading to nodes of high electron density is shown. These nodes are spaced far enough from each other to minimized the electronic repulsion of the electrons, while still providing adequate enough attraction so as to bind the excess elections into orbitals. We have found that even with relativistic considerations these species are stably bound within the field. It was also found that performing the dimensional scaling calculations for systems within the confines of laser fields to be a much simpler and more cost-effective method than the supporting D=3 SCF method. The dimensional scaling method is general and can be extended to include relativistic corrections to describe the stability of simple molecular systems in super-intense laser fields.}, number={3}, journal={Journal of Chemical Physics}, author={Hoehn, R.D. and Wang, J. and Kais, S.}, year={2012} }
 @article{antillon_wehefritz-kaufmann_kais_2012, title={Finite-size scaling for quantum criticality using the finite-element method}, volume={85}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859069681&partnerID=MN8TOARS}, DOI={10.1103/PhysRevE.85.036706}, abstractNote={Finite size scaling for the Schr\"{o}dinger equation is a systematic approach to calculate the quantum critical parameters for a given Hamiltonian. This approach has been shown to give very accurate results for critical parameters by using a systematic expansion with global basis-type functions. Recently, the finite element method was shown to be a powerful numerical method for ab initio electronic structure calculations with a variable real-space resolution. In this work, we demonstrate how to obtain quantum critical parameters by combining the finite element method (FEM) with finite size scaling (FSS) using different ab initio approximations and exact formulations. The critical parameters could be atomic nuclear charges, internuclear distances, electron density, disorder, lattice structure, and external fields for stability of atomic, molecular systems and quantum phase transitions of extended systems. To illustrate the effectiveness of this approach we provide detailed calculations of applying FEM to approximate solutions for the two-electron atom with varying nuclear charge; these include Hartree-Fock, density functional theory under the local density approximation, and an "exact"' formulation using FEM. We then use the FSS approach to determine its critical nuclear charge for stability; here, the size of the system is related to the number of elements used in the calculations. Results prove to be in good agreement with previous Slater-basis set calculations and demonstrate that it is possible to combine finite size scaling with the finite-element method by using ab initio calculations to obtain quantum critical parameters. The combined approach provides a promising first-principles approach to describe quantum phase transitions for materials and extended systems.}, number={3}, journal={Physical Review E - Statistical, Nonlinear, and Soft Matter Physics}, author={Antillon, E. and Wehefritz-Kaufmann, B. and Kais, S.}, year={2012} }
 @article{serra_kais_2012, title={Ground-state stability and criticality of two-electron atoms with screened Coulomb potentials using the B-splines basis set}, volume={45}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870467688&partnerID=MN8TOARS}, DOI={10.1088/0953-4075/45/23/235003}, abstractNote={We applied the finite-size scaling method using the B-splines basis set to construct the stability diagram for two-electron atoms with a screened Coulomb potential. The results of this method for two electron atoms are very accurate in comparison with previous calculations based on Gaussian, Hylleraas, and finite-element basis sets. The stability diagram for the screened two-electron atoms shows three distinct regions: a two-electron region, a one-electron region, and a zero-electron region, which correspond to stable, ionized and double ionized atoms. In previous studies, it was difficult to extend the finite size scaling calculations to large molecules and extended systems because of the computational cost and the lack of a simple way to increase the number of Gaussian basis elements in a systematic way. Motivated by recent studies showing how one can use B-splines to solve Hartree-Fock and Kohn-Sham equations, this combined finite size scaling using the B-splines basis set, might provide an effective systematic way to treat criticality of large molecules and extended systems. As benchmark calculations, the two-electron systems show the feasibility of this combined approach and provide an accurate reference for comparison.}, number={23}, journal={Journal of Physics B: Atomic, Molecular and Optical Physics}, author={Serra, P. and Kais, S.}, year={2012} }
 @article{zhu_kais_aspuru-guzik_rodriques_brock_love_2012, title={Multipartite quantum entanglement evolution in photosynthetic complexes}, volume={137}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865524311&partnerID=MN8TOARS}, DOI={10.1063/1.4742333}, abstractNote={We investigate the evolution of entanglement in the Fenna-Matthew-Olson (FMO) complex based on simulations using the scaled hierarchy equation of motion (HEOM) approach. We examine the role of multipartite entanglement in the FMO complex by direct computation of the convex roof optimization for a number of measures, including some that have not been previously evaluated. We also consider the role of monogamy of entanglement in these simulations. We utilize the fact that the monogamy bounds are saturated in the single exciton subspace. This enables us to compute more measures of entanglement exactly and also to validate the evaluation of the convex roof. We then use direct computation of the convex roof to evaluate measures that are not determined by monogamy. This approach provides a more complete account of the entanglement in these systems than has been available to date. Our results support the hypothesis that multipartite entanglement is maximum primary along the two distinct electronic energy transfer pathways.}, number={7}, journal={Journal of Chemical Physics}, author={Zhu, J. and Kais, S. and Aspuru-Guzik, A. and Rodriques, S. and Brock, B. and Love, P.J.}, year={2012} }
 @article{yeh_zhu_kais_2012, title={Population and coherence dynamics in light harvesting complex II (LH2)}, volume={137}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865793276&partnerID=MN8TOARS}, DOI={10.1063/1.4747622}, abstractNote={The electronic excitation population and coherence dynamics in the chromophores of the photosynthetic light harvesting complex 2 (LH2) B850 ring from purple bacteria (Rhodopseudomonas acidophila) have been studied theoretically at both physiological and cryogenic temperatures. Similar to the well-studied Fenna-Matthews-Olson (FMO) protein, oscillations of the excitation population and coherence in the site basis are observed in LH2 by using a scaled hierarchical equation of motion (HEOM) approach. However, this oscillation time (300 fs) is much shorter compared to the FMO protein (650 fs) at cryogenic temperature. Both environment and high temperature are found to enhance the propagation speed of the exciton wave packet yet they shorten the coherence time and suppress the oscillation amplitude of coherence and the population. Our calculations show that a long-lived coherence between chromophore electronic excited states can exist in such a noisy biological environment.}, number={8}, journal={Journal of Chemical Physics}, author={Yeh, S.-H. and Zhu, J. and Kais, S.}, year={2012} }
 @article{cao_daskin_frankel_kais_2012, title={Quantum circuit design for solving linear systems of equations}, volume={110}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865595761&partnerID=MN8TOARS}, DOI={10.1080/00268976.2012.668289}, abstractNote={Recently, it has been demonstrated that quantum computers can be used for solving linear systems of algebraic equations with exponential speedup compared with classical computers. Here, we present an efficient and generic quantum circuit design for implementing the algorithm for solving linear systems. In particular, we show the detailed construction of a quantum circuit which solves a 4 × 4 linear system with seven qubits. It consists of only the basic quantum gates that can be realized with present physical devices, implying great possibility for experimental implementation. Furthermore, the performance of the circuit is numerically simulated and its ability to solve the intended linear system is verified.}, number={15-16}, journal={Molecular Physics}, author={Cao, Y. and Daskin, A. and Frankel, S. and Kais, S.}, year={2012}, pages={1675–1680} }
 @article{friedrich_kais_mazziotti_2012, title={Scaling mount impossible: A festschrift for Dudley Herschbach}, volume={110}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865580889&partnerID=MN8TOARS}, DOI={10.1080/00268976.2012.695049}, abstractNote={The title of this Festschrift is intended to reflect threedicta that the jubilarian has invoked with collegiatespirit throughout his career, and has passed on to hisscientific progeny: ‘‘The difficult we do immediately.The impossible takes a little longer’’ (adopted from theSeabees [1]); ‘‘Ontogeny recapitulates phylogeny’’(amplified in Richard Dawkins’ masterpiece on evolu-tion, Climbing Mount Improbable [2]); and the‘‘scaling laws and form factors’’ (includingDimensional Scaling) that Dudley has discovered inhis scientific work and that have revealed so muchabout why things molecular – and other – are the waythey are. The title also implies Dudley’s fourth dictum,‘‘Keep going!’’ by which he has lived with anunmatched vigour and fortitude.This Festschrift, comprised of papers written inDudley Herschbach’s honour by his colleagues andformer students and associates, celebrates his distin-guished career. In addition, it offers a rich collectionof prefatory material, including an extensive interview,a list of Dudley’s students and coworkers, as well asthe Eighty Papers, a list of his own favouritepublications that he has compiled for this occasion.The full list can be found at Dudley’s website atHarvard [3] or at Texas A&M [4], along with twodefinitive reviews of his scientific work [5,6], essayson issues ranging from teaching to science funding,and much more – including some of his treasuredlecture notes. The prefatory section closes with DanKleppner’s essay on aspects of the well-known‘‘Herschbach effect.’’Richard Feynman once asked himself ‘‘If we couldpass on to posterity only one short sentence, whatshould it be?’’ His reply was ‘‘There really are atoms[7]’’. Although a fervent believer in the immensevalue of science, Dudley offered a different answer:‘‘The most important thing really is love!’’ And here iswhy: ‘‘With love comes awe, reverence, and compas-sion. With love, you know that whatever disappoint-ments or struggles lie behind or ahead, you are blessed.Beyond talent and energy, you can bring passion,vision, commitment to whatever you undertake. Youcan foster in yourself and others the awareness ofopportunity, the fellowship of striving, the joy ofdiscovery, the satisfactions of genuine service’’. And headded ‘‘So I’m glad to confess unabashedly my love formy life in science and the many wonderful people,ideas, and molecules that have come with it [8]’’.We, in turn, likewise confess a similar affection,and on this special occasion of his grand personalanniversary it is all directed towards the jubilarian:Happy birthday, Dudley!Bretislav Friedrich (Berlin)Sabre Kais (West Lafayette)David Mazziotti (Chicago)References}, number={15-16}, journal={Molecular Physics}, author={Friedrich, B. and Kais, S. and Mazziotti, D.}, year={2012} }
 @article{sadiek_xu_kais_2012, title={Tuning entanglement and ergodicity in two-dimensional spin systems using impurities and anisotropy}, volume={85}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84860178809&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.85.042313}, abstractNote={We consider the entanglement in a two-dimensional $XY$ model in an external magnetic field $h$. The model consists of a set of seven localized spin-$\frac{1}{2}$ particles in a two-dimensional triangular lattice coupled through nearest-neighbor exchange interaction $J$. We examine the effect of single and double impurities in the system as well as the degree of anisotropy on the nearest-neighbor entanglement and ergodicity of the system. We have found that the entanglement of the system at the different degrees of anisotropy mimics that of the one-dimensional spin systems at the extremely small and large values of the parameter $\ensuremath{\lambda}=h/J$. The entanglement of the Ising and partially anisotropic systems shows phase transition in the vicinity of $\ensuremath{\lambda}=2$, whereas, the entanglement of the isotropic system suddenly vanishes there. Also, we investigate the dynamic response of the system containing single and double impurities to an external exponential magnetic field at different degrees of anisotropy. We have demonstrated that the ergodicity of the system can be controlled by varying the strength and location of the impurities as well as the degree of anisotropy of the coupling.}, number={4}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Sadiek, G. and Xu, Q. and Kais, S.}, year={2012} }
