When two superconductors are electrically connected by a weak link—such as a tunnel barrier—a zero-resistance supercurrent can flow1,2. This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e. The 2e charge quantum is clearly visible in the height of voltage steps in Josephson junctions under microwave irradiation, and in the magnetic flux periodicity of h/2e (where h is Planck's constant) in superconducting quantum interference devices2. Here we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Owing to strong Coulomb interaction, electrons only tunnel one-by-one through the discrete energy levels of the quantum dot. This nevertheless can yield a supercurrent when subsequent tunnel events are coherent3,4,5,6,7. These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, that is, in a normal or a π-junction8,9,10, respectively. We demonstrate that the supercurrent reverses sign by adding a single electron spin to the quantum dot. When excited states of the quantum dot are involved in transport, the supercurrent sign also depends on the character of the orbital wavefunctions.
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We thank Y.-J. Doh and L. Glazman for discussions, G. Immink for nanowire growth, and A. van der Enden and R. Schouten for technical support. Financial support was obtained from the Dutch Foundation for Fundamental Research on Matter (FOM), the Dutch Organisation for Scientific Research (NWO), the EU programmes HYSWITCH and NODE, and the Japanese International Cooperative Research Project (ICORP). Author Contributions J.A.v.D., S.D.F. and L.P.K. are responsible for quantum transport experiments, Y.V.N. for numerical simulations, and E.P.A.M.B. for nanowire growth.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This file contains Supplementary Methods (nanowire growth and device fabrication), Supplementary Data (data for a seconds device), Supplementary Discussion (supercurrent reversal) and Supplementary Methods (numerical evaluation of supercurrents). (DOC 122 kb)
Scanning Electron Microscopy images. (PDF 758 kb)
Supercurrent reversal in the second device. (PDF 102 kb)
Energy diagrams illustrating transport through a multi-level quantum dot. (PDF 85 kb)
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van Dam, J., Nazarov, Y., Bakkers, E. et al. Supercurrent reversal in quantum dots. Nature 442, 667–670 (2006). https://doi.org/10.1038/nature05018
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