Qubit dephasing due to Quasiparticle Tunneling

TL;DR

This paper investigates the pure dephasing of superconducting qubits caused by quasiparticle tunneling, going beyond simple approximations to accurately model the dephasing behavior over different time scales.

Contribution

The study introduces a method to calculate qubit dephasing due to quasiparticles beyond the lowest order, revealing non-linear short-time dephasing dynamics.

Findings

Dephasing follows a non-linear exponential decay at short times.

Long-term dephasing rate can be self-consistently calculated.

Pure longitudinal coupling causes significant dephasing effects.

Abstract

We study dephasing of a superconducting qubit due to quasiparticle tunneling through a Josephson junction. While qubit decay due to tunneling processes is well understood within a golden rule approximation, pure dephasing due to BCS quasiparticles gives rise to a divergent golden rule rate. We calculate qubit dephasing due to quasiparticle tunneling beyond lowest order approximation in coupling between qubit and quasiparticles. Summing up a certain class of diagrams we show that qubit dephasing due to purely longitudinal coupling to quasiparticles leads to a dephasing $\sim \exp(-x(t))$ where $x(t)$ is not linear in time on short time scales while it tends towards a selfconsistent calculated dephasing rate for longer times.