超导量子比特中无磁通可调性时的磁通噪声

中文摘要 AI 辅助

磁通噪声被公认为是磁通可调超导量子比特的主要退相机制，但我们的微观理解仍不完整，法拉第电磁感应定律等基础效应直到最近才受到关注。基于法拉第效应的量子几何描述，我们结合器件几何结构，对一般磁源与薄膜超导结构的耦合进行了深入推导。我们将所得框架应用于随时间变化的磁偶极子（描述表面或衬底自旋）以及载流通量线。研究表明，磁通噪声不仅会影响退相，还会对量子比特的品质因子构成基本限制——值得注意的是，即使量子比特不含环路、名义上不具备磁通可调性，这一限制依然存在。假设表面自旋是普遍磁通噪声的来源，我们预计该品质因子限制可能在近期达到。对于载流通量线，我们确定了维持量子比特性能所需的最小安全距离，这可能会限制量子硬件的规模化；由于transmon（传输线谐振子量子比特）典型的大电容极片会增强电动势场的透镜效应，且该效应在很大程度上与迈斯纳屏蔽无关，因此该安全距离会随之增大。

英文摘要

Flux noise is unanimously recognised as a leading dephasing mechanism for flux-tunable superconducting qubits. However, our microscopic understanding remains incomplete, and basic effects like Faraday's law of induction have only very recently come into focus. Based on a quantum geometric description of the Faraday effect, we provide an in-depth derivation of the coupling of generic magnetic sources to thin film superconducting structures, under appropriate consideration of the device geometry. We apply the resulting framework to time-varying magnetic dipoles, describing surface or substrate spins, as well as current-carrying flux lines. We show that flux noise not only affects dephasing, but also provides a fundamental limit for the qubit quality factor - notably, even when the qubit contains no loops and is thus nominally not flux-tunable. Assuming surface spins as the origin for universal flux noise, we expect that this quality factor limit might be reached in the near term. For flux lines, we formulate a minimal safety distance to conserve the qubit performance, potentially constraining the scale-up of quantum hardware. This distance is boosted in the presence of large capacitor wings typical for transmons, due to a lensing of the electromotive field which is largely independent of Meissner screening.

发表机构

• Peter Grünberg Institute (PGI-2), Forschungszentrum Jülich（于利希研究中心彼得·格林贝格研究所（PGI-2））

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