 @article{daskin_grama_kollias_kais_2012, title={Universal programmable quantum circuit schemes to emulate an operator}, volume={137}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84871939999&partnerID=MN8TOARS}, DOI={10.1063/1.4772185}, abstractNote={Unlike fixed designs, programmable circuit designs support an infinite number of operators. The functionality of a programmable circuit can be altered by simply changing the angle values of the rotation gates in the circuit. Here, we present a new quantum circuit design technique resulting in two general programmable circuit schemes. The circuit schemes can be used to simulate any given operator by setting the angle values in the circuit. This provides a fixed circuit design whose angles are determined from the elements of the given matrix–which can be non-unitary–in an efficient way. We also give both the classical and quantum complexity analysis for these circuits and show that the circuits require a few classical computations. For the electronic structure simulation on a quantum computer, one has to perform the following steps: prepare the initial wave function of the system; present the evolution operator U = e−iHt for a given atomic and molecular Hamiltonian H in terms of quantum gates array and apply the phase estimation algorithm to find the energy eigenvalues. Thus, in the circuit model of quantum computing for quantum chemistry, a crucial step is presenting the evolution operator for the atomic and molecular Hamiltonians in terms of quantum gate arrays. Since the presented circuit designs are independent from the matrix decomposition techniques and the global optimization processes used to find quantum circuits for a given operator, high accuracy simulations can be done for the unitary propagators of molecular Hamiltonians on quantum computers. As an example, we show how to build the circuit design for the hydrogen molecule.}, number={23}, journal={Journal of Chemical Physics}, author={Daskin, A. and Grama, A. and Kollias, G. and Kais, S.}, year={2012} }
 @article{daskin_kais_2011, title={Decomposition of unitary matrices for finding quantum circuits: Application to molecular Hamiltonians}, volume={134}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79954579639&partnerID=MN8TOARS}, DOI={10.1063/1.3575402}, abstractNote={Constructing appropriate unitary matrix operators for new quantum algorithms and finding the minimum cost gate sequences for the implementation of these unitary operators is of fundamental importance in the field of quantum information and quantum computation. Evolution of quantum circuits faces two major challenges: complex and huge search space and the high costs of simulating quantum circuits on classical computers. Here, we use the group leaders optimization algorithm to decompose a given unitary matrix into a proper-minimum cost quantum gate sequence. We test the method on the known decompositions of Toffoli gate, the amplification step of the Grover search algorithm, the quantum Fourier transform, and the sender part of the quantum teleportation. Using this procedure, we present the circuit designs for the simulation of the unitary propagators of the Hamiltonians for the hydrogen and the water molecules. The approach is general and can be applied to generate the sequence of quantum gates for larger molecular systems.}, number={14}, journal={Journal of Chemical Physics}, author={Daskin, A. and Kais, S.}, year={2011} }
 @article{xu_sadiek_kais_2011, title={Dynamics of entanglement in a two-dimensional spin system}, volume={83}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79961050740&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.83.062312}, abstractNote={We consider the time evolution of entanglement in a finite two-dimensional transverse Ising model. The model consists of a set of seven localized spin-$\frac{1}{2}$ particles in a two-dimensional triangular lattice coupled through nearest-neighbor exchange interaction in the presence of an external time-dependent magnetic field. The magnetic field is applied in different function forms: step, exponential, hyperbolic, and periodic. We found that the time evolution of the entanglement shows an ergodic behavior under the effect of the time-dependent magnetic fields. Also, we found that while the step magnetic field causes great disturbance to the system, creating rapid oscillations, the system shows great controllability under the effects of the other magnetic fields where the entanglement profile follows closely the shape of the applied field even with the same frequency for periodic fields. This follow-up trend breaks down as the strength of the field, the transition constant for the exponential and hyperbolic forms, or the frequency for periodic field increase leading to rapid oscillations. We observed that the entanglement is very sensitive to the initial value of the applied periodic field: the smaller the initial value is, the less distorted the entanglement profile is. Furthermore, the effect of thermal fluctuations is very devastating to the entanglement, which decays very rapidly as the temperature increases. Interestingly, although a large value of the magnetic field strength may yield a small entanglement, the magnetic field strength was found to be more persistent against thermal fluctuations than the small field strengths.}, number={6}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Xu, Q. and Sadiek, G. and Kais, S.}, year={2011} }
 @article{alkurtass_sadiek_kais_2011, title={Entanglement dynamics of one-dimensional driven spin systems in time-varying magnetic fields}, volume={84}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80051638737&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.84.022314}, abstractNote={We study the dynamics of entanglement for a one-dimensional spin chain with a nearest neighbor time dependent Heisenberg coupling J(t) between the spins in presence of a time dependent external magnetic field h(t) at zero and finite temperatures. We consider different forms of time dependence for the coupling and magnetic field; exponential, hyperbolic and periodic. We examined the system size effect on the entanglement asymptotic value. It was found that for a small system size the entanglement starts to fluctuate within a short period of time after applying the time dependent coupling. The period of time increases as the system size increases and disappears completely as the size goes to infinity. We also found that when J(t) is periodic the entanglement shows a periodic behavior with the same period, which disappears upon applying periodic magnetic field with the same frequency. Solving the particular case where J(t) and h(t) are proportional exactly, we showed that the asymptotic value of entanglement depends only on the initial conditions regardless of the form of J(t) and h(t) applied at t > 0.}, number={2}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Alkurtass, B. and Sadiek, G. and Kais, S.}, year={2011} }
 @article{wei_kais_friedrich_herschbach_2011, title={Entanglement of polar molecules in pendular states}, volume={134}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79953305859&partnerID=MN8TOARS}, DOI={10.1063/1.3567486}, abstractNote={In proposals for quantum computers using arrays of trapped ultracold polar molecules as qubits, a strong external field with appreciable gradient is imposed in order to prevent quenching of the dipole moments by rotation and to distinguish among the qubit sites. That field induces the molecular dipoles to undergo pendular oscillations, which markedly affect the qubit states and the dipole-dipole interaction. We evaluate entanglement of the pendular qubit states for two linear dipoles, characterized by pairwise concurrence, as a function of the molecular dipole moment and rotational constant, strengths of the external field and the dipole-dipole coupling, and ambient temperature. We also evaluate a key frequency shift, △ω, produced by the dipole-dipole interaction. Under conditions envisioned for the proposed quantum computers, both the concurrence and △ω become very small for the ground eigenstate. In principle, such weak entanglement can be sufficient for operation of logic gates, provided the resolution is high enough to detect the △ω shift unambiguously. In practice, however, for many candidate polar molecules it appears a challenging task to attain adequate resolution. Simple approximate formulas fitted to our numerical results are provided from which the concurrence and △ω shift can be obtained in terms of unitless reduced variables.}, number={12}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Friedrich, B. and Herschbach, D.}, year={2011} }
 @article{wei_kais_friedrich_herschbach_2011, title={Entanglement of polar symmetric top molecules as candidate qubits}, volume={135}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80155137339&partnerID=MN8TOARS}, DOI={10.1063/1.3649949}, abstractNote={Proposals for quantum computing using rotational states of polar molecules as qubits have previously considered only diatomic molecules. For these the Stark effect is second-order, so a sizable external electric field is required to produce the requisite dipole moments in the laboratory frame. Here we consider use of polar symmetric top molecules. These offer advantages resulting from a first-order Stark effect, which renders the effective dipole moments nearly independent of the field strength. That permits use of much lower external field strengths for addressing sites. Moreover, for a particular choice of qubits, the electric dipole interactions become isomorphous with NMR systems for which many techniques enhancing logic gate operations have been developed. Also inviting is the wider chemical scope, since many symmetric top organic molecules provide options for auxiliary storage qubits in spin and hyperfine structure or in internal rotation states.}, number={15}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Friedrich, B. and Herschbach, D.}, year={2011} }
 @article{daskin_kais_2011, title={Group leaders optimization algorithm}, volume={109}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952645832&partnerID=MN8TOARS}, DOI={10.1080/00268976.2011.552444}, abstractNote={We present a new global optimization algorithm in which the influence of the leaders in social groups is used as an inspiration for the evolutionary technique which is designed into a group architecture. To demonstrate the efficiency of the method, a standard suite of single and multidimensional optimization functions along with the energies and the geometric structures of Lennard-Jones clusters are given as well as the application of the algorithm on quantum circuit design problems. We show that as an improvement over previous methods, the algorithm scales as N^2.5 for the Lennard-Jones clusters of N-particles. In addition, an efficient circuit design is shown for two qubit Grover search algorithm which is a quantum algorithm providing quadratic speed-up over the classical counterpart.}, number={5}, journal={Molecular Physics}, author={Daskin, A. and Kais, S.}, year={2011}, pages={761–772} }
 @article{zhu_kais_rebentrost_aspuru-guzik_2011, title={Modified scaled hierarchical equation of motion approach for the study of quantum coherence in photosynthetic complexes}, volume={115}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79951533126&partnerID=MN8TOARS}, DOI={10.1021/jp109559p}, abstractNote={We present a detailed theoretical study of the transfer of electronic excitation energy through the Fenna−Matthews−Olson (FMO) pigment−protein complex, using the newly developed modified scaled hierarchical approach (Shi, Q.; et al. J. Chem. Phys.2009, 130, 084105). We show that this approach is computationally more efficient than the original hierarchical approach. The modified approach reduces the truncation levels of the auxiliary density operators and the correlation function. We provide a systematic study of how the number of auxiliary density operators and the higher-order correlation functions affect the exciton dynamics. The time scales of the coherent beating are consistent with experimental observations. Furthermore, our theoretical results exhibit population beating at physiological temperature. Additionally, the method does not require a low-temperature correction to obtain the correct thermal equilibrium at long times.}, number={6}, journal={Journal of Physical Chemistry B}, author={Zhu, J. and Kais, S. and Rebentrost, P. and Aspuru-Guzik, A.}, year={2011}, pages={1531–1537} }
 @article{lemeshko_mustafa_kais_friedrich_2011, title={Supersymmetric factorization yields exact solutions to the molecular Stark-effect problem for stretched states}, volume={83}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79960631119&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.83.043415}, abstractNote={By invoking supersymmetry, we found a condition under which the Stark-effect problem for a polar and polarizable molecule subject to nonresonant electric fields becomes exactly solvable for the $|\mathrm{J\ifmmode \tilde{}\else \~{}\fi{}}=m,m\ensuremath{\rangle}$ family of stretched states. The analytic expressions for the wave function and eigenenergy and other expectation values allow one to readily reverse-engineer the problem of finding the values of the interaction parameters required for creating quantum states with preordained characteristics. The method also allows the construction of families of isospectral potentials, realizable with combined fields.}, number={4}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Lemeshko, M. and Mustafa, M. and Kais, S. and Friedrich, B.}, year={2011} }
 @article{lemeshko_mustafa_kais_friedrich_2011, title={Supersymmetry identifies molecular Stark states whose eigenproperties can be obtained analytically}, volume={13}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79959670048&partnerID=MN8TOARS}, DOI={10.1088/1367-2630/13/6/063036}, abstractNote={We made use of supersymmetric (SUSY) quantum mechanics to find the condition under which the Stark effect problem for a polar and polarizable closed-shell diatomic molecule subjected to collinear electrostatic and nonresonant radiative fields becomes exactly solvable. The condition connects values of the dimensionless parameters ω and Δω that characterize the strengths of the permanent and induced dipole interactions of the molecule with the respective fields. The exact solutions are obtained for the family of 'stretched' states. The field-free and strong-field limits of the combined-fields problem were found to exhibit supersymmetry and shape invariance, which is indeed the reason why they are analytically solvable. By making use of the analytic form of the wavefunctions, we obtained simple formulae for the expectation values of the space-fixed electric dipole moment, the alignment cosine and the angular momentum squared, and derived a 'sum rule' that combines the above expectation values into a formula for the eigenenergy. The analytic expressions for the characteristics of the strongly oriented and aligned states provide direct access to the values of the interaction parameters required for creating such states in the laboratory.}, journal={New Journal of Physics}, author={Lemeshko, M. and Mustafa, M. and Kais, S. and Friedrich, B.}, year={2011} }
