Destructive Little-Parks Effect in a Full-Shell Nanowire-Based Transmon

Abstract

A semiconductor transmon with an epitaxial Al shell fully surrounding an InAs nanowire core is investigated in the low E J =E C regime. Little-Parks oscillations as a function of flux along the hybrid wire axis are destructive, creating lobes of reentrant superconductivity separated by a metallic state at a half quantum of applied flux. In the first lobe, phase winding around the shell can induce topological superconductivity in the core. Coherent qubit operation is observed in both the zeroth and first lobes. Splitting of parity bands by coherent single-electron coupling across the junction is not resolved beyond line broadening, placing a bound on Majorana coupling, E M =h < 10 MHz, much smaller than the Josephson coupling E J =h ∼ 4.7 GHz.

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

“…We interpret this reduction of f sim 01 as resulting from the overall reduced gap for n ¼ 1, as discussed previously in Ref. [24].…”

“…Here, we investigate low-energy ABSs in superconducting full-shell NW-based gatemons, which exhibit a destructive Little-Parks effect [20,23,24]. An applied magnetic field parallel to the NW induces a sequence of reentrant superconducting lobes, each associated with a different winding number, n, of the superconducting phase around the shell [21,22,25].…”

“…We measure both the in-phase (I) and quadrature (Q) components of the transmission signal, defining V H by rotating the data in the IQ plane to maximize the signal along the real axis [32,33]. Two-tone measurement of the qubit frequency spectrum as a function of B [24], through the zeroth and first lobes (n ¼ 0, 1) is shown in Fig. 1(c).…”

Protocol to identify a topological superconducting phase in a three-terminal device

“…Finally, photon-assisted tunneling consistent with the coherent hybridization of MZMs was also observed [78]. However, such microwave signatures have not been consistently detected in systems showing ZBPs [79] and, since they rely on a local coupling between MZMs, they are equally susceptible to the problem of distinguishing MZMs from trivial ABSs. Thus, these experiments are not suited either to reliably tune into a topological phase.…”

Comparing tunneling spectroscopy and charge sensing of Andreev bound states in a semiconductor-superconductor hybrid nanowire structure

“…Experiments with superconductor-semiconductor hybrid structures have demonstrated the presence of Andreev bound states (ABSs) in proximitized semiconductors by a variety of methods, including tunneling spectroscopy in NIS junctions (N is normal, S is superconductor, I is an insulating tunnel barrier), Coulomb blockade spectroscopy in superconducting islands with normal leads (NISIN) [1][2][3][4][5], and SNS spectroscopy of Josephson junctions [6][7][8][9][10][11][12][13]. These methods reveal distinct spectroscopic features and raise different technical challenges.…”