两比特门中残余条件相位的分支分辨泡利块光谱学
Branch-resolved Pauli-block spectroscopy of residual conditional phase in two-qubit gates
浏览论文内容
中文总结 AI 辅助
研究两比特门高精度控制中残余条件相位难以分辨的问题，引入分支分辨泡利块光谱学，通过重复探测、测量泡利块及形成分支相干性来估计残余ZZ旋转角度及其符号，实现低开销的符号估计，优于标准保真度基准。
中文摘要 AI 辅助
量子物理和量子技术的进展推动量子计算从噪声中等规模（NISQ）时代迈向容错操作。两比特门的高精度控制是这一转变的关键要求，依赖于精确的两比特校准。对于受控相位和CZ型操作，残余条件相位（局部补偿后的非局部ZZ型偏差）在平均保真度和随机基准测试中难以在主导阶分辨，重复拉姆齐放大也无法可靠地将其与普通目标失谐、SPAM误差和长序列中的对比度损失区分开。我们引入分支分辨泡利块光谱学来估计每个周期的残余ZZ旋转角度θ_c及其符号，受控相位残余可通过固定约定得出。该协议对N个周期重复固定探测，测量封闭泡利块IX、IY、ZX和ZY，并根据控制量子比特形成分支相干性C+和C-；θ_c以相反方向分裂两个分支相位斜率，而局部目标相位β_c使它们一起移动。回声周期变体抑制可移除的局部项，同时保留非局部贡献。数值模拟证实了在标量扇区替代方法失败时的无偏符号读出，并在相等保真度下区分相反符号的误差。在一个超导云量子比特-耦合器对上，脉冲级校准闭环显示出近线性注入、保留的分支对比度以及通过一次迭代跟踪原生残余条件相位。该方法产生了低开销的、每个周期的残余条件相位符号估计，而标准保真度基准在主导阶无法分辨。
英文摘要
Recent progress in quantum physics and quantum technologies is driving quantum computing from the noisy intermediate-scale (NISQ) era toward fault-tolerant operation. High-precision control of two-qubit gates is among the most critical requirements in this transition and hinges on accurate two-qubit calibration. For controlled-phase and CZ-style operations, the residual conditional phase (the nonlocal ZZ-type deviation after local compensation) is weakly resolved at leading order in average infidelity and randomized benchmarking, and repeated Ramsey amplification does not reliably isolate it from ordinary target detuning, SPAM errors, and contrast loss in long sequences. We introduce branch-resolved Pauli-block spectroscopy to estimate the per-cycle residual ZZ-rotation angle theta_c with its sign, from which the controlled-phase residual follows by a fixed convention. The protocol repeats a fixed probe for N cycles, measures the closed Pauli block IX, IY, ZX, and ZY, and forms branch coherences C+ and C- conditioned on the control qubit; theta_c splits the two branch phase slopes in opposite directions, while local target phase beta_c shifts them together. An echoed-cycle variant suppresses removable local terms while preserving the nonlocal contribution. Numerical simulations with injected theta_c, detuning, damping, and SPAM confirm unbiased signed readout where scalar-sector alternatives fail and distinguish opposite-sign errors at equal infidelity. On one superconducting cloud qubit-coupler pair, a pulse-level calibration closed loop shows near-linear injection, preserved branch contrast, and tracking of the native residual conditional phase through one iteration. The approach yields a low-overhead, signed per-cycle estimate of residual conditional phase that standard fidelity benchmarks underresolve at leading order.
