Even-odd asymmetry of a superconductor revealed by the Coulomb blockade of Andreev reflection

We have measured at low temperatures the current through a submicrometer superconducting island connected to two normal metal leads by ultrasmall tunnel junctions. As the bias voltage is lowered well below twice the superconducting energy gap, the current changes from being e periodic with gate charge to 2e periodic. This behavior is clear evidence that there is a difference in the total energy between the ground states of an even and odd number of electrons on the island. The 2e-periodic current peaks are the first observation of the Coulomb blockade of Andreev reflection.