@inproceedings{li_liu_li_zhou_2022, place={New Jersey}, title={Exploiting Quantum Assertions for Error Mitigation and Quantum Program Debugging}, ISSN={["1063-6404"]}, DOI={10.1109/ICCD56317.2022.00028}, abstractNote={An assertion is a predicate that should be evaluated true during program execution. In this paper, we present the development of quantum assertion schemes and show how they are used for hardware error mitigation and software debugging. Compared to assertions in classical programs, quantum assertions are challenging due to the no-cloning theorem and potentially destructive measurement. We discuss how these challenges can be circumvented such that certain properties of quantum states can be verified non-destructively during program execution. Furthermore, we show that besides detecting program bugs, dynamic assertion circuits can mitigate noise effects via post-selection of the assertion results. Our case studies demonstrate the use of quantum assertions in various quantum algorithms.}, booktitle={2022 IEEE 40TH INTERNATIONAL CONFERENCE ON COMPUTER DESIGN (ICCD 2022)}, publisher={IEEE}, author={Li, Peiyi and Liu, Ji and Li, Yangjia and Zhou, Huiyang}, year={2022}, pages={124–131} }
 @article{liu_li_zhou_2022, title={Not All SWAPs Have the Same Cost: A Case for Optimization-Aware Qubit Routing}, ISSN={["1530-0897"]}, DOI={10.1109/HPCA53966.2022.00058}, abstractNote={Despite rapid advances in quantum computing technologies, the qubit connectivity limitation remains to be a critical challenge. Both near-term NISQ quantum computers and relatively long-term scalable quantum architectures do not offer full connectivity. As a result, quantum circuits may not be directly executed on quantum hardware, and a quantum compiler needs to perform qubit routing to make the circuit compatible with the device layout. During the qubit routing step, the compiler inserts SWAP gates and performs circuit transformations. Given the connectivity topology of the target hardware, there are typically multiple qubit routing candidates. The state-of-the-art compilers use a cost function to evaluate the number of SWAP gates for different routes and then select the one with the minimum number of SWAP gates. After qubit routing, the quantum compiler performs gate optimizations upon the circuit with the newly inserted SWAP gates.In this paper, we observe that the aforementioned qubit routing is not optimal, and qubit routing should not be independent on subsequent gate optimizations. We find that with the consideration of gate optimizations, not all of the SWAP gates have the same basis-gate cost. These insights lead to the development of our qubit routing algorithm, NASSC (Not All Swaps have the Same Cost). NASSC is the first algorithm that considers the subsequent optimizations during the routing step. Our optimization-aware qubit routing leads to better routing decisions and benefits subsequent optimizations. We also propose a new optimization-aware decomposition for the inserted SWAP gates. Our experiments show that the routing overhead compiled with our routing algorithm is reduced by up to 69.30% (21.30% on average) in the number of CNOT gates and up to 43.50% (7.61% on average) in the circuit depth compared with the state-of-the-art scheme, SABRE.}, journal={2022 IEEE INTERNATIONAL SYMPOSIUM ON HIGH-PERFORMANCE COMPUTER ARCHITECTURE (HPCA 2022)}, author={Liu, Ji and Li, Peiyi and Zhou, Huiyang}, year={2022}, pages={709–725} }
 @article{liu_bello_zhou_2021, title={Relaxed Peephole Optimization: A Novel Compiler Optimization for Quantum Circuits}, ISSN={["2164-2397"]}, DOI={10.1109/CGO51591.2021.9370310}, abstractNote={As in classical computing, compilers play an important role in quantum computing. Quantum processors typically support a limited set of primitive operations or quantum gates and have certain hardware-related limitations. A quantum compiler is responsible for adapting a quantum program to these constraint environments and decomposing quantum gates into a sequence of the primitive ones. During the compilation process, it is also critical for the compiler to optimize the quantum circuits in order to reduce the noise in the computation results. Since the noise is introduced by operations and decoherence, reducing the gate count is the key for improving performance. In this paper, we propose a novel quantum compiler optimization, named relaxed peephole optimization (RPO) for quantum computers. RPO leverages the single-qubit state information that can be determined statically by the compiler. We define that a qubit is in a basis state when, at a given point in time, its state is either in the X-, Y-, or Z-basis (|+) / |-〉, |L〉 / R〉 and 10〉 / |1〉). When basis qubits are used as inputs to quantum gates, there exist opportunities for strength reduction, which replaces quantum operations with equivalent but less expensive ones. Compared to the existing peephole optimization for quantum programs, the difference is that