Credit: © 2006 Nature

The well-defined energy levels of quantum dots (QD) can be used to control transport, one electron at a time, even when the QD is sandwiched between two metal leads. The electron transport is more complex, however, when the QD separates two superconducting leads. In a superconductor, electrons are bound in pairs and so conduction through a QD, one electron at a time, forces the pair to temporarily break apart.

Jorden van Dam and co-workers1 at Delft University in the Netherlands explore the peculiarities of passing a superconducting current through a QD by inserting two 60 nm indium arsenide wires as junctions in an aluminium loop. The aluminium is a superconductor below 1.1 K, and one of the InAs wires functions as the QD (the other wire serves as a reference). Changing the voltage across the InAs wire brings its energy levels in and out of registry with those of the aluminium leads and controls the number of electrons on it.

The most significant result of this study is to show that the sign of the super-current changes from positive to negative when the number of electrons on the QD changes from even to odd. The effect itself is not simple, but follows from considering that the number of electrons on the dot affects the possible quantum mechanical paths that the paired electrons can take to tunnel from one superconducting lead to the other.