Andreev Modes from Phase Winding in a Full-Shell Nanowire-Based Transmon

Abstract

We investigate 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.

Magnetic-Field-Compatible Superconducting Transmon Qubit

“…The observation of anomalous qubit-resonance splittings has also been reported in Ref. [18]. This has been attributed to Andreev transitions emerging from phase winding associated with the reentrant lobe spectrum of the Little-Parks effect.…”

“…We note that the field dependence is strongly influenced by the nanowire charge distribution and the oscillations observed here are for a particular range of plunger-gate values [35,38]. Periodic oscillations in qubit frequency have also been observed for gatemons with nanowire junctions where the Al shell has fully enclosed the leads and have been interpreted as the Little-Parks effect [17,18].…”

“…Another key difference is the use of nanowires where not all facets are covered with Al, allowing voltage control of the charge density in the proximitized semiconductor leads. We speculate that the differences in T 1 times between the three lobes is the result of a varying density of subgap states [18]. While not presently under full experimental control, this difference is likely specific to the charge distribution in the wire for a given device tuning.…”

“…Comparing with previous work, we attribute the enhanced magnetic field resilience to the use of thin-shell nanowires (approximately 7 nm) as opposed to the thicker Al shells (approximately 30-40 nm) used in Refs. [16][17][18].…”

“…Recent work has demonstrated the coherent operation of gatemons with S-Sm-S nanowire junctions at moderate magnetic fields of approximately 100 mT [16][17][18]. Spectroscopy of S-Sm-S nanowire fluxonium qubits [19] and graphene-based gatemons [20] at high magnetic fields (approximately 1 T) has also been shown.…”