Microwave Sensing of Andreev Bound States in a Gate-Defined Superconducting Quantum Point Contact

Andreev bound states (ABSs) are coherent superpositions of electrons and holes bound at a junction between two superconductors. Hybrid superconductor-semiconductor junctions allow exquisite control over the number and energy of ABSs using capacitively-coupled gate electrodes. Here we use circuit quantum electrodynamics (cQED) to monitor ABSs in a quantum point contact defined by surface gates over a proximitized two-dimensional electron gas. We infer the ABS structure by comparing a microphysical model of the junction to fits of the cavity frequency as a function of phase difference and gate voltage. Close to pinch-off, we identify features consistent with the presence of a single ABS and describe the Andreev-cavity interaction using a dispersive Jaynes-Cummings model. By fitting the weak Andreev-cavity coupling, we extract ~GHz decoherence consistent with charge noise and the transmission dispersion associated with a single localized state.