Quasiparticles in superconducting qubits with asymmetric junctions

TL;DR

This paper investigates how asymmetry in superconducting gaps across Josephson junctions affects quasiparticle behavior in superconducting qubits, revealing regimes that influence qubit relaxation and excited state populations.

Contribution

It explicitly models quasiparticle effects considering gap asymmetry, highlighting different regimes and their impact on qubit performance, which was not thoroughly analyzed before.

Findings

Quasiparticles can be either evenly distributed or confined to the lower-gap lead.

Different regimes affect qubit relaxation rates and excited state populations.

Trade-offs exist between qubit coherence and population based on quasiparticle distribution.

Abstract

Designing the spatial profile of the superconducting gap -- gap engineering -- has long been recognized as an effective way of controlling quasiparticles in superconducting devices. In aluminum films, their thickness modulates the gap; therefore, standard fabrication of Al/AlOx/Al Josephson junctions, which relies on overlapping a thicker film on top of a thinner one, always results in gap-engineered devices. Here we reconsider quasiparticle effects in superconducting qubits to explicitly account for the unavoidable asymmetry in the gap on the two sides of a Josephson junction. We find that different regimes can be encountered in which the quasiparticles have either similar densities in the two junction leads, or are largely confined to the lower-gap lead. Qualitatively, for similar densities the qubit's excited state population is lower but its relaxation rate higher than when the quasiparticles are confined; therefore, there is a potential trade-off between two desirable properties in a qubit.