Two-level systems in superconducting quantum devices due to trapped quasiparticles

Abstract

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.

Microscopic relaxation channels in materials for superconducting qubits

“…Specifically, the plateauing of internal quality factor at low power suggests that the resonators are dominated by TLS loss at the single-photon level. Also, our observation of temporal fluctuations in both resonator quality factor and qubit T 1 is consistent with the TLS mechanism 10,11,13,14,16 . The scaling of T 1 fluctuations with relaxation time and the correlation to Nb film characteristics connects those fluctuations to TLS centers associated with the Nb pads.…”

“…8). Additionally, we observe shifts in Q i over time, especially in the HiPIMS resonators, which we attribute to individual near-resonant TLS 16 . These findings are consistent with TLS being a relevant loss channel.…”

“…However, performance improvements by these means have started to plateau, indicating that the dominant sources of decoherence are not well understood 1 . This has led to a recent surge of interest in understanding the limiting loss mechanisms in qubit materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] .…”

Engineering high-coherence superconducting qubits

“…Although the standard tunneling model of TLS defects can account for a wide range of decoherence phenomena observed in superconducting circuits, precisely identifying the detailed microscopic mechanism underlying such defects has proven tremendously challenging. There are a number of candidates for TLS defects: hydrogenated Al vacancies, interstitial hydrogen defects, dangling OH bonds on the surface that act as rotors, electrons dressed by phononic interactions, electrons trapped in metal-insulator gap states, quasiparticles hopping between disorder-mediated Andreev states 95 , and several others 28 . Note that not all of these candidates are simply linked to lossy dielectrics.…”

Ultrahigh-inductance materials from spinodal decomposition

“…However, despite the continuing optimization in the material synthesis, it remains a fundamental challenge to control the homogeneity of ultrathin films where a slight variation in thickness will inevitably result in substantial changes in the material properties. It is also known that ultrathin and disordered superconducting films are accompanied by large spacial fluctuations on the superconducting order parameter [28,[30][31][32], which could trap quasiparticles in the superconductors and act as detrimental two-level systems [33]. Moreover, the high defect density at the surfaces and edges of ultrathin nanowires typically plays a more significant role in deteriorating the microwave performance due to the larger filing factors of electric-field energy at these two regions [17,23,25,34,35].…”