 @article{wei_kais_chen_2010, title={Communications: Entanglement switch for dipole arrays}, volume={132}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77950421825&partnerID=MN8TOARS}, DOI={10.1063/1.3366522}, abstractNote={We propose a new entanglement switch of qubits consisting of electric dipoles oriented along or against an external electric field and coupled by the electric dipole-dipole interaction. The pairwise entanglement can be tuned and controlled by the ratio of the Rabi frequency and the dipole-dipole coupling strength. Tuning the entanglement can be achieved for one, two, and three-dimensional arrangements of the qubits. The feasibility of building such an entanglement switch is also discussed.}, number={12}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Chen, Y.P.}, year={2010} }
 @article{xu_kais_naumov_sameh_2010, title={Exact calculation of entanglement in a 19-site two-dimensional spin system}, volume={81}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77249122201&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.81.022324}, abstractNote={Using the trace minimization algorithm, we carried out an exact calculation of entanglement in a 19-site two-dimensional transverse Ising model. This model consists of a set of localized spin-$\frac{1}{2}$ particles in a two-dimensional triangular lattice coupled through exchange interaction $J$ and subject to an external magnetic field of strength $h$. We demonstrate, for such a class of two-dimensional magnetic systems, that entanglement can be controlled and tuned by varying the parameter $\ensuremath{\lambda}=h/J$ in the Hamiltonian and by introducing impurities into the systems. Examining the derivative of the concurrence as a function of $\ensuremath{\lambda}$ shows that the system exhibits a quantum phase transition at about ${\ensuremath{\lambda}}_{c}=3.01$, a transition induced by quantum fluctuations at the absolute zero of temperature.}, number={2}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Xu, Q. and Kais, S. and Naumov, M. and Sameh, A.}, year={2010} }
 @article{antillon_moy_wei_kais_2009, title={Comparison study of finite element and basis set methods for finite size scaling}, volume={131}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349283276&partnerID=MN8TOARS}, DOI={10.1063/1.3207909}, abstractNote={We compare two methods of obtaining critical parameters for a quantum Hamiltonian using a finite size scaling approach. A finite element and basis set method were used in conjunction with the finite size scaling to obtain the critical parameters for the Hulthen potential. The critical parameters obtained analytically were the coupling constant λc=12, the critical exponents for the energy α=2 and for the “correlation length” ν=1. The extrapolated results for finite size scaling with the basis set method are λc=0.49999, α=1.9960, and ν=0.99910. The results for the finite element solutions are λc=0.50184, α=1.99993, and ν=1.00079 for the linear interpolation and λc=0.50000, α=2.00011, and ν=1.00032 for the Hermite interpolation. The results for each method compare very well with the analytical results obtained for the Hulthen potential. However, the finite element method is easier to implement and may be combined with ab initio and density functional theory to obtain quantum critical parameters for more complex systems.}, number={10}, journal={Journal of Chemical Physics}, author={Antillon, E. and Moy, W. and Wei, Q. and Kais, S.}, year={2009} }
 @article{zhu_huang_kais_2009, title={Simulated quantum computation of global minima}, volume={107}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349443473&partnerID=MN8TOARS}, DOI={10.1080/00268970903117126}, abstractNote={Abstract Finding the optimal solution to a complex optimisation problem is of great importance in practically all fields of science, technology, technical design and econometrics. We demonstrate that a modified Grover's quantum algorithm can be applied to real problems of finding a global minimum using modest numbers of quantum bits. Calculations of the global minimum of simple test functions and Lennard-Jones clusters have been carried out on a quantum computer simulator using a modified Grover's algorithm. The number of function evaluations N reduced from O(N) in classical simulation to O(N 1/2) in quantum simulation. We also show how the Grover's quantum algorithm can be combined with the classical Pivot method for global optimisation to treat larger systems. Keywords: quantum computationGrover's quantum algorithmPivot methodclustersglobal optimisation This article is part of the following collections: Molecular Physics Early Career Researcher Prize Acknowledgements We thank Jonathan Baugh for useful discussions and the Army Research Office (ARO) for funding.}, number={19}, journal={Molecular Physics}, author={Zhu, J. and Huang, Z. and Kais, S.}, year={2009}, pages={2015–2023} }
 @article{du_francisco_kais_2009, title={Study of electronic structure and dynamics of interacting free radicals influenced by water}, volume={130}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-63649091311&partnerID=MN8TOARS}, DOI={10.1063/1.3100549}, abstractNote={We present a study of electronic structure, stability, and dynamics of interaction and recombination of free radicals such as HO(2) and OH influenced by water. As simple model calculations, we performed ab initio and density functional calculations for the interaction of HO(2) and OH in the presence of water cluster. Results indicate that a significant interaction, overcoming the repulsive Columbic barrier, occurs at a separation distance between the radicals of 5.7 A. This confirms early predictions of the minimum size of molecular dianions stable in the gas phase. It is well known that atomic dianions are unstable in the gas phase but molecular dianions are stable when the size of the molecule is larger than 5.7 A. Ab initio molecular dynamics calculations with Car-Parrinello scheme show that the reaction is very fast and occurs on a time scale of about 1.5 ps. The difference in stability and dynamics of the interacting free radicals on singlet and triplet potential energy surfaces is also discussed.}, number={12}, journal={Journal of Chemical Physics}, author={Du, S. and Francisco, J.S. and Kais, S.}, year={2009} }
 @article{carignano_mohammad_kais_2009, title={Temperature dependent electron binding in (H2O)8}, volume={113}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70349906205&partnerID=MN8TOARS}, DOI={10.1021/jp901047y}, abstractNote={We combine classical molecular dynamics simulations and quantum density functional theory calculations to study the temperature effects on the electron affinity of the water octamer. The atomistic simulations provide a sample of the cluster's conformations as a function of the temperature, on which the density functional calculations are carried on. As the temperature increases, the cluster undergoes its characteristic phase change from a cubic, solidlike structure to a liquidlike state. This phase change is also reflected by an increase on the total dipole moment of the cluster. The quantum calculations indicate that the large dipole moment conformations have a positive electron affinity. Relaxing the high temperature conformations of the cluster anion to its local minimum, the average vertical detachment energy is calculated and shows a clear tendency to increase as the temperature increases. The analysis of the high temperatures conformations reveals that origin of higher values of the vertical detachment energy is not the stability of the negative octamer but the high energy of the corresponding neutral cluster.}, number={41}, journal={Journal of Physical Chemistry A}, author={Carignano, M.A. and Mohammad, A. and Kais, S.}, year={2009}, pages={10886–10890} }
 @article{ferrón_serra_kais_2008, title={Critical conditions for stable dipole-bound dianions}, volume={128}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-38849130765&partnerID=MN8TOARS}, DOI={10.1063/1.2822285}, abstractNote={We present finite size scaling calculations of the critical parameters for binding two electrons to a finite linear dipole field. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. A complete ground state stability diagram for the dipole-bound dianion is obtained using accurate variational and finite size scaling calculations. We also study the near threshold behavior of the ground state energy by calculating its critical exponent.}, number={4}, journal={Journal of Chemical Physics}, author={Ferrón, A. and Serra, P. and Kais, S.}, year={2008} }
 @article{ferrón_serra_kais_2008, title={Dimensional scaling for stability of two particles in a dipole field}, volume={461}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48149111947&partnerID=MN8TOARS}, DOI={10.1016/j.cplett.2008.06.061}, number={1-3}, journal={Chemical Physics Letters}, author={Ferrón, A. and Serra, P. and Kais, S.}, year={2008}, pages={127–130} }
 @article{wei_kais_herschbach_2008, title={Dimensional scaling treatment of stability of simple diatomic molecules induced by superintense, high-frequency laser fields}, volume={129}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57349195975&partnerID=MN8TOARS}, DOI={10.1063/1.3027451}, abstractNote={We present results obtained using dimensional scaling with high-frequency Floquet theory to evaluate the stability of gas phase simple diatomic molecules in superintense laser fields. The large-D limit provides a simple model that captures the main physics of the problem, which imposes electron localization along the polarization direction of the laser field. This localization markedly reduces the ionization probability and can enhance chemical bonding when the laser strength becomes sufficiently strong. We find that energy and structure calculations at the large-dimensional limit (D-->infinity) for stabilities of H(2) (+), H(2), and He(2) in superintense laser fields are much simpler than at D=3, yet yield similar results to those found from demanding ab initio calculations. We also use the large-D model to predict the stability of H(2) (-) and the field strength needed to bind the "extra" electron to the H(2) molecule.}, number={21}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Herschbach, D.}, year={2008} }
 @article{osenda_serra_kais_2008, title={Dynamics of entanglement for two-electron atoms}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44049091780&partnerID=MN8TOARS}, DOI={10.1142/S0219749908003463}, abstractNote={We studied the dynamics of the entanglement for two electron atoms with initial states created from a superposition of the eigenstates of the two-electron Hamiltonian. We present numerical evidence that the pairwise entanglement for the two electrons evolves in a way that is strongly related with the time evolution of the Coulombic interaction between the two electrons.}, number={2}, journal={International Journal of Quantum Information}, author={Osenda, O. and Serra, P. and Kais, S.}, year={2008}, pages={303–316} }
 @article{oh_huang_peskin_kais_2008, title={Entanglement, Berry phases, and level crossings for the atomic Breit-Rabi Hamiltonian}, volume={78}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57649107211&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.78.062106}, abstractNote={The relation between level crossings, entanglement, and Berry phases is investigated for the Breit-Rabi Hamiltonian of hydrogen and sodium atoms, describing a hyperfine interaction of electron and nuclear spins in a magnetic field. It is shown that the entanglement between nuclear and electron spins is maximum at avoided crossings. An entangled state encircling avoided crossings acquires a marginal Berry phase of a subsystem like an instantaneous eigenstate moving around real crossings accumulates a Berry phase. Especially, the nodal points of a marginal Berry phase correspond to the avoided crossing points.}, number={6}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Oh, S. and Huang, Z. and Peskin, U. and Kais, S.}, year={2008} }
 @article{moy_carignano_kais_2008, title={Finite element method for finite-size scaling in quantum mechanics}, volume={112}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-53349118514&partnerID=MN8TOARS}, DOI={10.1021/jp800346z}, abstractNote={We combined the finite-size scaling method with the finite element method to provide a systematic procedure for obtaining quantum critical parameters for a quantum system. We present results for the Yukawa potential solved with the finite element approach. The finite-size scaling approach was then used to find the critical parameters of the system. The critical values lambda c, alpha, and nu were found to be 0.83990345, 2.0002, and 1.002, respectively, for l = 0. These results compare well with the theoretically exact values for alpha and nu and with the best numerical estimations for lambda c. The finite element method is general and can be extended to larger systems.}, number={24}, journal={Journal of Physical Chemistry A}, author={Moy, W. and Carignano, M.A. and Kais, S.}, year={2008}, pages={5448–5452} }
