Transmon performance with different electrode energy gaps*

We have repeatedly measured the behavior of gap-engineered Al/AlOx/Al transmons in a 3-D cavity and observed significant fluctuations. In our devices, one electrode was formed from nominally pure aluminum while the other electrode was formed from oxygen-doped aluminum. The superconducting energy gap of the aluminum films depends on the grain size, which depends on the oxygen doping as well as the layer thickness. In a device with a thin first layer of pure Al and thick doped second Al layer, T₁ varied from about 100 to 300 μs at 20 mK. A device with a thin doped-Al first layer and thick pure Al second layer showed T₁ fluctuations of a similar size. This device also showed large fluctuations in T₂ , with a maximum value over 100 μs. Observation of the transition spectrum in this low E_j/E_c device showed significant  charge dispersion, with quasiparticle switching and periods of slow drifting (hour) in the offset charge.