Opportunities for the direct manipulation of a phase-driven Andreev spin qubit

TL;DR

This paper demonstrates that spin-flip scattering processes in semiconducting nanowire Josephson junctions enable direct manipulation of phase-driven Andreev spin qubits, eliminating the need for complex Raman processes.

Contribution

It introduces a scattering matrix formalism showing that spin-flip scattering couples Andreev states, allowing for direct qubit control without Raman processes.

Findings

Spin-flip scattering couples Andreev states with opposite spins.

Direct manipulation of phase-driven Andreev spin qubits is possible.

Circumvents the need for Raman processes in qubit control.

Abstract

In a Josephson junction, the transfer of Cooper pairs from one superconductor to the other one can be associated with the formation of Andreev bound states. In a Josephson junction made with a semiconducting nanowire, the spin degeneracy of these Andreev states can be broken thanks to the presence of spin-orbit coupling and a finite phase difference between the two superconducting electrodes. The lifting of the spin degeneracy opened the way to the realization of Andreev spin qubits that do not require the application of a large magnetic field. So far the operation of these qubits relied on a Raman process involving two microwave tones and a third Andreev state [M. Hays et al., Science 373, 430 (2021)]. Still, time-reversal preserving impurities in the nanowire allow for spin-flip scattering processes. Here, using the formalism of scattering matrices, we show that these processes generically couple Andreev states with opposite spins. In particular, the non-vanishing current matrix element between them allows for the direct manipulation of phase-driven Andreev spin qubits, thereby circumventing the use of the above-mentioned Raman process.