 @article{moy_kais_serra_2008, title={Finite size scaling with gaussian basis sets}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-42149125586&partnerID=MN8TOARS}, DOI={10.1080/00268970701528714}, abstractNote={We combined finite size scaling method with the well-developed electronic structure methods, such as ab initio and density functional methods, to provide a systematic procedure for obtaining quantum critical parameters for atoms and molecules using Gaussian basis sets. The finite size scaling method is based on taking the number of elements in a complete basis set as the size of the system, to calculate the critical parameters for a given quantum system. We present results for the Yukawa potential and helium-like systems by expanding the wave function with a Gaussian basis. The finite size scaling approach was then used with the ab initio methods to find the critical parameters of two-electron atoms. The critical values of λc and α were found to be 1.0578 and 1.0711 respectively using Møller–Plesset (MP2) level of theory. We then applied configuration interaction single and doubles excitation (CISD) to the helium system to improve upon the results. The critical parameters at the CISD level of theory were α = 1.2891 and λc = 1.1259. With time-dependent density functional theory (TDDFT) using the hybrid functional B3LYP resulted in λc = 1.0160. The ab initio results compare well with the exact results α = 1 and λc = 1.0971. The method is general and can be extended to calculate critical parameters for larger systems.}, number={2-4}, journal={Molecular Physics}, author={Moy, W. and Kais, S. and Serra, P.}, year={2008}, pages={203–212} }
 @article{kais_handy_2008, title={Molecular Physics: Preface}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-42149167390&partnerID=MN8TOARS}, DOI={10.1080/00268970801892671}, number={2-4}, journal={Molecular Physics}, author={Kais, S. and Handy, N.C.}, year={2008} }
 @article{sadiek_huang_aldossary_kais_2008, title={Nuclear-induced time evolution of entanglement of two-electron spins in anisotropically coupled quantum dot}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-53449086631&partnerID=MN8TOARS}, DOI={10.1080/00268970802290313}, abstractNote={We study the time evolution of entanglement of two spins in an anisotropically coupled quantum dot interacting with the unpolarised nuclear spins environment. We assume that the exchange coupling strength in the z direction J z is different from the lateral one J l . We observe that the entanglement decays as a result of the coupling to the nuclear environment and reaches a saturation value, which depends on the value of the exchange interaction difference J = ‖J l − J z ‖ between the two spins and the strength of the applied external magnetic field. We find that the entanglement exhibits a critical behaviour controlled by the competition between the exchange interaction J and the external magnetic field. The entanglement shows a quasi-symmetric behaviour above and below a critical value of the exchange interaction. It becomes more symmetric as the external magnetic field increases. The entanglement reaches a large saturation value, close to unity, when the exchange interaction is far above or below its critical value and a small one as it closely approaches the critical value. Furthermore, we find that the decay rate profile of entanglement is linear when the exchange interaction is much higher or lower than the critical value but converts to a power law and finally to a Gaussian as the critical value is approached from both directions. The dynamics of entanglement is found to be independent of the exchange interaction for an isotropically coupled quantum dot.}, number={14}, journal={Molecular Physics}, author={Sadiek, G. and Huang, Z. and Aldossary, O. and Kais, S.}, year={2008}, pages={1777–1786} }
 @article{wang_kais_aspuru-guzik_hoffmann_2008, title={Quantum algorithm for obtaining the energy spectrum of molecular systems}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50849106399&partnerID=MN8TOARS}, DOI={10.1039/b804804e}, abstractNote={Simulating a quantum system is more efficient on a quantum computer than on a classical computer. The time required for solving the Schr\"odinger equation to obtain molecular energies has been demonstrated to scale polynomially with system size on a quantum computer, in contrast to the well-known result of exponential scaling on a classical computer. In this paper, we present a quantum algorithm to obtain the energy spectrum of molecular systems based on the multi-configurational self-consistent field (MCSCF) wave function. By using a MCSCF wave function as the initial guess, the excited states are accessible; Entire potential energy surfaces of molecules can be studied more efficiently than if the simpler Hartree-Fock guess was employed. We show that a small increase of the MCSCF space can dramatically increase the success probability of the quantum algorithm, even in regions of the potential energy surface that are far from the equilibrium geometry. For the treatment of larger systems, a multi-reference configuration interaction approach is suggested. We demonstrate that such an algorithm can be used to obtain the energy spectrum of the water molecule.}, number={35}, journal={Physical Chemistry Chemical Physics}, author={Wang, H. and Kais, S. and Aspuru-Guzik, A. and Hoffmann, M.R.}, year={2008}, pages={5388–5393} }
 @article{ferrón_serra_kais_2008, title={Stability conditions for hydrogen-antihydrogen-like quasimolecules}, volume={77}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44449125032&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.77.052505}, abstractNote={We present a detailed study of the stability conditions of hydrogen-antihydrogen--like quasimolecules using both variational and finite-size scaling calculations. The stability diagram of the nuclear charge $Z$ as a function of the internuclear distance $R$ shows bound and unbound regions separated by a first-order critical line. Calculations of the leptonic annihilation rate show a peculiar behavior for nuclear charges $Z\ensuremath{\ge}2$, which was not observed for the hydrogen-antihydrogen quasimolecule; it goes through a maximum before it decays exponentially for large interhadronic distances. This might have a practical impact on the study of stability of matter-antimatter systems.}, number={5}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Ferrón, A. and Serra, P. and Kais, S.}, year={2008} }
 @book{kais_2007, title={Chapter 18: Entanglement, electron correlation, and density matrices}, volume={134}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247532062&partnerID=MN8TOARS}, journal={Advances in Chemical Physics}, author={Kais, S.}, year={2007}, pages={493–535} }
 @article{wei_kais_herschbach_2007, title={Dimensional scaling treatment of stability of atomic anions induced by superintense, high-frequency laser fields}, volume={127}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34548475735&partnerID=MN8TOARS}, DOI={10.1063/1.2768037}, abstractNote={We show that dimensional scaling, combined with the high-frequency Floquet theory, provides useful means to evaluate the stability of gas phase atomic anions in a superintense laser field. At the large-dimension limit (D→∞), in a suitably scaled space, electrons become localized along the polarization direction of the laser field. We find that calculations at large D are much simpler than D=3, yet yield similar results for the field strengths needed to bind an “extra” one or two electrons to H and He atoms. For both linearly and circularly polarized laser fields, the amplitude of quiver motion of the electrons correlates with the detachment energy. Despite large differences in scale, this correlation is qualitatively like that found between internuclear distances and dissociation energies of chemical bonds.}, number={9}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Herschbach, D.}, year={2007} }
 @article{wei_kais_moiseyev_2007, title={Frequency-dependent stabilization of He- by a superintense laser field}, volume={76}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547164883&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.76.013407}, abstractNote={Using the alternative representation of time-dependent Hamiltonians that describe laser-driven systems [I. Gilary and N. Moiseyev, Phys. Rev. A 66, 063415 (2002)], we have performed calculations for the stability of ${\text{He}}^{\ensuremath{-}}$ in a superintense linearly polarized laser field. We estimated the laser parameters, amplitude, and frequency needed to stabilize ${\text{He}}^{\ensuremath{-}}$. If we choose the laser frequency $\ensuremath{\omega}=5\text{ }\text{eV}$, the laser intensity needed for stabilization is ${I}^{\text{critical}}=9.0\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{W}∕{\text{cm}}^{2}$ and the maximal detachment energy is 1.0 eV. Our results show that high frequencies and large intensities are preferred for stabilizing multiply negative atomic ions in superintense laser fields.}, number={1}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Wei, Q. and Kais, S. and Moiseyev, N.}, year={2007} }
 @article{peskin_huang_kais_2007, title={Internal entanglement amplification by external interactions}, volume={76}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547264925&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.76.012102}, abstractNote={We propose a scheme to control the level of entanglement between two fixed spin-$1∕2$ systems by interaction with a third particle. For specific designs, entanglement is shown to be ``pumped'' into the system from the surroundings even when the spin-spin interaction within the system is small or nonexistent. The effect of the external particle on the system is introduced by including a dynamic spinor in the Hamiltonian. Controlled amplification of the internal entanglement to its maximum value is demonstrated. The possibility of entangling noninteracting spins in a stationary state is also demonstrated by coupling each one of them to a flying qubit in a quantum wire.}, number={1}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Peskin, U. and Huang, Z. and Kais, S.}, year={2007} }
 @article{wang_kais_2007, title={Quantum entanglement and electron correlation in molecular systems}, volume={47}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-40349112873&partnerID=MN8TOARS}, DOI={10.1560/IJC.47.1.59}, abstractNote={Abstract We study the relation between quantum entanglement and electron correlation in quantum chemistry calculations. We prove that the Hartree–Fock (HF) wave function does not violate Bell's inequality, and thus is not entangled, whereas the configuration interaction (CI) wave function is entangled since it violates Bell's inequality. Entanglement is related to electron correlation and might be used as an alternative measure of the electron correlation in quantum chemistry calculations. As an example we show the calculations of entanglement for the H 2 molecule and how it correlates with the traditional electron correlation, which is the difference between the exact and the HF energies.}, number={1}, journal={Israel Journal of Chemistry}, author={Wang, H. and Kais, S.}, year={2007}, pages={59–65} }
 @article{huang_wang_kais_2006, title={Entanglement and electron correlation in quantum chemistry calculations}, volume={53}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33750530210&partnerID=MN8TOARS}, DOI={10.1080/09500340600955674}, abstractNote={Electron–electron correlation in quantum chemistry calculations can be analysed in terms of entanglement between electrons. Two exactly solvable models: two fixed spin-1/2 particles and two-electron two-site Hubbard model are used to define and discuss the entanglement as a function of the system parameters. Ab initio configuration interaction calculation for entanglement is presented for the H2 molecule. Qualitatively, entanglement and electron–electron correlation have similar behaviour. Thus, entanglement might be used as an alternative measure of electron correlation in quantum chemistry calculations.}, number={16-17}, journal={Journal of Modern Optics}, author={Huang, Z. and Wang, H. and Kais, S.}, year={2006}, pages={2543–2558} }
 @article{wang_kais_2006, title={Entanglement and quantum phase transition in a one-dimensional system of quantum dots with disorder}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33845548014&partnerID=MN8TOARS}, DOI={10.1142/S0219749906002183}, abstractNote={We study the entanglement of formation and quantum phase transition in a one-dimensional quantum dot system with disorder modeled by the Hubbard Hamiltonian. The entanglement for three different cases is studied: the homogeneous case, the impurity case of symmetric electron hopping, and asymmetric electron hopping. We find that the local entanglement of the system can be tuned by introducing different impurities characterized by the physical parameters of the system. In particular, for certain parameters, the entanglement is negligible up to a critical point U c , where a quantum phase transition occurs, and is different from zero above U c .}, number={5}, journal={International Journal of Quantum Information}, author={Wang, H. and Kais, S.}, year={2006}, pages={827–835} }
