Energy decay and frequency shift of a superconducting qubit from non-equilibrium quasiparticles

TL;DR

This paper investigates how non-equilibrium quasiparticles affect superconducting qubits, demonstrating that their impact on decay rate and frequency aligns with theoretical predictions, enabling quasiparticle density estimation.

Contribution

It experimentally verifies the effect of injected quasiparticles on qubit decay and frequency, confirming the applicability of a generalized admittance theory for quasiparticle-induced decoherence.

Findings

Relative change in decay rate and frequency matches theory within 4%.

Quasiparticle injection alters qubit properties as predicted.

Settling of decay rate can indicate quasiparticle-limited relaxation.

Abstract

Quasiparticles are an important decoherence mechanism in superconducting qubits, and can be described with a complex admittance that is a generalization of the Mattis-Bardeen theory. By injecting non-equilibrium quasiparticles with a tunnel junction, we verify qualitatively the expected change of the decay rate and frequency in a phase qubit. With their relative change in agreement to within 4% of prediction, the theory can be reliably used to infer quasiparticle density. We describe how settling of the decay rate may allow determination of whether qubit energy relaxation is limited by non-equilibrium quasiparticles.