The Josephson effect describes the flow of supercurrent in a weak link—such as a tunnel junction, nanowire or molecule—between two superconductors1. It is the basis for a variety of circuits and devices, with applications ranging from medicine2 to quantum information3. Experiments using Josephson circuits that behave like artificial atoms4 are now revolutionizing the way we probe and exploit the laws of quantum physics5,6. 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 gap7,8,9,10. 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 qubits11,12,13.
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We acknowledge technical assistance from P. Sénat and P.-F. Orfila, theoretical input from M. Houzet, help in the experiments from L. Tosi, and discussions with V. Shumeiko, A. Levy-Yeyati and within the Quantronics group. This work was supported by ANR contracts DOCFLUC and MASH, and by C’Nano. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement no. PIIF-GA-2011-298415.
The authors declare no competing financial interests.
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Bretheau, L., Girit, Ç., Pothier, H. et al. Exciting Andreev pairs in a superconducting atomic contact. Nature 499, 312–315 (2013). https://doi.org/10.1038/nature12315
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