 @article{huang_kais_2006, title={Entanglement evolution of one-dimensional spin systems in external magnetic fields}, volume={73}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33344469140&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.73.022339}, abstractNote={We study the dynamics of entanglement for a one-dimensional spin system, where spins are coupled through nearest-neighbor exchange interaction and subject to different external magnetic fields. First we examine the system size effect on the entanglement with three different external magnetic fields changing with time $t$: an exponential function ${e}^{[\ensuremath{-}Kt]}$ and two periodic $\mathrm{sin}[Kt]$ and $\mathrm{cos}[Kt]$ functions, where $K$ is a control parameter. We have found that the entanglement fluctuates shortly after a disturbance by an external magnetic field when the system size is small. For larger system size, the entanglement reaches a stable state for a long time before it fluctuates. However, this fluctuation of entanglement disappears when the system size goes to infinity. We also show that in a periodic external magnetic field, the nearest-neighbor entanglement displays a periodic structure with a period related to that of the magnetic field. Moreover, changing the direction of the magnetic field will destroy the concurrence in the system.}, number={2}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Huang, Z. and Kais, S.}, year={2006} }
 @article{wei_kais_moiseyev_2006, title={New stable multiply charged negative atomic ions in linearly polarized superintense laser fields}, volume={124}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547648321&partnerID=MN8TOARS}, DOI={10.1063/1.2207619}, abstractNote={Singly charged negative atomic ions exist in the gas phase and are of fundamental importance in atomic and molecular physics. However, theoretical calculations and experimental results clearly exclude the existence of any stable doubly-negatively-charged atomic ion in the gas phase, only one electron can be added to a free atom in the gas phase. In this report, using the high-frequency Floquet theory, we predict that in a linear superintense laser field one can stabilize multiply charged negative atomic ions in the gas phase. We present self-consistent field calculations for the linear superintense laser fields needed to bind extra one and two electrons to form He-, He2-, and Li2-, with detachment energies dependent on the laser intensity and maximal values of 1.2, 0.12, and 0.13 eV, respectively. The fields and frequencies needed for binding extra electrons are within experimental reach. This method of stabilization is general and can be used to predict stability of larger multiply charged negative atomic ions.}, number={20}, journal={Journal of Chemical Physics}, author={Wei, Q. and Kais, S. and Moiseyev, N.}, year={2006} }
 @article{kais_wenger_wei_2006, title={Quantum criticality at the infinite complete basis set limit: A thermodynamic analog of the Yang and Lee theorem}, volume={423}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646175029&partnerID=MN8TOARS}, DOI={10.1016/j.cplett.2006.03.035}, number={1-3}, journal={Chemical Physics Letters}, author={Kais, S. and Wenger, C. and Wei, Q.}, year={2006}, pages={45–49} }
 @article{wang_kais_2006, title={Quantum teleportation in one-dimensional quantum dots system}, volume={421}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33645864938&partnerID=MN8TOARS}, DOI={10.1016/j.cplett.2006.01.093}, number={4-6}, journal={Chemical Physics Letters}, author={Wang, H. and Kais, S.}, year={2006}, pages={338–342} }
 @article{huang_sadiek_kais_2006, title={Time evolution of a single spin inhomogeneously coupled to an interacting spin environment}, volume={124}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547139808&partnerID=MN8TOARS}, DOI={10.1063/1.2192778}, abstractNote={We study the time evolution of a single spin coupled by exchange interaction to an environment of interacting spin bath modeled by the XY Hamiltonian. By evaluating the spin correlator of the single spin, we observed that the decay rate of the spin oscillations strongly depends on the relative magnitude of the exchange coupling between the single spin and its nearest neighbor J(') and coupling among the spins in the environment J. The decoherence time varies significantly based on the relative coupling magnitudes of J and J('). The decay rate law has a Gaussian profile when the two exchange couplings are of the same order J(') approximately J but converts to exponential and then a power law as we move to the regimes of J(')>J and J(')<J. We also show that the spin oscillations propagate from the single spin to the environmental spins with a certain speed. The effect of varying the anisotropic parameter, external magnetic field, and temperature on the decaying rate of the spin state is also discussed.}, number={14}, journal={Journal of Chemical Physics}, author={Huang, Z. and Sadiek, G. and Kais, S.}, year={2006} }
 @article{shi_kais_2005, title={Discontinuity of Shannon information entropy for two-electron atoms}, volume={309}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-12944322206&partnerID=MN8TOARS}, DOI={10.1016/j.chemphys.2004.08.020}, number={2-3}, journal={Chemical Physics}, author={Shi, Q. and Kais, S.}, year={2005}, pages={127–131} }
 @article{huang_kais_2005, title={Dynamics of entaglement for one-dimensional spin system in an external time-dependent magnetic field}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33344465015&partnerID=MN8TOARS}, DOI={10.1142/S0219749905001055}, abstractNote={We study the dynamics of entanglement for the XY-model, one-dimensional spin systems coupled through the nearest neighbor exchange interaction and subject to an external time-dependent magnetic field. Using the two-site density matrix, we calculate the time-dependent entanglement of formation between nearest neighbor qubits. We investigate the effect of varying the temperature, the anisotropy parameter and the external time-dependent magnetic field on the entanglement. We have found that the entanglement can be localized between nearest neighbor qubits for certain values of the external time-dependent magnetic field. Moreover, as known for the magnetization of this model, the entanglement shows nonergodic behavior, it does not approach its equilibrium value at the infinite time limit.}, number={3}, journal={International Journal of Quantum Information}, author={Huang, Z. and Kais, S.}, year={2005}, pages={483–500} }
 @article{huang_kais_2005, title={Entanglement as measure of electron-electron correlation in quantum chemistry calculations}, volume={413}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-24344476691&partnerID=MN8TOARS}, DOI={10.1016/j.cplett.2005.07.045}, number={1-3}, journal={Chemical Physics Letters}, author={Huang, Z. and Kais, S.}, year={2005}, pages={1–5} }
 @article{shi_kais_francisco_2005, title={Graph theory for fused cubic clusters of water dodecamer}, volume={109}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-30344439814&partnerID=MN8TOARS}, DOI={10.1021/jp0550154}, abstractNote={The stable structures of the fused cubic water cluster (H2O)12 are examined using graph theoretical techniques and ab initio calculations. The calculations are obtained by scanning the symmetry of digraph structures of hydrogen-bond network spanning 12 oxygen atom vertexes. Using the Pólya theorem the cycle index expressions for 12 vertexes and 20 edges of a cuboid in point-group symmetry D4h are developed. A total of 91 energy-allowed fused cubic structures are obtained, which are classified by 8 point-group symmetries: 1 D2h, 2 S4, 5 C4, 1 D2, 11 C2, 10 Ci, 1 Cs, and 60 C1. An energy level diagram of the structures reveals 14 bands that correspond to 14 unique two-colored graphs derived from the distributions of four free hydrogens of the cluster.}, number={51}, journal={Journal of Physical Chemistry A}, author={Shi, Q. and Kais, S. and Francisco, J.S.}, year={2005}, pages={12036–12045} }
 @article{shi_kais_herschbach_2004, title={Electron localization-delocalization transitions in dissociation of the C 4 - anion: A large-D analysis}, volume={120}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1442355587&partnerID=MN8TOARS}, number={5}, journal={Journal of Chemical Physics}, author={Shi, Q. and Kais, S. and Herschbach, D.R.}, year={2004}, pages={2199–2207} }
 @article{huang_osenda_kais_2004, title={Entanglement of formation for one-dimensional magnetic systems with defects}, volume={322}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1242272057&partnerID=MN8TOARS}, DOI={10.1016/j.physleta.2004.01.022}, number={3-4}, journal={Physics Letters, Section A: General, Atomic and Solid State Physics}, author={Huang, Z. and Osenda, O. and Kais, S.}, year={2004}, pages={137–145} }
 @article{shi_kais_2004, title={Finite size scaling for the atomic Shannon-information entropy}, volume={121}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-5544237576&partnerID=MN8TOARS}, DOI={10.1063/1.1785773}, abstractNote={We have developed the finite size scaling method to treat the criticality of Shannon-information entropy for any given quantum Hamiltonian. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. To illustrate this approach we present a study to estimate the critical exponents of the Shannon-information entropy S approximately (lambda-lambda(c))(alpha(S) ), the electronic energy E approximately (lambda-lambda(c))(alpha(E) ), and the correlation length xi approximately mid R:lambda-lambda(c)mid R:(-nu) for atoms with the variable lambda=1/Z, which is the inverse of the nuclear charge Z. This was realized by approximating the multielectron atomic Hamiltonian with a one-electron model Hamiltonian. This model is very accurate for describing the electronic structure of the atoms near their critical points. For several atoms in their ground electronic states, we have found that the critical exponents (alpha(E),nu,alpha(S)) for He (Z=2), C (Z=6), N (Z=7), F (Z=9), and Ne (Z=10), respectively, are (1, 0, 0). At the critical points lambda(c)=1/Z(c), the bound state energies become absorbed or degenerate with continuum states and the entropies reach their maximum values, indicating a maximal delocalization of the electronic wave function.}, number={12}, journal={Journal of Chemical Physics}, author={Shi, Q. and Kais, S.}, year={2004}, pages={5611–5617} }
 @article{ferrón_serra_kais_2004, title={Finite-size scaling for critical conditions for stable quadrupole-bound anions}, volume={120}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-2542504537&partnerID=MN8TOARS}, DOI={10.1063/1.1695552}, abstractNote={We present finite-size scaling calculations of the critical parameters for binding an electron to a finite linear quadrupole field. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. The model Hamiltonian consists of a charge Q located at the origin of the coordinates and k charges -Q/k located at distances R(i), i=1, em leader,k. After proper scaling of distances and energies, the rescaled Hamiltonian depends only on one free parameter q=QR. Two different linear charge configurations with q>0 and q<0 are studied using basis sets in both spherical and prolate spheroidal coordinates. For the case with q>0, the finite size scaling calculations give an extrapolated critical value of q(c)=1.469 70+/-0.000 05 a.u. by using a basis set with prolate spheroidal coordinates. For the quadrupole case with q<0, we obtained an extrapolated critical value of mid R:q(c)mid R:=3.982 51+/-0.000 01 a.u. for stable quadrupole bound anions. The corresponding critical exponent for the ground state energy alpha=1.9964+/-0.0005, with E approximately (q-q(c))(alpha).}, number={18}, journal={Journal of Chemical Physics}, author={Ferrón, A. and Serra, P. and Kais, S.}, year={2004}, pages={8412–8419} }
 @article{wang_kais_2004, title={Scaling of entanglement at a quantum phase transition for a two-dimensional array of quantum dots}, volume={70}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-19544364448&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.70.022301}, abstractNote={Using the Hubbard model, the entanglement scaling behavior in a two-dimensional itinerant system is investigated. It has been found that, on the two sides of the critical point denoting an inherent quantum phase transition (QPT), the entanglement follows different scalings with the size, just as an order parameter does. This fact reveals the subtle role played by the entanglement in QPT as a fungible physical resource.}, number={2}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Wang, J. and Kais, S.}, year={2004} }
 @article{wang_kais_2003, title={Combined effects of disorders and electron-electron interactions upon metal-insulator transition in 2D nonbipartite lattice}, volume={316}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17344385590&partnerID=MN8TOARS}, DOI={10.1016/j.physleta.2003.08.002}, number={3-4}, journal={Physics Letters, Section A: General, Atomic and Solid State Physics}, author={Wang, J. and Kais, S.}, year={2003}, pages={265–270} }
 @article{serra_kais_2003, title={Finite size scaling for critical conditions for stable dipole-bound anions}, volume={372}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037461194&partnerID=MN8TOARS}, DOI={10.1016/S0009-2614(03)00371-3}, number={1-2}, journal={Chemical Physics Letters}, author={Serra, P. and Kais, S.}, year={2003}, pages={205–209} }
 @book{kais_serra_2003, title={Finite-size scaling for atomic and molecular systems}, volume={125}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1442347461&partnerID=MN8TOARS}, journal={Advances in Chemical Physics}, author={Kais, S. and Serra, P.}, year={2003}, pages={1–99} }
 @article{wang_kais_2003, title={Metal-insulator transition in the hubbard model on a triangular lattice with disorders: Renormalization group approach}, volume={93}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037514590&partnerID=MN8TOARS}, DOI={10.1002/qua.10572}, abstractNote={Abstract A multistages block renormalization group approach to study the metal–insulator transition in the Hubbard model on a triangular lattice with hexagonal blocks is presented and implemented. A second‐order phase transition with a critical point at U/t = 12.5 is obtained (the coupling parameters U and t correspond to the repulsive charging energy and to the nearest‐neighbor exchange coupling terms, respectively). In the presence of disorder the phase diagram for the system exhibits a metallic phase, an insulating phase, and a domain‐localized phase that separates them in the Mott regime. The subtle influence of electron–electron interactions upon inverse participation rate in the Anderson regime is also investigated. The results are discussed in light of experimental evidence for arrays of metalic quantum dots and exact numerical diagonalization of the Hubbard Hamiltonian. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 360–374, 2003}, number={5}, journal={International Journal of Quantum Chemistry}, author={Wang, J.X. and Kais, S.}, year={2003}, pages={360–374} }
