QUANTUM COHERENCE WITH A SINGLE COOPER-PAIR

A metallic electrode connected to electron reservoirs by tunnel juncti ons has a series of charge states corresponding to the number of exces s electrons in the electrode. In contrast with the charge state of an atomic or molecular ion, the charge states of such an ''island'' invol ve a macroscopic number of conduction electrons of the island. Island charge states bear some resemblance with the photon number states of t he cavity in cavity QED, the phase conjugate to the number of electron s being analogous to the phase of the field in the cavity. For a norma l island charge states decay irreversibly into charge states of lower energies. However, the ground state of a superconducting island connec ted to superconducting reservoirs can be a coherent superposition of c harge states differing by two electrons (i.e. a Cooper pair). We descr ibe an experiment in which this Josephson effect involving only one Co oper pair is measured.