Collaborative Research: Proximal Digital Control and Stabilization of Superconducting Qubits

虽然最近在构建由几个量子比特（或“量子位”）组成的小规模量子电路方面取得了巨大进展，但一台超越现有经典机器性能的容错量子计算机将需要数千或数百万个量子比特的网络，远远超出当前的能力。控制和测量下一代量子机器的可靠方法尚未开发。该项目包括一个实验项目，旨在开发数字控制、测量和反馈技术，以稳定超导电路元件形成的量子比特的量子状态。该计划将把量子控制和反馈从模拟领域转移到数字领域；在经典状态下与数字控制相关的所有鲁棒性应该延续到量子状态。该项目将涉及本科生和研究生研究人员的广泛参与，并在这两个层面提供丰富的教育机会。当地学校的科学推广将在麦迪逊和锡拉丘兹的当地社区产生更广泛的影响。本项目包括一个实验程序，将最优控制理论应用于超导量子比特的操作，并实现基于数字量子比特读出的低延迟反馈，以稳定计算基态。该小组将展示超导量子比特的高保真相干控制，使用源自最优控制理论的单通量量子（SFQ）电压脉冲的复杂序列。这些脉冲中的每一个都为量子位提供了一个类似delta函数的踢，在布洛赫球上形成了一个复杂的轨迹，可以将泄漏误差降到最低。同时，该团队将采用最近开发的量子比特测量方法，该方法基于准备适当的微波腔指针状态和微波光子计数，结合基于sfq的控制，实现低延迟反馈。这将为监测和纠正计算子空间外的泄漏提供一条途径，这对提出的容错量子计算方法尤其有害。该程序将把量子控制和反馈从模拟领域转移到数字领域，并且在经典状态下与数字控制相关的所有鲁棒性将延续到量子状态。

While there has been tremendous recent progress in constructing small-scale quantum circuits comprising several quantum bits (or "qubits"), a fault-tolerant quantum computer that exceeds the performance of existing classical machines will require a network of thousands or millions of qubits, far beyond current capabilities. Robust approaches to the control and measurement of next-generation quantum machines have yet to be developed. This project consists of an experimental program to develop digital control, measurement, and feedback techniques for stabilizing the quantum state of a qubit formed from superconducting circuit elements. The program will move quantum control and feedback from the analog realm to the digital realm; all of the robustness associated with digital control in the classical regime should carry over to the quantum regime. The project will involve the extensive participation of undergraduate and graduate researchers, and is rich in educational opportunities at both of these levels. Science outreach in local schools will extend a broader impact throughout the local communities of Madison and Syracuse. This project consists of an experimental program to apply optimal control theory to the manipulation of superconducting qubits and to implement low-latency feedback based on digital qubit readout for stabilizing computational basis states. The group will demonstrate high-fidelity coherent control of a superconducting qubit using complex trains of Single Flux Quantum (SFQ) voltage pulses derived from optimal control theory. Each of these pulses provides a delta function-like kick to the qubit, inducing a complex trajectory on the Bloch sphere that is tailored to minimize leakage errors. In parallel, the team will employ a recently developed approach to qubit measurement based on the preparation of appropriate microwave cavity pointer states and microwave photon counting, combined with SFQ-based control, to implement low-latency feedback. This will provide a path to monitor and correct leakage out of the computational subspace, which is particularly damaging to proposed approaches to fault-tolerant quantum computing. The program will move quantum control and feedback from the analog realm to the digital realm, and all of the robustness associated with digital control in the classical regime will carry over to the quantum regime.