Transmon Qubit Using Sn as a Junction Superconductor

Superconductor qubits typically use aluminum-aluminum oxide tunnel junctions to provide the nonlinear inductance. Junctions with semiconductor barriers make it possible to vary the superconductor material and explore beyond aluminum. We use InAs semiconductor nanowires coated with thin superconducting shells of β-Sn to realize transmon qubits. By tuning the Josephson energy with a gate voltage, we adjust the qubit frequency over a range of 3 GHz. The longest energy relaxation time, T₁ = 27 μs, is obtained at the lowest qubit frequencies, while the longest echo dephasing time, T₂ = 1.8 μs, is achieved at higher frequencies. We assess the possible factors limiting coherence times in these devices and discuss steps to enhance performance through improvements in materials fabrication and circuit design.