Energy relaxation of superconducting charge qubit via Andreev processes

TL;DR

This paper investigates the fundamental limits on energy relaxation in superconducting charge qubits, focusing on how Andreev processes contribute to relaxation when quasiparticle poisoning is suppressed by a large gap mismatch.

Contribution

It provides a detailed evaluation of the qubit relaxation rate caused by Andreev processes, highlighting a relaxation mechanism distinct from quasiparticle poisoning.

Findings

Quasiparticle poisoning becomes ineffective with large gap mismatch.

Andreev processes can still cause qubit relaxation despite suppression of poisoning.

The paper quantifies the relaxation rate due to Andreev processes.

Abstract

We study fundamental limitations on the energy relaxation rate of a superconducting charge qubit with a large-gap Cooper-pair box, $\Delta_b > \Delta_r$. At a sufficiently large mismatch between the gap energies in the box $\Delta_b$ and in the reservoir $\Delta_r$,"quasiparticle poisoning" becomes ineffective even in the presence of nonequilibrium quasiparticles in the reservoir. The qubit relaxation still may occur due to higher-order (Andreev) processes. In this paper we evaluate the qubit energy relaxation rate due to Andreev processes.