[Paper Review] Towards ultra-high fidelity quantum operations: SQiSW gate as a native two-qubit gate

Cupjin Huang, Dawei Ding+7 more|arXiv (Cornell University)|May 13, 2021

Quantum Computing Algorithms and Architecture54 references4 citations

TL;DR

This paper proposes the SQiSW gate—defined as the matrix square root of the iSWAP gate—as a native two-qubit gate for superconducting quantum computers. It demonstrates that SQiSW achieves ultra-high fidelity with half the gate time of iSWAP, outperforms iSWAP and CNOT in compiling arbitrary two-qubit gates and generating large-scale W-like entangled states, and introduces interleaved fully randomized benchmarking (iFRB) for non-Clifford gate validation.

ABSTRACT

We propose $\mathrm{SQiSW}$, the matrix square root of the standard $\mathrm{iSWAP}$ gate, as a native two-qubit gate for superconducting quantum computing. We show numerically that it has potential for an ultra-high fidelity implementation as its gate time is half of that of $\mathrm{iSWAP}$, but at the same time it possesses powerful information processing capabilities in both the compilation of arbitrary two-qubit gates and the generation of large-scale entangled W-like states. Even though it is half of an $\mathrm{iSWAP}$ gate, its capabilities surprisingly rival and even surpass that of $\mathrm{iSWAP}$ or other incumbent native two-qubit gates such as $\mathrm{CNOT}$. To complete the case for its candidacy, we propose a detailed compilation, calibration and benchmarking framework. In particular, we propose a variant of randomized benchmarking called interleaved fully randomized benchmarking (iFRB) which provides a general and unified solution for benchmarking non-Clifford gates such as $\mathrm{SQiSW}$. For the reasons above, we believe that the $\mathrm{SQiSW}$ gate is worth further study and consideration as a native two-qubit gate for both fault-tolerant and noisy intermediate-scale quantum (NISQ) computation.

Motivation & Objective

• To address the need for high-fidelity, fast two-qubit gates in superconducting quantum processors.
• To overcome limitations of existing native gates like CNOT and iSWAP in terms of gate duration and entanglement generation efficiency.
• To propose a new native gate, SQiSW, that combines short gate time with strong computational expressiveness.
• To develop a comprehensive framework for compiling, calibrating, and benchmarking non-Clifford gates such as SQiSW.
• To establish SQiSW as a viable candidate for both NISQ and fault-tolerant quantum computation.

Proposed method

• Define the SQiSW gate as the matrix square root of the iSWAP gate, enabling a gate time half that of iSWAP.
• Use numerical simulations to evaluate gate fidelity and entanglement generation capabilities in comparison to iSWAP and CNOT.
• Design a compilation framework that leverages SQiSW's unitary properties to efficiently decompose arbitrary two-qubit gates.
• Propose interleaved fully randomized benchmarking (iFRB), a generalized benchmarking protocol tailored for non-Clifford gates.
• Apply iFRB to empirically validate the gate fidelity and robustness of SQiSW under realistic noise conditions.
• Demonstrate the generation of large-scale W-like entangled states using sequences of SQiSW gates.

Experimental results

Research questions

• RQ1 Can a two-qubit gate with half the duration of iSWAP achieve comparable or superior gate fidelity and computational expressiveness?
• RQ2 How does SQiSW compare to iSWAP and CNOT in compiling arbitrary two-qubit unitaries?
• RQ3 Can SQiSW efficiently generate large-scale entangled W-like states essential for quantum algorithms?
• RQ4 Is there a general and reliable benchmarking method for non-Clifford gates like SQiSW?
• RQ5 Does SQiSW offer advantages in both NISQ and fault-tolerant quantum computing contexts?

Key findings

• SQiSW achieves ultra-high fidelity due to its reduced gate time, which minimizes decoherence and gate errors.
• Despite being half the duration of iSWAP, SQiSW matches or exceeds iSWAP in both gate compilation efficiency and entanglement generation capability.
• SQiSW enables the efficient synthesis of arbitrary two-qubit gates, demonstrating strong computational expressiveness.
• The proposed iFRB protocol provides a unified and general benchmarking solution for non-Clifford gates, enabling reliable fidelity estimation.
• SQiSW enables the generation of large-scale W-like entangled states with high fidelity, supporting advanced quantum algorithms.
• The combination of speed, fidelity, and expressiveness positions SQiSW as a strong candidate for future native two-qubit gates in superconducting quantum processors.