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 $\mathrm{In}\mathrm{As}$ 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 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 those of the best devices previously reported, suggestive that surface atomic-scale disorder and quasiparticle poisoning are not linked to spin relaxation in $\mathrm{In}\mathrm{As}$ nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits, such as germanium and carbon.

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• Andreev bound states
• Josephson effect
• Optoelectronics
• Quantum information with solid state qubits
• III-V semiconductors
• Superconducting qubits
• Superconductors
• Microwave techniques