1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands

Correspondence to: Pablo Jarillo-Herrero¹ Correspondence and requests for materials should be addressed to P.J.-H. (Email: Pablo@qt.tn.tudelft.nl).

Abstract

Electronic transport through nanostructures is greatly affected by the presence of superconducting leads^{1, 2, 3}. If the interface between the nanostructure and the superconductors is sufficiently transparent, a dissipationless current (supercurrent) can flow through the device owing to the Josephson effect^{4, 5}. A Josephson coupling, as measured by the zero-resistance supercurrent, has been obtained using tunnel barriers, superconducting constrictions, normal metals and semiconductors. The coupling mechanisms vary from tunnelling to Andreev reflection^{5, 6, 7, 8}. The latter process has hitherto been observed only in normal-type systems with a continuous density of electronic states. Here we investigate a supercurrent flowing through a discrete density of states—that is, the quantized single particle energy states of a quantum dot⁹, or 'artificial atom', placed between superconducting electrodes. For this purpose, we exploit the quantum properties of finite-sized carbon nanotubes¹⁰. By means of a gate electrode, successive discrete energy states are tuned on- and off-resonance with the Fermi energy in the superconducting leads, resulting in a periodic modulation of the critical current and a non-trivial correlation between the conductance in the normal state and the supercurrent. We find, in good agreement with existing theory¹¹, that the product of the critical current and the normal state resistance becomes an oscillating function, in contrast to being constant as in previously explored regimes.

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