Large fluctuations of T₁ in long-lived transmon qubits

Recently the relaxation time T₁ of transmons has increased, as has the size of the fluctuations in T₁. To investigate the source of these fluctuations, we measured the T₁ of transmons made with an electrode layer of pure aluminum and a counter-electrode layer made with either pure Al or oxygen-doped granular Al. The superconducting energy gap of the counter-electrode depends on the grain size, which depends on the oxygen doping as well as the layer thickness. At 20 mK, an oxygen-doped device showed T₁ variations between about 80 and 300 μs, while an un-doped device on the same chip showed uncorrelated T₁ variations between 50 and 100 ms. Measurements of the fluctuations versus temperature reveal that the standard deviation of T₁ is proportional to T₁, even above 150 mK, where the transmon relaxation is dominated by thermally-generated quasiparticles. We discuss why this behavior is not consistent with the two most commonly proposed mechanisms, fluctuations in two-level-system dielectric loss and fluctuations in the density of non-equilibrium quasiparticles, and propose an alternative mechanism that is consistent with the observed behaviors.