Long-lived transmons with different electrode layouts

Abstract

To test the contribution of non-equilibrium quasiparticles to qubit relaxation, we have repeatedly measured the relaxation time T1 in Al/AlOx/Al transmons with electrodes that have different superconducting gaps. In one device, the first layer electrode was formed by thermal evaporation of nominally pure Al, while the counter-electrode was formed by deposition of oxygen-doped Al, which gave a larger gap value. The relaxation time was long, but showed large fluctuations, with T1 varying between about 100 and 300 μs at 20 mK. In other transmons, we formed the first layer electrode by deposition of oxygen-doped Al, while the counter-electrode was formed by deposition of nominally pure Al. These devices showed a similar range of large and fluctuating T1 values, with maximum T1 values over 200 μs. The relaxation time of the devices did not depend strongly on temperature below about 150 mK, but dropped rapidly above this due to thermally generated quasiparticles.Graphical abstract