 @article{shi_belair_francisco_kais_2003, title={On the interactions between atmospheric radicals and cloud droplets: A molecular picture of the interface}, volume={100}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0041691232&partnerID=MN8TOARS}, DOI={10.1073/pnas.1733696100}, abstractNote={How gas-phase materials become incorporated with cloud droplets has been an intriguing subject for decades, and considerable work has been done to understand the interactions between closed-shell molecules and liquid water. The interactions between open-shell radical species and liquid-phase cloud droplets, however, are not well understood. To probe these interactions we used quantum chemistry calculations to predict the energetics of the hydroperoxy radical (HO2) in the presence of an (H2O)20 spherical water cage. Our calculations show that it is energetically favorable for the radical to bind to the outside of the cage. This configuration has the hydrogen and the terminal oxygen of the radical as its primary bonding sites. Free-energy calculations suggest that, at atmospheric conditions, there will be a partitioning between HO2 radicals that are surface-bound and HO2 radicals that dissolve into the bulk. This may have important ramifications for our understanding of radical chemistry and may lend insight into the role that clouds and aerosols play in atmospheric chemical processes.}, number={17}, journal={Proceedings of the National Academy of Sciences of the United States of America}, author={Shi, Q. and Belair, S.D. and Francisco, J.S. and Kais, S.}, year={2003}, pages={9686–9690} }
 @article{osenda_huang_kais_2003, title={Tuning the entanglement for a one-dimensional magnetic system with anisotropic coupling and impurities}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0041508929&partnerID=MN8TOARS}, number={6}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Osenda, O. and Huang, Z. and Kais, S.}, year={2003}, pages={623211–623214} }
 @inproceedings{kais_2002, title={Finite size scaling in quantum mechanics}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-6344246156&partnerID=MN8TOARS}, booktitle={2002 International Conference on Computational Nanoscience and Nanotechnology - ICCN 2002}, author={Kais, S.}, year={2002}, pages={474–477} }
 @article{wang_kais_2002, title={Finite-size scaling for Mott metal-insulator transition on a half filled nonpartite lattice}, volume={66}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037104287&partnerID=MN8TOARS}, number={8}, journal={Physical Review B - Condensed Matter and Materials Physics}, author={Wang, J.X. and Kais, S.}, year={2002}, pages={811011–811014} }
 @article{shi_kais_2002, title={Lifetimes of metastable spherical carbon cluster dianions}, volume={100}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037138924&partnerID=MN8TOARS}, DOI={10.1080/00268970110086327}, abstractNote={Semiclassical calculations are made of the lifetimes of metastable spherical carbon cluster dianions using a model potential based on electrostatics. The metastability is attributed to shape resonances resulting from the combination of the long range Coulomb repulsion and the shorter range electron-molecule interaction. Results for fullerene dianions show a strong dependence of the lifetimes on the molecular size. The transition from stable to metastable dianions is estimated to occur when the radius of the spherical molecule is about 5.5 Å. This simple model potential might provide a starting point for experimental and theoretical searches for the existence of stable dianions}, number={4}, journal={Molecular Physics}, author={Shi, Q. and Kais, S.}, year={2002}, pages={475–481} }
 @article{belair_kais_francisco_2002, title={Potential energy surface for the hydroperoxy and water (HO2·H2O) radical complex}, volume={100}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037138185&partnerID=MN8TOARS}, DOI={10.1080/00268970110086309}, abstractNote={A potential energy surface for the system of a hydroperoxy radical and a water molecule is presented. The surface was sampled using constrained density functional theory optimizations performed at the B3LYP level of theory using a 6–311 ++G(3df,3pd) basis set. The data points were fitted to an analytical function based on a common 4-point model for water and a 5-point model for the peroxy radical. A weighted least-squares fit of the parameters was performed using the nearest neighbour pivot method.}, number={2}, journal={Molecular Physics}, author={Belair, S.D. and Kais, S. and Francisco, J.S.}, year={2002}, pages={247–253} }
 @article{wang_kais_levine_2002, title={Real-space renormalization group study of the Hubbard model on anon-bipartite lattice}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0002567832&partnerID=MN8TOARS}, number={1}, journal={International Journal of Molecular Sciences}, author={Wang, J.X. and Kais, S. and Levine, R.D.}, year={2002}, pages={4–16} }
 @article{wang_kais_remade_levine_2002, title={Size effects in the electronic properties of finite arrays of exchange-coupled quantum dots}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037137664&partnerID=MN8TOARS}, DOI={10.1021/jp026452a}, abstractNote={Transport properties of arrays of metallic quantum dots are governed by the distance-dependent exchange coupling between the dots. It is shown that the effective value of the exchange coupling, as measured by the charging energy per dot, depends monotonically on the size of the array. The effect saturates for hexagonal arrays of over 75 unit cells. The discussion uses a multistage block renormalization group approach applied to the Hubbard Hamiltonian. A first-order phase transition occurs upon compression of the lattice, and the size dependence is qualitatively different for the two phases.}, number={50}, journal={Journal of Physical Chemistry B}, author={Wang, J.X. and Kais, S. and Remade, F. and Levine, R.D.}, year={2002}, pages={12847–12850} }
 @article{shi_kais_2002, title={The repulsive Coulomb barrier along a dissociation path of the BeC4 2- dianion}, volume={124}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037009984&partnerID=MN8TOARS}, DOI={10.1021/ja020116q}, abstractNote={We present ab initio calculations of the repulsive Coulomb barrier for several geometrically stable isomers of the BeC(2-)(4) dianion. We describe how the deformation of certain isomers can account for the experimental Coulomb explosion images of the dianion. For the most stable linear isomer, C(-)(2)BeC(-)(2), we examined the electron tunneling process along the dissociation path to obtain C(-)(2) plus BeC(-)(2). We found the crossing point for autodetachment to be R(c)(dis)= 3.25 A. R(dis) is the bond length between C(-)(2) and BeC(-)(2); at this point, the electron tunneling energy is equal to the maximum of the repulsive Coulomb barrier. In the framework of the Wenzel-Kramer-Brioullin theory, the electron-loss lifetime of the metastable C(-)(2)BeC(-)(2) dianion at the equilibrium geometry, R(dis) = 1.64 A, was estimated to be about 5 ms. This lower limit is in agreement with the experimental results in which the BeC(2-)(4) dianion has a lifetime much longer than 5 micros.}, number={39}, journal={Journal of the American Chemical Society}, author={Shi, Q. and Kais, S.}, year={2002}, pages={11723–11729} }
 @article{serra_kais_moiseyev_2001, title={Crossover phenomena and resonances in quantum systems}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035676058&partnerID=MN8TOARS}, number={6}, journal={Physical Review A. Atomic, Molecular, and Optical Physics}, author={Serra, P. and Kais, S. and Moiseyev, N.}, year={2001} }
 @article{shi_kais_remacle_levine_2001, title={Electronic isomerism: Symmetry breaking and electronic phase diagrams for diatomic molecules at the large-dimension limit}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1442298441&partnerID=MN8TOARS}, number={7}, journal={ChemPhysChem}, author={Shi, Q. and Kais, S. and Remacle, F. and Levine, R.D.}, year={2001}, pages={434–442} }
 @article{shi_kais_remacle_levine_2001, title={On the crossing of electronic energy levels of diatomic molecules at the large-D limit}, volume={114}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035827118&partnerID=MN8TOARS}, DOI={10.1063/1.1372181}, abstractNote={Analytical and numerical results are presented for the intersection of electronic energies of the same space symmetry for electrons in the field of two Coulomb centers in D-dimensions. We discuss why such crossings are allowed and may be less “exceptional” than one could think because even for a diatomic molecule there is more than one parameter in the electronic Hamiltonian. For a one electron diatomic molecule at the large-D limit, the electronic energies are shown analytically to diverge quadratically from the point of their intersection. The one electron two Coulomb centers problem allows a separation of variables even when the charges on the two centers are not equal. The case of two electrons, where their Coulombic repulsion precludes an exact symmetry, is therefore treated in the large-D limit. It is then found that, in addition to the quadratic intersection, there is also a curve crossing where the energies diverge linearly.}, number={22}, journal={Journal of Chemical Physics}, author={Shi, Q. and Kais, S. and Remacle, F. and Levine, R.D.}, year={2001}, pages={9697–9705} }
 @article{sauerwein_kais_2001, title={Quantum critical phenomena in the Schrödinger formulation: Mapping to classical lattices}, volume={333}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0006978834&partnerID=MN8TOARS}, DOI={10.1016/S0009-2614(00)01411-1}, number={6}, journal={Chemical Physics Letters}, author={Sauerwein, R.A. and Kais, S.}, year={2001}, pages={451–458} }
 @article{shi_kais_2001, title={Quantum criticality at the large-dimensional limit: Three-body Coulomb systems}, volume={85}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035890723&partnerID=MN8TOARS}, DOI={10.1002/qua.1542}, abstractNote={Abstract We present quantum phase transitions and critical phenomena at the large‐dimension (D) limit for three‐body ABA Coulomb systems with charges ( Q , q , Q ) and masses ( M , m , M ). The Hamiltonian depends linearly on two parameters λ=∣ Q / q ∣ and κ=[1+( m / M )] −1 . The system exhibits critical points with mean field critical exponents (α=0, β=½, δ=3, γ=1). We calculate the critical curve λ c (κ) through which all systems undergo a continuous‐phase transition from the symmetrical configuration, the two like particles have the same distance from the reference particle, to the unsymmetrical phase. The critical curve at D →∞ limit is a convex function of κ and very similar to the one obtained at D =3 with variational calculations. We also calculated the line of zero angular correlation in the mass polarization term, which separates the symmetrical phase to an atom‐like region and a molecule‐like region. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001}, number={4-5}, journal={International Journal of Quantum Chemistry}, author={Shi, Q. and Kais, S.}, year={2001}, pages={307–314} }
 @article{sauerwein_kais_2001, title={Quantum criticality for few-body systems: Path-integral approach}, volume={64}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17944389944&partnerID=MN8TOARS}, number={5 II}, journal={Physical Review E - Statistical, Nonlinear, and Soft Matter Physics}, author={Sauerwein, R.A. and Kais, S.}, year={2001} }
 @article{sergeev_kais_2001, title={Resonance states of atomic anions}, volume={82}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035917428&partnerID=MN8TOARS}, DOI={10.1002/qua.1047}, abstractNote={Abstract We study destabilization of an atom in its ground state with decrease of its nuclear charge. By analytic continuation from bound to resonance states, we obtain complex energies of unstable atomic anions with nuclear charge that is less than the minimum “critical” charge necessary to bind N electrons. We use an extrapolating scheme with a simple model potential for the electron, which is loosely bound outside the atomic core. Results for O 2− and S 2− are in good agreement with earlier estimates. Alternatively, we use the Hylleraas basis variational technique with three complex nonlinear parameters to find accurately the energy of two‐electron atoms as the nuclear charge decreases. Results are used to check the less accurate one‐electron model. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 255–261, 2001}, number={5}, journal={International Journal of Quantum Chemistry}, author={Sergeev, A.V. and Kais, S.}, year={2001}, pages={255–261} }
 @article{nigra_carignano_kais_2001, title={Study of phase changes of the water octamer using parallel tempering and multihistogram methods}, volume={115}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035828054&partnerID=MN8TOARS}, DOI={10.1063/1.1385795}, abstractNote={Parallel tempering Monte Carlo and multihistogram methods are combined to study the phase changes of the water octamer. The heat capacity is calculated continuously from very low temperatures up to T=230 K. We find the melting temperature to be 178.5 K. In addition, a solid–solid phase change is found at 12 K. We introduce an order parameter to monitor this low temperature phase change.}, number={6}, journal={Journal of Chemical Physics}, author={Nigra, P. and Carignano, M.A. and Kais, S.}, year={2001}, pages={2621–2628} }