our proposed optimization does not require an identical unitary matrix, thereby named ‘relaxed’ peephole optimization. We also extend our approach to optimize the quantum gates when some input qubits are in known pure states. Both optimizations, namely the Quantum Basis-state Optimization (QBO) and the Quantum Pure-state Optimization (QPO), are implemented in the IBM's Qiskit transpiler. Our experimental results show that our proposed optimization pass is fast and effective. The circuits optimized with our compiler optimizations obtain up to 18.0% (11.7% on average) fewer CNOT gates and up to 8.2% (7.1% on average) lower transpilation time than that of the most aggressive optimization level in the Qiskit compiler. When running on real quantum computers, the success rates of 3-qubit quantum phase estimation algorithm improve by 2.30X due to the reduced gate counts.}, journal={CGO '21: PROCEEDINGS OF THE 2021 IEEE/ACM INTERNATIONAL SYMPOSIUM ON CODE GENERATION AND OPTIMIZATION (CGO)}, author={Liu, Ji and Bello, Luciano and Zhou, Huiyang}, year={2021}, pages={301–314} }
 @article{liu_zhou_2021, title={Systematic Approaches for Precise and Approximate Quantum State Runtime Assertion}, ISSN={["1530-0897"]}, DOI={10.1109/HPCA51647.2021.00025}, abstractNote={With the rapid growth of quantum computing technology, programmers need new tools for debugging quantum programs. Recent works show that assertions are a promising way for debugging quantum programs. However, there are two main drawbacks with the existing schemes. First, the existing schemes, including both statistical and dynamic assertions are only capable of asserting limited types of states, namely classical, superposition, and specific entanglement states. Second, the use cases of these assertions are limited, since the programmer has to know the exact/precise state to assert.In this work, we propose two systematic approaches for dynamic quantum state assertion and they can assert a much broader range of quantum states including both pure states and mixed states. We also introduce the idea of approximate quantum state assertion for the cases where the programmers only have limited knowledge of the quantum states. Approximate assertion is capable of checking membership in a set of states $\{|\psi\rangle,\ |\phi\rangle,\ldots\}$. While precise quantum state assertion can check a specific quantum state, approximate assertion enables a way to check whether the qubits of interest are in a super-set of some expected states, which is analogous to the well-known Bloom filter for membership checking in classical computing. Our experiments demonstrate that our systematic approaches can assert many more quantum states and can be used in various assertion locations for qubit state checking.}, journal={2021 27TH IEEE INTERNATIONAL SYMPOSIUM ON HIGH-PERFORMANCE COMPUTER ARCHITECTURE (HPCA 2021)}, author={Liu, Ji and Zhou, Huiyang}, year={2021}, pages={179–193} }
 @article{liu_zhou_2020, title={Reliability Modeling of NISQ-Era Quantum Computers}, DOI={10.1109/IISWC50251.2020.00018}, abstractNote={Recent developments in quantum computers have been pushing up the number of qubits. However, the state-of-the-art Noisy Intermediate Scale Quantum (NISQ) computers still do not have enough qubits to accommodate the error correction circuit. Noise in quantum gates limits the reliability of quantum circuits. To characterize the noise effects, prior methods such as process tomography, gateset tomography and randomized benchmarking have been proposed. However, the challenge is that these methods do not scale well with the number of qubits. Noise models based on the understanding of underneath physics have also been proposed to study different kinds of noise in quantum computers. The difficulty is that there is no widely accepted noise model that incorporates all different kinds of errors. The realworld errors can be very complicated and it remains an active area of research to produce accurate noise models. In this paper, instead of using noise models to estimate the reliability, which is measured with success rates or inference strength, we treat the NISQ quantum computer as a black box. We use several quantum circuit characteristics such as the number of qubits, circuit depth, the number of CNOT gates, and the connection topology of the quantum computer as inputs to the black box and derive a reliability estimation model using (1) polynomial fitting and (2) a shallow neural network. We propose randomized benchmarks with random numbers of qubits and basic gates to generate a large data set for neural network training. We show that the estimated reliability from our black-box model outperforms the noise models from Qiskit. We also showcase that our black-box model can be used to guide quantum circuit optimization at compile time.}, journal={2020 IEEE INTERNATIONAL SYMPOSIUM ON WORKLOAD CHARACTERIZATION (IISWC 2020)}, author={Liu, Ji and Zhou, Huiyang}, year={2020}, pages={94–105} }