Quantum coherence of charge states in the single electron Box

A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast to the charge state of art atomic or molecular ion, the charge states of such an "island" are mesoscopic states involving all the conduction electrons of the island. Island charge states bear some resemblance to the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the field in the cavity. For a normal island, charge states decay irreversibly into charge stares of lower energies. However, the gound state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states differing by two electrons (i.e., a Cooper pair). We describe an experiment in which this Josephson effect involving only one Cooper pair is measured.