 @article{ladadwa_kais_2000, title={Critical behavior of electron impact ionization of atoms}, volume={80}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034320032&partnerID=MN8TOARS}, number={4-5}, journal={International Journal of Quantum Chemistry}, author={Ladadwa, I. and Kais, S.}, year={2000}, pages={575–581} }
 @article{serra_kais_2000, title={Data collapse for the Schrödinger equation}, volume={319}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0011059885&partnerID=MN8TOARS}, number={3-4}, journal={Chemical Physics Letters}, author={Serra, P. and Kais, S.}, year={2000}, pages={273–277} }
 @article{shi_kais_2000, title={Finite size scaling for critical parameters of simple diatomic molecules}, volume={98}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034634010&partnerID=MN8TOARS}, DOI={10.1080/002689700419716}, number={19}, journal={Molecular Physics}, author={Shi, Q. and Kais, S.}, year={2000}, pages={1485–1493} }
 @article{serra_kais_neirotti_2000, title={Finite-size scaling method for the stability of atomic and molecular ions}, volume={283}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033691943&partnerID=MN8TOARS}, DOI={10.1016/S0378-4371(00)00129-1}, number={1}, journal={Physica A: Statistical Mechanics and its Applications}, author={Serra, P. and Kais, S. and Neirotti, J.P.}, year={2000}, pages={65–73} }
 @article{kais_serra_2000, title={Quantum critical phenomena and stability of atomic and molecular ions}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033653798&partnerID=MN8TOARS}, DOI={10.1080/014423500229873}, abstractNote={In this review we discuss quantum phase transitions and the mapping between symmetry breaking of electronic structure configurations at the large-dimension limit and mean-field theory of phase transitions. We show that the finite size scaling method can be used for the calculations of the critical parameters of the few-body Schrodinger equation. In this approach, the finite size corresponds to the number of elements in a complete basis set used to expand the exact eigenfunction of a given Hamiltonian. The critical parameters such as the critical nuclear charges will be used to explain and predict the stability of atomic and molecular negative ions. For N-electron atoms with 2 N 86, results show that, at most, only one electron can be added to a free atom in the gas phase. However, doubly charged atomic negative ions might exist in a strong magnetic field.}, number={1}, journal={International Reviews in Physical Chemistry}, author={Kais, S. and Serra, P.}, year={2000}, pages={97–121} }
 @article{kais_shi_2000, title={Quantum criticality and stability of three-body Coulomb systems}, volume={62}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4243868576&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.62.060502}, abstractNote={We present quantum phase transitions and critical phenomena of three-body Coulomb systems with charges $(Q,$ q, $Q)$ and masses $(M,$ m, $M).$ Full numerical results, using the finite-size scaling method, for an arbitrary mass ratio $0<~\ensuremath{\kappa}{=(1+m/M)}^{\ensuremath{-}1}<~1$ over the range $1<~\ensuremath{\lambda}=|Q/q|<~1.25,$ show that there exists a transition curve ${\ensuremath{\lambda}}_{c}(\ensuremath{\kappa})$ through which all systems undergo a first-order phase transition from stable to unstable. Particularly, ${\ensuremath{\lambda}}_{c}(\ensuremath{\kappa})$ has a minimum at ${\ensuremath{\kappa}}_{m}=0.35,$ which leads to a new proposed classification of the three-body Coulomb systems: moleculelike systems, $\ensuremath{\kappa}>{\ensuremath{\kappa}}_{m},$ such as ${\mathrm{Ps}}^{\ensuremath{-}}$ $(\ensuremath{\kappa}=0.5)$ and atomlike systems, $\ensuremath{\kappa}<{\ensuremath{\kappa}}_{m},$ such as $\overline{p}\overline{p}d$ $(\ensuremath{\kappa}=0.33).$}, number={6}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Kais, S. and Shi, Q.}, year={2000}, pages={060502–060501} }
 @article{sergeev_kais_1999, title={Critical nuclear charges for n-electron atoms}, volume={75}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001530336&partnerID=MN8TOARS}, number={4-5}, journal={International Journal of Quantum Chemistry}, author={Sergeev, A.V. and Kais, S.}, year={1999}, pages={533–542} }
 @article{nigra_kais_1999, title={Pivot method for global optimization: A study of water clusters (H2O)N with 2 ≤ N ≤ 33}, volume={305}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000564441&partnerID=MN8TOARS}, number={5-6}, journal={Chemical Physics Letters}, author={Nigra, P. and Kais, S.}, year={1999}, pages={433–438} }
 @article{sergeev_kais_1999, title={Variational principle for critical parameters of quantum systems}, volume={32}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033208843&partnerID=MN8TOARS}, DOI={10.1088/0305-4470/32/39/312}, abstractNote={The variational principle for eigenvalue problems with a nonidentity weight operator is used to establish upper or lower bounds on critical parameters of quantum systems. Three problems from atomic physics are considered as examples. Critical screening parameters for the exponentially screened Coulomb potential are found using a trial function with one nonlinear variational parameter. The critical charge for the helium isoelectronic series is found using a Hylleraas-type trial function. Finally, critical charges for the same system subjected to a magnetic field are found using a product of two hydrogen-like basis sets.}, number={39}, journal={Journal of Physics A: Mathematical and General}, author={Sergeev, A.V. and Kais, S.}, year={1999}, pages={6891–6896} }
 @article{neirotti_serra_kais_1998, title={Critical parameters for the heliumlike atoms: A phenomenological renormalization study}, volume={108}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-5544224963&partnerID=MN8TOARS}, number={7}, journal={Journal of Chemical Physics}, author={Neirotti, J.P. and Serra, P. and Kais, S.}, year={1998}, pages={2765–2770} }
 @article{serra_neirotti_kais_1998, title={Electronic structure critical parameters for the lithium isoelectronic series}, volume={80}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001419331&partnerID=MN8TOARS}, number={24}, journal={Physical Review Letters}, author={Serra, P. and Neirotti, J.P. and Kais, S.}, year={1998}, pages={5293–5296} }
 @article{el-batanouny_murthy_willis_kais_staemmler_1998, title={Feasibility of measuring surface electron spin dynamics by inelastic scattering of metastable helium atoms}, volume={58}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0009664243&partnerID=MN8TOARS}, number={11}, journal={Physical Review B - Condensed Matter and Materials Physics}, author={El-Batanouny, M. and Murthy, G. and Willis, C.R. and Kais, S. and Staemmler, V.}, year={1998}, pages={7391–7402} }
 @article{serra_neirotti_kais_1998, title={Finite size scaling in quantum mechanics}, volume={102}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0006789264&partnerID=MN8TOARS}, number={47}, journal={Journal of Physical Chemistry A}, author={Serra, P. and Neirotti, J.P. and Kais, S.}, year={1998}, pages={9518–9522} }
 @article{serra_neirotti_kais_1998, title={Finite-size scaling approach for the Schrödinger equation}, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000576810&partnerID=MN8TOARS}, number={3}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Serra, P. and Neirotti, J.P. and Kais, S.}, year={1998} }
 @article{shi_zhang_cho_xu_li_kais_1998, title={Relativistic structure description and relaxation effect on krypton 4p5(2P3/2,1/2)5s excitation at small squared momentum transfer}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0013228681&partnerID=MN8TOARS}, DOI={10.1088/0953-4075/31/18/010}, abstractNote={Relativistic generalized oscillator strengths (GOSs) at small squared momentum transfer for krypton excitations are studied in detail under the first-order Born approximation based on the Dirac-Fock (DF) and Dirac-Fock-Slater (DS) theories. Comparisons are made with the existing experiment at 300 and 500 eV. The large GOS difference between the local-type and non-local-type exchange interactions in the target, and the large influence of the relativistic effects on the GOS are shown at less than 0.1. Particularly, the relaxation effect between the ground- and excited-state single configurations, and the Coulomb correlation effect by a model potential as a function of local density are investigated. Comparisons with the experiment show that the relaxation effect is more important than the correlation effect. The present DF calculation, although not involving the non-local correlation, yields both the GOS and the excitation energy closer to the experimental values than the DS calculation, due to the very small correlation effect in the outer-shell excitations. Results of krypton show that the average-configuration-state approximation in the DF theory is suitable to describe the excited-state single configuration including and subshells.}, number={18}, journal={Journal of Physics B: Atomic, Molecular and Optical Physics}, author={Shi, Q. and Zhang, S. and Cho, H. and Xu, K. and Li, J.-M. and Kais, S.}, year={1998}, pages={4123–4135} }
 @article{murthy_kais_1998, title={Renormalization group approach for electronic excitations in atoms}, volume={290}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032568710&partnerID=MN8TOARS}, number={1-3}, journal={Chemical Physics Letters}, author={Murthy, G. and Kais, S.}, year={1998}, pages={199–204} }
 @article{stanton_bleil_kais_1997, title={A new approach to global minimization}, volume={18}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0005658571&partnerID=MN8TOARS}, number={4}, journal={Journal of Computational Chemistry}, author={Stanton, A.F. and Bleil, R.E. and Kais, S.}, year={1997}, pages={594–599} }
 @article{serra_stanton_kais_bleil_1997, title={Comparison study of pivot methods for global optimization}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000591831&partnerID=MN8TOARS}, number={17}, journal={Journal of Chemical Physics}, author={Serra, P. and Stanton, A.F. and Kais, S. and Bleil, R.E.}, year={1997}, pages={7170–7177} }
 @article{germann_kais_1997, title={Dimensional perturbation theory for Regge poles}, volume={106}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001056898&partnerID=MN8TOARS}, number={2}, journal={Journal of Chemical Physics}, author={Germann, T.C. and Kais, S.}, year={1997}, pages={599–604} }
 @article{neirotti_serra_kais_1997, title={Electronic structure critical parameters from finite-size scaling}, volume={79}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031588258&partnerID=MN8TOARS}, number={17}, journal={Physical Review Letters}, author={Neirotti, J.P. and Serra, P. and Kais, S.}, year={1997}, pages={3142–3145} }
 @article{chen_chen_kais_freiser_1997, title={Gas-phase reactions of Fe(CH2O)+ and Fe(CH2S)+ with small alkanes: An experimental and theoretical study}, volume={119}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031593112&partnerID=MN8TOARS}, DOI={10.1021/ja964234n}, abstractNote={The gas-phase reactions of Fe(CH2O)+ and Fe(CH2S)+ with a series of aliphatic alkanes were studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Like bare Fe+, C−C insertion, particularly terminal C−C insertion, is predominant for the reactions of Fe(CH2O)+, while C−H insertion is preferred for Fe(CH2S)+. About 90% of the Fe(CH2O)+ reaction products are formed by C−C insertion with small alkane loss. For Fe(CH2S)+, after initial C−H insertion, the proposed mechanism includes hydrogen transfer to sulfur, followed by migratory insertion of methylene into the metal−alkyl bond and formation of an activated H2S−Fe+−olefin complex, which dissociates by H2S elimination. The structures of the reaction products were probed by collision-induced dissociation, ion−molecule reactions, and use of labeled compounds, yielding information about the reaction mechanism. Collision-induced dissociation and ligand displacement reactions yield the brackets D0(Fe+−C3H6) = 37 ± 2 kcal/mol < D0(Fe+−CH2S) < D0(Fe+−C6H6) = 49.6 ± 2.3 kcal/mol and D0(Fe+−CH2O) < D0(Fe+−C2H4) = 34 ± 2 kcal/mol. The optimized geometry of Fe(CH2O)+, obtained by density functional calculations, has C2v symmetry with a nearly undisturbed formaldehyde unit. The Fe+−CH2O bonding is found to be predominantly electrostatic with a calculated bond energy of 32.2 kcal/mol. However, the optimized Fe(CH2S)+ structure has Cs symmetry with dative bonding between Fe+ and CH2S. D0(Fe+−CH2S) is calculated at 41.5 kcal/mol. The differences in geometry and chemical bonding between Fe(CH2O)+ and Fe(CH2S)+ are correlated with the different reaction pathways observed.}, number={52}, journal={Journal of the American Chemical Society}, author={Chen, Q. and Chen, H. and Kais, S. and Freiser, B.S.}, year={1997}, pages={12879–12888} }
 @article{march_kais_1997, title={Kinetic energy functional derivative for the Thomas-Fermi atom in D dimensions}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-5444263126&partnerID=MN8TOARS}, number={5}, journal={International Journal of Quantum Chemistry}, author={March, N.H. and Kais, S.}, year={1997}, pages={411–413} }
 @article{serra_kais_1997, title={Mean field phase diagrams for one-electron molecules}, volume={30}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031557315&partnerID=MN8TOARS}, DOI={10.1088/0305-4470/30/5/016}, abstractNote={We describe a simple model for symmetry breaking of electronic structure configurations of one-electron systems. This model involves generalizing the problem to D-dimensional space and finding the solution at , a semiclassical limit which can be solved exactly. The large-D limit model reduces the problem to a variational calculation which is equivalent to mean-field theories of critical phenomena in statistical mechanics. We show that symmetry breaking of electronic structure configurations can be described as standard phase transitions. Rich phase diagrams with multicritical points are reported for both linear and planar one-electron systems.}, number={5}, journal={Journal of Physics A: Mathematical and General}, author={Serra, P. and Kais, S.}, year={1997}, pages={1483–1493} }
