Proximity superconductivity in atom-by-atom crafted quantum dots

Gapless materials in electronic contact with superconductors acquire proximity-induced superconductivity in a region near the interface(1,2). Numerous proposals build on this addition of electron pairing to originally non-superconducting systems and predict intriguing phases of matter, including topological(3-7), odd-frequency(8), nodal-point(9) or Fulde-Ferrell-Larkin-Ovchinnikov(10) superconductivity. Here we investigate the most miniature example of the proximity effect on only a single spin-degenerate quantum level of a surface state confined in a quantum corral(11) on a superconducting substrate, built atom by atom by a scanning tunnelling microscope. Whenever an eigenmode of the corral is pitched close to the Fermi energy by adjusting the size of the corral, a pair of particle-hole symmetric states enters the gap of the superconductor. We identify these as spin-degenerate Andreev bound states theoretically predicted 50 years ago by Machida and Shibata(12), which had-so far-eluded detection by tunnel spectroscopy but were recently shown to be relevant for transmon qubit devices(13,14). We further find that the observed anticrossings of the in-gap states are a measure of proximity-induced pairing in the eigenmodes of the quantum corral. Our results have direct consequences on the interpretation of impurity-induced in-gap states in superconductors, corroborate concepts to induce superconductivity into surface states and further pave the way towards superconducting artificial lattices.