Electronic transport through nanostructures is greatly affected by thepresence of superconducting leads. If the interface between the nanostructureand the superconductors is sufficiently transparent, a dissipationless current(supercurrent) can flow through the device due to the Josephson effect. AJosephson coupling, as measured via the zero-resistance supercurrent, has beenobtained via tunnel barriers, superconducting constrictions, normal metals, andsemiconductors. The coupling mechanisms vary from tunneling to Andreevreflection. The latter process has always occurred via a normal-type systemwith a continuous density of states. Here we investigate a supercurrent flowingvia a discrete density of states, i.e., the quantized single particle energystates of a quantum dot, or artificial atom, placed in between superconductingelectrodes. For this purpose, we exploit the quantum properties of finite-sizedcarbon nanotubes (CNTs). By means of a gate electrode, successive discreteenergy states are tuned ON and OFF resonance with the Fermi energy in thesuperconducting leads, resulting in a periodic modulation of the criticalcurrent and a non-trivial correlation between the conductance in the normalstate and the supercurrent. We find, in good agreement with existing theory,that the product of the critical current and the normal state resistancebecomes an oscillating function, in contrast to being constant as in previouslyexplored regimes.
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