Non-equilibrium quasiparticles in superconducting circuits: photons vs. phonons

TL;DR

This paper investigates how non-equilibrium quasiparticles, influenced by photon and phonon interactions, affect superconducting qubits, providing a simple model for their stationary distribution and implications for qubit relaxation and excitation.

Contribution

It introduces a simple non-thermal stationary solution for quasiparticle distribution considering heating and cooling processes, linking theory with recent experimental observations.

Findings

Quasiparticle heating and cooling balance determines their stationary distribution.

Quasiparticles significantly impact qubit relaxation and excitation rates.

The model aligns with recent experimental data on superconducting circuits.

Abstract

We study the effect of non-equilibrium quasiparticles on the operation of a superconducting device (a qubit or a resonator), including heating of the quasiparticles by the device operation. Focusing on the competition between heating via low-frequency photon absorption and cooling via photon and phonon emission, we obtain a remarkably simple non-thermal stationary solution of the kinetic equation for the quasiparticle distribution function. We estimate the influence of quasiparticles on relaxation and excitation rates for transmon qubits, and relate our findings to recent experiments.