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1. Cross-Platform Comparison of Arbitrary Quantum Computations

   Daiwei Zhu, Ze-Pei Cian (January 2021, ArXivorg)

   As we approach the era of quantum advantage, when quantum computers (QCs) can outperform any classical computer on particular tasks, there remains the difficult challenge of how to validate their performance. While algorithmic success can be easily verified in some instances such as number factoring or oracular algorithms, these approaches only provide pass/fail information for a single QC. On the other hand, a comparison between different QCs on the same arbitrary circuit provides a lower-bound for generic validation: a quantum computation is only as valid as the agreement between the results produced on different QCs. Such an approach is also at the heart of evaluating metrological standards such as disparate atomic clocks. In this paper, we report a cross-platform QC comparison using randomized and correlated measurements that results in a wealth of information on the QC systems. We execute several quantum circuits on widely different physical QC platforms and analyze the cross-platform fidelities. 

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2. Cross-platform comparison of arbitrary quantum states

   https://doi.org/10.1038/s41467-022-34279-5  

   Zhu, D.; Cian, Z. P.; Noel, C.; Risinger, A.; Biswas, D.; Egan, L.; Zhu, Y.; Green, A. M.; Alderete, C. Huerta; Nguyen, N. H.; et al (December 2022, Nature Communications)

   Abstract As we approach the era of quantum advantage, when quantum computers (QCs) can outperform any classical computer on particular tasks, there remains the difficult challenge of how to validate their performance. While algorithmic success can be easily verified in some instances such as number factoring or oracular algorithms, these approaches only provide pass/fail information of executing specific tasks for a single QC. On the other hand, a comparison between different QCs preparing nominally the same arbitrary circuit provides an insight for generic validation: a quantum computation is only as valid as the agreement between the results produced on different QCs. Such an approach is also at the heart of evaluating metrological standards such as disparate atomic clocks. In this paper, we report a cross-platform QC comparison using randomized and correlated measurements that results in a wealth of information on the QC systems. We execute several quantum circuits on widely different physical QC platforms and analyze the cross-platform state fidelities. 

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3. Quantum Image Processing: A Comparative Study of NEQR and FRQI Encoding Schemes with Hybrid Processing

   https://doi.org/10.1145/3716368.3735286  

   Masum, Abu; Moghadam, Mehran; Kouhalvandi, Lida; Najafi, M Hassan; Aygun, Sercan (June 2025, ACM)

   Quantum image processing (QIP) is an emerging field that integrates image processing with the principles of quantum computing (QC). As quantum technologies advance, researchers face new opportunities and challenges in developing efficient QIP techniques. This paper provides an overview of quantum image representations, with a focus on two prominent encoding schemes: Novel Enhanced Quantum Representation (NEQR) and Fourier-based Quantum Image Representation (FRQI). We compare their performance in noisy quantum environments by evaluating qubit requirements, image quality, and computational efficiency. The study further analyzes the impact of quantum gate errors and qubit limitations on image reconstruction fidelity. We also compare GPU and QPU performance to highlight their strengths and weaknesses. Our findings stress the importance of error mitigation, advancements in quantum hardware, and the advancements of quantum-classical hybrid systems to drive future progress in QIP. 

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4. Not All SWAPs Have the Same Cost: A Case for Optimization-Aware Qubit Routing

   https://doi.org/10.1109/HPCA53966.2022.00058  

   Liu, Ji; Li, Peiyi; Zhou, Huiyang (April 2022, 2022 IEEE International Symposium on High-Performance Computer Architecture (HPCA))

   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. 

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5. A universal quantum gate set for transmon qubits with strong ZZ interactions

   Junling Long, Tongyu Zhao (March 2021, ArXivorg)

   null (Ed.)

   High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time tg=53.8 ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography. 

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