To test the contribution of non-equilibrium quasiparticles to qubit relaxation, we have repeatedly measured the relaxation time T_1 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 T_1 varying between about 100 and 300 {\mu}s 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 T_1 values, with maximum T_1 values over 200 {\mu}s. 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.
Cite
@article{arxiv.2301.04555,
title = {Long-lived transmons with different electrode layouts},
author = {Kungang Li and S. K. Dutta and Zachary Steffen and Dylan Poppert and Shahriar Keshvari and Jeffery Bowser and B. S. Palmer and C. J. Lobb and F. C. Wellstood},
journal= {arXiv preprint arXiv:2301.04555},
year = {2023}
}