Andreev states taken to the next level

Abstract

The Andreev bound states are a hallmark of the theory of superconducting weak links. Using the versatile Josephson effect as source, device and detector, these spin-1/2 states have been directly observed. See Letter p.312 Brian Josephson proposed in 1962 that a dissipationless 'supercurrent' should flow between two superconductors separated by a weak link such as a tunnel junction. He was right, and the Josephson effect launched a new field of research with applications to magnetometry, medicine and astronomy. This paper describes an aspect of the Josephson effect that has previously been overlooked. Existing Josephson junction applications are based on properties of just the ground state, in which the electron pairs that carry the supercurrent localize at the weak link and form so-called Andreev doublets of ground and excited pair states. Bretheau et al. establish the existence of excited Andreev pair states through spectroscopic measurements of superconducting atomic contacts. This previously overlooked degree of freedom for tunneling electron pairs is a new quantum resource that could be exploited in novel types of superconducting qubits.