Long-lived transmons with different electrode layouts

TL;DR

This study investigates how different electrode layouts in Al/AlOx/Al transmons affect their relaxation times, revealing long, fluctuating T_1 times influenced by quasiparticles and electrode material properties.

Contribution

It demonstrates that electrode material composition and gap differences impact transmon relaxation times, providing insights into quasiparticle effects on qubit coherence.

Findings

T_1 times ranged from 100 to 300 μs at 20 mK.

Electrode material composition influences T_1 fluctuations.

T_1 drops rapidly above 150 mK due to quasiparticles.

Abstract

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.