Disentangling the Impact of Quasiparticles and Two-Level Systems on the Statistics of Superconducting Qubit Lifetime

TL;DR

This paper investigates how quasiparticles and two-level systems influence superconducting qubit lifetime fluctuations, providing insights for improving qubit stability through analysis of noise spectra and qubit geometry effects.

Contribution

It disentangles the effects of TLSs and QPs on qubit lifetime fluctuations using noise spectrum analysis across different geometries and temperatures, advancing understanding of decoherence mechanisms.

Findings

Qubit lifetime fluctuations are more affected by QPs and TLSs in small-footprint qubits.

QP-induced variances align with QP diffusion and fluctuation theories.

Surface dielectric and geometry influence susceptibility to decoherence sources.

Abstract

Temporal fluctuations in the superconducting qubit lifetime, $T_1$, bring up additional challenges in building a fault-tolerant quantum computer. While the exact mechanisms remain unclear, $T_1$ fluctuations are generally attributed to the strong coupling between the qubit and a few near-resonant two-level systems (TLSs) that can exchange energy with an assemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report $T_1$ measurements on the qubits with different geometrical footprints and surface dielectrics as a function of the temperature. By analyzing the noise spectrum of the qubit depolarization rate, $\Gamma_1 = 1/T_1$, we can disentangle the impact of TLSs, non-equilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs on the variance in $\Gamma_1$. We find that $\Gamma_1$ variances in the qubit with a small footprint are more susceptible to the QP and TLS fluctuations than those in the large-footprint qubits. Furthermore, the QP-induced variances in all qubits are consistent with the theoretical framework of QP diffusion and fluctuation. We suggest these findings can offer valuable insights for future qubit design and engineering optimization.