EVEN-ODD SYMMETRY-BREAKING IN THE NSN COULOMB-BLOCKADE ELECTROMETER

We have measured at low temperature the current through a submicrometer superconducting island connected to two normal metal leads by ultra-small tunnel junctions. As the bias voltage is lowered below the superconducting gap of this Coulomb blockade electrometer, the current changes from being e-periodic with gate charge to 2e-periodic. We interpret the 2e-periodic current at low voltages as a manifestation of a sequence of Andreev reflections which shuttles two electrons at a time through the island. This process can only exist if the island favors a state with a definite parity of the number of conduction electrons.