Nonequilibrium regimes for quasiparticles in superconducting qubits with gap-asymmetric junctions

TL;DR

This paper explores how gap asymmetry in superconducting qubits influences quasiparticle behavior across different temperature regimes, providing insights to improve qubit coherence by understanding nonequilibrium effects.

Contribution

It characterizes four distinct temperature-dependent regimes of quasiparticle states in gap-asymmetric junctions, clarifying the limitations of assuming global quasiequilibrium in experiments.

Findings

Four temperature regimes identified: nonequilibrium, local quasiequilibrium, global quasiequilibrium, full equilibrium.

Measurement techniques involving magnetic fields can improve parameter determination.

Highlighting the importance of nonequilibrium understanding for qubit coherence enhancement.

Abstract

Superconducting qubits hold promise for quantum computing, but their operation is challenged by various sources of noise, including excitations known as quasiparticles. Qubits with gap asymmetry larger than their transition energy are less susceptible to quasiparticle decoherence as the quasiparticles are mostly trapped in the low-gap side of the junction. Because of this trapping, the gap asymmetry can contribute to maintaining the quasiparticles out of equilibrium. Here we address the temperature dependence of the quasiparticle densities in the two sides of the junction. We show that four qualitatively different regimes are possible with increasing temperature: i) nonequilibrium, ii) local quasiequilibrium, iii) global quasiequilibrium, and iv) full equilibrium. We identify shortcomings in assuming global quasiequilibrium when interpreting experimental data, highlighting how measurements in the presence of magnetic field can aid the accurate determination of the junction parameters, and hence the identification of the nonequilibrium regimes.