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, ${\mathrm{\Gamma }}_1=1/T_1$, we disentangle the contributions of TLSs, nonequilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs to the variance in ${\mathrm{\Gamma }}_1$. We find that the ${\mathrm{\Gamma }}_1$ 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.

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• Josephson junctions
• Open quantum systems & decoherence
• Optoelectronics
• Quantum information with solid state qubits
• Quasiparticles & collective excitations
• Superconducting qubits
• Microwave techniques
• Two-level models