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

TL;DR

This paper demonstrates microwave sensing of Andreev bound states in a superconducting quantum point contact using a microresonator, revealing quantum interference effects and characterizing decoherence mechanisms.

Contribution

It introduces a method to detect and analyze Andreev bound states via microwave absorption in a superconducting cavity, employing a dispersive Jaynes-Cummings model for interpretation.

Findings

Detection of phase-dependent microwave absorption linked to Andreev states

Observation of quantum interference effects modulating electron transmission

Quantification of decoherence consistent with charge noise

Abstract

We use a superconducting microresonator as a cavity to sense absorption of microwaves by a superconducting quantum point contact defined by surface gates over a proximitized two-dimensional electron gas. Renormalization of the cavity frequency with phase difference across the point contact is consistent with adiabatic coupling to Andreev bound states. Near $\pi$ phase difference, we observe random fluctuations in absorption with gate voltage, related to quantum interference-induced modulations in the electron transmission. We identify features consistent with the presence of single Andreev bound states 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 localized state.