Andreev spin qubit protected by Franck-Condon blockade

TL;DR

This paper proposes a method to enhance the relaxation lifetime of an Andreev spin qubit by leveraging Franck-Condon blockade in a transmon circuit, reducing spin relaxation at low temperatures.

Contribution

It introduces a theoretical approach to protect Andreev spin qubits by coupling spin flips to plasmon excitations, exploiting Franck-Condon blockade effects.

Findings

Spin relaxation can be suppressed at low temperatures.

Coupling to plasmons requires multiple excitations for spin flips.

Franck-Condon blockade enhances qubit coherence times.

Abstract

Andreev levels localized in a weak link between two superconductors can trap a superconducting quasiparticle. If there is a spin-orbit coupling in the link, the spin of the quasiparticle couples to the Josephson current. This effect can be leveraged to control and readout the spin of the quasiparticle thus using it as a qubit. One of the factors limiting the performance of such an Andreev spin qubit is spin relaxation. Here, we theoretically demonstrate that the relaxation lifetime can be enhanced by utilizing the coupling between the Andreev spin and the supercurrent in a transmon circuit. The coupling ensures that the flip of the quasiparticle spin can only happen if it is accompanied by the excitation of multiple plasmons, as dictated by the Franck-Condon principle. This blocks spin relaxation at temperatures small compared to plasmon energy.