Disentangling the impact of quasiparticles and two-level systems on the statistics of superconducting-qubit lifetime

Temporal fluctuations in the superconducting qubit lifetime, ${T}_{1}$, present additional challenges in the pursuit of fault-tolerant quantum computing. Although the exact mechanisms remain unclear, ${T}_{1}$ fluctuations are generally attributed to strong coupling between the qubit and a few near-resonant two-level systems (TLSs), which can exchange energy with an ensemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report ${T}_{1}$ measurements of qubits with varying geometrical footprints and surface dielectrics as a function of temperature. By analyzing the noise spectrum of the qubit depolarization rate, ${\mathrm{\ensuremath{\Gamma}}}_{1}=1/{T}_{1}$, we disentangle the contributions of TLSs, nonequilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs to the variance in ${\mathrm{\ensuremath{\Gamma}}}_{1}$. We find that the ${\mathrm{\ensuremath{\Gamma}}}_{1}$ variance in qubits with smaller footprints is more susceptible to QP and TLS fluctuations than that in larger-footprint qubits. Furthermore, the QP-induced variances in all qubits align with the theoretical framework of QP diffusion and fluctuation. These findings offer valuable insights for future qubit design and engineering optimization.