de Graaf S E, Faoro L, Ioffe L B, Mahashabde S, Burnett J J, Lindström T, Kubatkin S E, Danilov A V, Tzalenchuk A Ya
National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK.
Sorbonne Université, Laboratoire de Physique Théorique et Hautes Énergies, UMR 7589 CNRS, Tour 13, 5eme Etage, 4 Place Jussieu, F-75252 Paris 05, France.
Sci Adv. 2020 Dec 18;6(51). doi: 10.1126/sciadv.abc5055. Print 2020 Dec.
A major issue for the implementation of large-scale superconducting quantum circuits is the interaction with interfacial two-level system (TLS) defects that lead to qubit parameter fluctuations and relaxation. Another major challenge comes from nonequilibrium quasiparticles (QPs) that result in qubit relaxation and dephasing. Here, we reveal a previously unexplored decoherence mechanism in the form of a new type of TLS originating from trapped QPs, which can induce qubit relaxation. Using spectral, temporal, thermal, and magnetic field mapping of TLS-induced fluctuations in frequency tunable resonators, we identify a highly coherent subset of the general TLS population with a low reconfiguration temperature ∼300 mK and a nonuniform density of states. These properties can be understood if the TLS are formed by QPs trapped in shallow subgap states formed by spatial fluctutations of the superconducting order parameter. This implies that even very rare QP bursts will affect coherence over exponentially long time scales.
大规模超导量子电路实施中的一个主要问题是与界面两能级系统（TLS）缺陷的相互作用，这会导致量子比特参数波动和弛豫。另一个主要挑战来自非平衡准粒子（QP），它们会导致量子比特弛豫和退相。在这里，我们揭示了一种以前未被探索的退相干机制，其形式为源自捕获QP的新型TLS，它可以诱导量子比特弛豫。通过对频率可调谐谐振器中TLS诱导波动的光谱、时间、热和磁场映射，我们识别出一般TLS群体中的一个高度相干子集，其具有低至约300 mK的重新配置温度和非均匀的态密度。如果TLS是由捕获在由超导序参量的空间波动形成的浅子带隙态中的QP形成的，那么这些特性就可以得到理解。这意味着即使非常罕见的QP爆发也会在指数长的时间尺度上影响相干性。
