Temperature Dependence of Relaxation-Time Fluctuations in Transmon Qubits

As the lifetime of superconducting qubits has increased, large temporal fuctuations in the relaxation time T₁ of the excited state have been observed by different groups. We have measured such fluctuations in several Al/AlOx/Al transmons mounted in 3-D aluminum cavities. Our longest lived device showed T₁ that varied from 100 to 300 μs at 20 mK. Comparing with shorter lived transmons, we find that the size of the fluctuations in T₁ appears to scale with T₁. Measurements of T₁ versus temperature from 20 to 300 mK reveal that the fluctuations depend on temperature and support the idea that non-equilibrium quasiparticles are the dominant source of dissipation in these devices.