一句话：用软件方法刻画硬件串扰并调度量子指令，在真实 NISQ 设备上把应用电路错误率最高降低 5.6 倍。
串扰是 NISQ 系统的主要噪声源之一：当多条量子指令并行执行时，指令间的耦合会破坏量子态，导致程序执行出错。本文的目标是用软件技术降低串扰对应用的实际影响，其思路包含两部分：一是对硬件串扰进行准确刻画，二是通过智能指令调度把受影响的操作串行化。由于全对（all-pairs）串扰刻画在计算上代价高昂，作者提出若干优化以削减刻画开销。在 3 台 20-qubit 的 IBMQ 系统上，相比全对测量，刻画时间（量子设备上的计算时间）减少两个数量级。基于这些刻画结果，作者设计了一个调度器，审慎地将高串扰指令串行化，从而在缓解串扰与串行化带来的指数级退相干误差之间取得平衡。在 3 台 IBMQ 系统上的真实运行表明，该调度器相对 IBM 的指令调度器可将应用电路错误率最多改善 5.6 倍，并在实践中达到近乎最优的串扰缓解效果。
Crosstalk is a major source of noise in Noisy Intermediate-Scale Quantum (NISQ) systems and is a fundamental challenge for hardware design. When multiple instructions are executed in parallel, crosstalk between the instructions can corrupt the quantum state and lead to incorrect program execution. Our goal is to mitigate the application impact of crosstalk noise through software techniques. This requires (i) accurate characterization of hardware crosstalk, and (ii) intelligent instruction scheduling to serialize the affected operations. Since crosstalk characterization is computationally expensive, we develop optimizations which reduce the characterization overhead. On 3 20-qubit IBMQ systems, we demonstrate two orders of magnitude reduction in characterization time (compute time on the QC device) compared to all-pairs crosstalk measurements. Informed by these characterization, we develop a scheduler that judiciously serializes high crosstalk instructions balancing the need to mitigate crosstalk and exponential decoherence errors from serialization. On real-system runs on 3 IBMQ systems, our scheduler improves the error rate of application circuits by up to 5.6x, compared to the IBM instruction scheduler and offers near-optimal crosstalk mitigation in practice.
@inproceedings{paperbot5449,
title = {Software Mitigation of Crosstalk on Noisy Intermediate-Scale Quantum Computers},
author = {Prakash Murali and David C. Mckay and Margaret Martonosi and Ali Javadi-Abhari},
booktitle = {ASPLOS},
year = {2020},
doi = {10.1145/3373376.3378477}
}