A team of researchers has investigated transmon qubits made from semiconductor nanowires with a fully surrounding superconducting shell.
In the regime of reentrant superconductivity associated with the destructive Little-Parks effect, numerous coherent transitions are observed in the first reentrant lobe, where the shell carries 2π winding of superconducting phase, and are absent in the zeroth lobe.
As junction density was increased by gate voltage, qubit coherence was suppressed then lost in the first lobe.
These observations and numerical simulations highlight the role of winding-induced Andreev states in the junction.
The paper has been published in Physical Review Letters.