Exciting Andreev pairs in a superconducting atomic contact

A fundamental and previously unobserved aspect of the Josephson effect is revealed through spectroscopic measurements of the excited Andreev states in superconducting atomic contacts. A new twist on the Josephson effect 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. The Josephson effect describes the flow of supercurrent in a weak link—such as a tunnel junction, nanowire or molecule—between two superconductors 1 . It is the basis for a variety of circuits and devices, with applications ranging from medicine 2 to quantum information 3 . Experiments using Josephson circuits that behave like artificial atoms 4 are now revolutionizing the way we probe and exploit the laws of quantum physics 5 , 6 . Microscopically, the supercurrent is carried by Andreev pair states, which are localized at the weak link. These states come in doublets and have energies inside the superconducting gap 7 , 8 , 9 , 10 . Existing Josephson circuits are based on properties of just the ground state of each doublet, and so far the excited states have not been directly detected. Here we establish their existence through spectroscopic measurements of superconducting atomic contacts. The spectra, which depend on the atomic configuration and on the phase difference between the superconductors, are in complete agreement with theory. Andreev doublets could be exploited to encode information in novel types of superconducting qubits 11 , 12 , 13 .