 @article{serra_kais_1997, title={Phase transitions for N-electron atoms at the large-dimension limit}, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4243427253&partnerID=MN8TOARS}, number={1}, journal={Physical Review A - Atomic, Molecular, and Optical Physics}, author={Serra, P. and Kais, S.}, year={1997}, pages={238–247} }
 @article{serra_stanton_kais_1997, title={Pivot method for global optimization}, volume={55}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001166671&partnerID=MN8TOARS}, number={1 SUPPL. B}, journal={Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics}, author={Serra, P. and Stanton, A.F. and Kais, S.}, year={1997}, pages={1162–1165} }
 @article{serra_kais_1997, title={Symmetry breaking and stability of binary clusters}, volume={275}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031590122&partnerID=MN8TOARS}, number={3-4}, journal={Chemical Physics Letters}, author={Serra, P. and Kais, S.}, year={1997}, pages={211–214} }
 @article{serra_kais_1996, title={Critical phenomena for electronic structure at the large-dimension limit}, volume={77}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001140973&partnerID=MN8TOARS}, number={3}, journal={Physical Review Letters}, author={Serra, P. and Kais, S.}, year={1996}, pages={466–469} }
 @article{serra_kais_1996, title={Multicritical phenomena for the hydrogen molecule at the large-dimension limit}, volume={260}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030595193&partnerID=MN8TOARS}, number={1-2}, journal={Chemical Physics Letters}, author={Serra, P. and Kais, S.}, year={1996}, pages={302–308} }
 @article{kais_bleil_1995, title={Charge renormalization at the large-D limit for N-electron atoms and weakly bound systems}, volume={102}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000775704&partnerID=MN8TOARS}, number={19}, journal={The Journal of Chemical Physics}, author={Kais, S. and Bleil, R.}, year={1995}, pages={7472–7478} }
 @article{bleil_faliks_miletic_kais_1995, title={Charge renormalization at the large-D limit for diatomic molecules}, volume={103}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0345999500&partnerID=MN8TOARS}, number={15}, journal={The Journal of Chemical Physics}, author={Bleil, R. and Faliks, A. and Miletic, M. and Kais, S.}, year={1995}, pages={6529–6535} }
 @article{kais_germann_herschbach_1994, title={Large-dimension limit yields generic reduced potential curves for H2 +, H2, HHe+, and He2+}, volume={98}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0028521701&partnerID=MN8TOARS}, number={43}, journal={Journal of physical chemistry}, author={Kais, S. and Germann, T.C. and Herschbach, D.R.}, year={1994}, pages={11015–11017} }
 @article{lacks_kais_1994, title={Statistical model for delocalized π bonding in the C60 molecule}, volume={218}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-43949148854&partnerID=MN8TOARS}, number={3}, journal={Chemical Physics Letters}, author={Lacks, D.J. and Kais, S.}, year={1994}, pages={229–233} }
 @article{bleil_tao_kais_1994, title={Structure and stability of C13 carbon clusters}, volume={229}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001529521&partnerID=MN8TOARS}, number={4-5}, journal={Chemical Physics Letters}, author={Bleil, R. and Tao, F.-M. and Kais, S.}, year={1994}, pages={491–494} }
 @article{kais_herschbach_1994, title={The 1/Z expansion and renormalization of the large-dimension limit for many-electron atoms}, volume={100}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0001104850&partnerID=MN8TOARS}, number={6}, journal={The Journal of Chemical Physics}, author={Kais, S. and Herschbach, D.R.}, year={1994}, pages={4367–4376} }
 @article{kais_sung_herschbach_1993, title={Atomic energies from renormalization of the large-dimension limit}, volume={99}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000831132&partnerID=MN8TOARS}, number={7}, journal={The Journal of Chemical Physics}, author={Kais, S. and Sung, S.M. and Herschbach, D.R.}, year={1993}, pages={5184–5196} }
 @article{kais_herschbach_handy_murray_laming_1993, title={Density functionals and dimensional renormalization for an exactly solvable model}, volume={99}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0011498998&partnerID=MN8TOARS}, number={1}, journal={The Journal of Chemical Physics}, author={Kais, S. and Herschbach, D.R. and Handy, N.C. and Murray, C.W. and Laming, G.J.}, year={1993}, pages={417–425} }
 @article{kais_beltrame_1993, title={Dimensional scaling for Regge trajectories}, volume={97}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0011405367&partnerID=MN8TOARS}, number={10}, journal={Journal of Physical Chemistry}, author={Kais, S. and Beltrame, G.}, year={1993}, pages={2453–2456} }
 @article{kais_herschbach_1993, title={Dimensional scaling for quasistationary states}, volume={98}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000261521&partnerID=MN8TOARS}, number={5}, journal={The Journal of Chemical Physics}, author={Kais, S. and Herschbach, D.R.}, year={1993}, pages={3990–3998} }
 @article{germann_kais_1993, title={Large order dimensional perturbation theory for complex energy eigenvalues}, volume={99}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36449008617&partnerID=MN8TOARS}, number={10}, journal={The Journal of Chemical Physics}, author={Germann, T.C. and Kais, S.}, year={1993}, pages={7739–7747} }
 @article{kais_frantz_herschbach_1992, title={Electronic tunneling in H+ 2 evaluated from the large-dimension limit}, volume={161}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0011503574&partnerID=MN8TOARS}, number={3}, journal={Chemical Physics}, author={Kais, S. and Frantz, D.D. and Herschbach, D.R.}, year={1992}, pages={393–402} }
 @article{loeser_zhen_kais_herschbach_1991, title={Dimensional interpolation of hard sphere virial coefficients}, volume={95}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000610001&partnerID=MN8TOARS}, number={6}, journal={The Journal of Chemical Physics}, author={Loeser, J.G. and Zhen, Z. and Kais, S. and Herschbach, D.R.}, year={1991}, pages={4525–4544} }
 @article{electronic tunneling and exchange energy in the d-dimensional hydrogen-molecule ion_1991, volume={95}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0345998565&partnerID=MN8TOARS}, number={12}, journal={The Journal of Chemical Physics}, year={1991}, pages={9028–9041} }
 @article{kais_levine_1990, title={Coherent states for the Morse oscillator}, volume={41}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547664550&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.41.2301}, abstractNote={The explicit coordinate representation of coherent states for the Morse oscillator is derived and discussed, in the limit that the potential well is deep. Without this limit, such states can only be written as a finite sum that does evolve coherently but in a dilated time.}, number={5}, journal={Physical Review A}, author={Kais, S. and Levine, R.D.}, year={1990}, pages={2301–2305} }
 @article{kais_herschbach_levine_1989, title={Dimensional scaling as a symmetry operation}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0000290618&partnerID=MN8TOARS}, number={12}, journal={The Journal of Chemical Physics}, author={Kais, S. and Herschbach, D.R. and Levine, R.D.}, year={1989}, pages={7791–7796} }
 @article{kais_cohen_levine_1989, title={The perturbed hydrogen atom: Some new algebraic results}, volume={22}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36149034814&partnerID=MN8TOARS}, DOI={10.1088/0305-4470/22/7/012}, abstractNote={The authors have employed algebraic methods to calculate the bound-state spectra of a non-relativistic hydrogen atom subjected to a wide class of perturbations. Their procedure exploits the linearity of the complete (perturbed) Hamiltonian in the generators of the SO(2, 2) Lie algebra which follows naturally from the separation of variables in Schrodinger's equation in parabolic coordinates. Appropriate transformations then allow the Hamiltonian to be expressed as a linear combination of the compact generators of the two underlying SO(2, 1) algebras. They give some examples for which the bound-state spectra can be obtained completely analytically.}, number={7}, journal={Journal of Physics A: Mathematical and General}, author={Kais, S. and Cohen, M. and Levine, R.D.}, year={1989}, pages={803–809} }
 @article{kais_levine_1987, title={Directed states of molecules}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0039646468&partnerID=MN8TOARS}, number={21}, journal={Journal of Physical Chemistry}, author={Kais, S. and Levine, R.D.}, year={1987}, pages={5462–5465} }
 @article{gilmore_kais_levine_1986, title={Quantum cusp}, volume={34}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-24844467848&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.34.2442}, abstractNote={The quantum-mechanical system whose potential is the cusp catastrophe is studied. The qualitative features of this system, including the eigenvalue spectrum, density of states, wave-function localization and delocalization, probability distribution, operator expectation values, and matrix elements, are studied as functions of the cusp control parameter values. Canonical features preceding and accompanying the transition from the single-mode to the double-mode regime have been determined. The relationships between the qualitative properties of the quantum cusp and the canonical properties of the cusp catastrophes are exhibited.}, number={3}, journal={Physical Review A}, author={Gilmore, R. and Kais, S. and Levine, R.D.}, year={1986}, pages={2442–2452} }
 @article{cohen_kais_1986, title={Rayleigh-Schrodinger perturbation theory with a strong perturbation: Anharmonic oscillators}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-3042987649&partnerID=MN8TOARS}, DOI={10.1088/0305-4470/19/5/021}, abstractNote={The bound state solutions of Schrodinger's equation for the anharmonic oscillator potentials V=x2+ lambda x2k (k=2,3, . . . ) have been investigated, using elementary techniques of low-order variational perturbation theory. For the quartic oscillator (k=2) a scaled harmonic potential provides a remarkably accurate model for all lambda . Although this model is slightly less satisfactory for higher-order anharmonicities (k>or=3), the perturbation procedures remain effective, and can be applied successfully provided that higher-order terms are calculated.}, number={5}, journal={Journal of Physics A: Mathematical and General}, author={Cohen, M. and Kais, S.}, year={1986}, pages={683–690} }
 @article{kais_levine_1986, title={Square-well potential by an algebraic approach}, volume={34}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4244150459&partnerID=MN8TOARS}, DOI={10.1103/PhysRevA.34.4615}, abstractNote={The spectrum-generating algebra for the problem of a particle in a potential well is shown to be su(1,1). Both the infinitely deep and finite square wells are considered. The generators can also be derived via a systematic procedure for determining the time-dependent constants of the motion. The coherent states are explicitly constructed.}, number={6}, journal={Physical Review A}, author={Kais, S. and Levine, R.D.}, year={1986}, pages={4615–4620} }
 @article{kais_kim_1986, title={Unstable bound states of the Dirac equation by an algebraic approach}, volume={114}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-46149136767&partnerID=MN8TOARS}, number={4}, journal={Physics Letters A}, author={Kais, S. and Kim, S.K.}, year={1986}, pages={165–167} }
 @article{kais_1985, title={Unstable bound states by an algebraic method}, volume={112}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-46549090640&partnerID=MN8TOARS}, number={6-7}, journal={Physics Letters A}, author={Kais, S.}, year={1985}, pages={269–270} }
 @article{cohen_kais_1984, title={Rayleigh-Schrodinger perturbation theory with a strong perturbation: The quadratic Zeeman effect in hydrogen}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0040839300&partnerID=MN8TOARS}, DOI={10.1088/0022-3700/17/15/018}, abstractNote={The ground state energy of a hydrogen atom in a uniform magnetic field has been computed by means of low-order variational-perturbation theory. High accuracy is obtained for arbitrary strengths up to 1012 G by the simple expedient of including the leading effect of the field in the zero-order model.}, number={15}, journal={Journal of Physics B: Atomic and Molecular Physics}, author={Cohen, M. and Kais, S.}, year={1984}, pages={3049–3055} }
 @article{cohen_kais_1984, title={Scaling, renormalisation and accuracy of perturbation calculations}, volume={105}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48749136166&partnerID=MN8TOARS}, number={3}, journal={Chemical Physics Letters}, author={Cohen, M. and Kais, S.}, year={1984}, pages={295–298} }
 @article{cohen_feldmann_kais_1984, title={Stark effect of a rigid rotor}, volume={17}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36149043032&partnerID=MN8TOARS}, DOI={10.1088/0022-3700/17/17/023}, abstractNote={Energy levels of a rigid rotor in a uniform electric field of arbitrary strength have been computed using Rayleigh-Schrodinger perturbation theory. Rational fraction representations of the weak-field Taylor series, the strong-field asymptotic series, and of both series simultaneously are very accurate over limited ranges of the applied field strength. A scaled variational perturbation theory yields high accuracy over the entire range.}, number={17}, journal={Journal of Physics B: Atomic and Molecular Physics}, author={Cohen, M. and Feldmann, T. and Kais, S.}, year={1984}, pages={3535–3544} }