Few-mode to mesoscopic junctions in gatemon qubits

We investigate a semiconductor nanowire-based gatemon qubit with epitaxial Al on two facets of the nanowire, allowing gate control of wire density. Two segments have the Al removed, one forming a Josephson junction, and the other operating as a transistor, providing in situ switching between dc transport and qubit operation. Gating the nanowire (NW) changes the rate of decay of qubit frequency in magnetic field applied parallel to the NW. Gating the Josephson junction can lead additionally to nonmonotonic behavior of the qubit frequency in parallel magnetic field. A detailed model of the wire and junction yields behavior consistent with experiment, and allows us to propose that gating the bulk wire affects the rate of frequency decay in fields due to changes in the bulk potential distribution, while nonmonotonic behavior of the frequency is due to the interference of Andreev bound states in the junction. Gating the junction further allows access to the multimode regime, where fluctuations in qubit frequency are measured to be considerably smaller than the theoretical "universal" value, also smaller than numerics, and consistent with previous measurements of fluctuating critical currents.