Hot non-equilibrium quasiparticles in transmon qubits

TL;DR

This paper investigates the energy distribution of non-equilibrium quasiparticles in superconducting transmon qubits, revealing their significant impact on qubit decoherence and suggesting they are more energetic than thermal predictions.

Contribution

It provides the first systematic correlation between qubit relaxation/excitation and charge-parity switches to analyze quasiparticle energy distribution in transmons.

Findings

Quasiparticle-induced excitation dominates residual excited-state population.

Quasiparticle loss limits T1 to approximately 200 microseconds.

Quasiparticles are more energetic than thermal distribution predicts.

Abstract

Non-equilibrium quasiparticle excitations degrade the performance of a variety of superconducting circuits. Understanding the energy distribution of these quasiparticles will yield insight into their generation mechanisms, the limitations they impose on superconducting devices, and how to efficiently mitigate quasiparticle-induced qubit decoherence. To probe this energy distribution, we systematically correlate qubit relaxation and excitation with charge-parity switches in an offset-charge-sensitive transmon qubit, and find that quasiparticle-induced excitation events are the dominant mechanism behind the residual excited-state population in our samples. By itself, the observed quasiparticle distribution would limit $T_1$ to $\approx200~\mu\mathrm{s}$, which indicates that quasiparticle loss in our devices is on equal footing with all other loss mechanisms. Furthermore, the measured rate of quasiparticle-induced excitation events is greater than that of relaxation events, which signifies that the quasiparticles are more energetic than would be predicted from a thermal distribution describing their apparent density.