Microwave spectroscopy of spinful Andreev bound states in ballistic semiconductor Josephson junctions

TL;DR

This study uses microwave spectroscopy to directly observe spinful Andreev bound states in ballistic semiconductor Josephson junctions, revealing high transmission channels, spin splitting, and symmetry-breaking effects due to spin-orbit coupling.

Contribution

First direct microwave spectroscopy detection of spin-split Andreev bound states in ballistic semiconductor Josephson junctions, highlighting high transmission and spin-orbit effects.

Findings

Detection of high transmission probabilities up to 0.9

Observation of spin-split Andreev bound states

Identification of symmetry-broken ABS due to spin-orbit coupling

Abstract

The superconducting proximity effect in semiconductor nanowires has recently enabled the study of new superconducting architectures, such as gate-tunable superconducting qubits and multiterminal Josephson junctions. As opposed to their metallic counterparts, the electron density in semiconductor nanosystems is tunable by external electrostatic gates providing a highly scalable and in-situ variation of the device properties. In addition, semiconductors with large $g$-factor and spin-orbit coupling have been shown to give rise to exotic phenomena in superconductivity, such as $\varphi_0$ Josephson junctions and the emergence of Majorana bound states. Here, we report microwave spectroscopy measurements that directly reveal the presence of Andreev bound states (ABS) in ballistic semiconductor channels. We show that the measured ABS spectra are the result of transport channels with gate-tunable, high transmission probabilities up to $0.9$, which is required for gate-tunable Andreev qubits and beneficial for braiding schemes of Majorana states. For the first time, we detect excitations of a spin-split pair of ABS and observe symmetry-broken ABS, a direct consequence of the spin-orbit coupling in the semiconductor.