Andreev spin relaxation time in a shadow-evaporated InAs weak link

TL;DR

This paper introduces a new InAs nanowire weak link design for Andreev spin qubits, enabling detailed characterization of spin relaxation and coherence, and demonstrating that surface disorder and quasiparticle poisoning are not primary relaxation sources.

Contribution

The study presents a microwave readout circuit sensitive to spin states, a gap-engineering strategy to reduce quasiparticle poisoning, and a fabrication method via shadow evaporation, advancing InAs Andreev qubit technology.

Findings

Spin relaxation and dephasing rates are comparable to best devices.

Surface atomic-scale disorder and quasiparticle poisoning are not primary relaxation sources.

Design strategies are applicable to other materials like germanium and carbon.

Abstract

Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link so that higher orbital states are not necessary for readout -- this resulted in larger windows in parameter space in which the spin state properties can be probed. Second, we implemented a successful gap-engineering strategy to mitigate quasiparticle poisoning. Third, the weak link was fabricated by \textit{in situ} shadow evaporation, which has been shown to improve atomic-scale disorder. We show how our design allows characterization of the spin stability and coherence over the full range of magnetic flux and gate voltage of an odd parity bias point. The spin relaxation and dephasing rates are comparable with the best devices previously reported, suggestive that surface atomic-scale disorder and QP poisoning are not linked to spin relaxation in InAs nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits such as germanium and carbon.