Spectrum of Andreev Bound States in a Molecule Embedded Inside a Microwave-Excited Superconducting Junction

TL;DR

This paper proposes a new microwave spectroscopy method to directly observe Andreev bound states in a molecular superconducting junction, revealing polariton excitations and phase-dependent shifts.

Contribution

It introduces a novel microwave QED cavity technique for spectroscopic detection of Andreev bound states in molecular junctions, providing direct experimental access.

Findings

Detection of polariton excitation at twice the Andreev bound state energy

Observation of superconducting-phase dependent ac Stark shift

Proposal of dispersive measurement for Andreev state spectroscopy

Abstract

Non-dissipative Josephson current through nanoscale superconducting constrictions is carried by spectroscopically sharp energy states, so-called Andreev bound states. Although theoretically predicted almost 40 years ago, no direct spectroscopic evidence of these Andreev bound states exists to date. We propose a novel type of spectroscopy based on embedding a superconducting constriction, formed by a single-level molecule junction, in a microwave QED cavity environment. In the electron-dressed cavity spectrum we find a polariton excitation at twice the Andreev bound state energy, and a superconducting-phase dependent ac Stark shift of the cavity frequency. Dispersive measurement of this frequency shift can be used for Andreev bound state spectroscopy.