Coherent transfer of Cooper pairs by a movable grain

Superconducting circuits that incorporate Josephson junctions are of consid erable experimental and theoretical interest, particularly in the context o f quantum computing(1-5). A nanometre-sized superconducting grain (commonly referred to as a Cooper-pair box(2)) connected to a reservoir by a Josephs on junction is an important example of such a system. Although the grain co ntains a large number of electrons, it has been experimentally demonstrated (6) that its states are given by a superposition of only two charge states (differing by 2e, where e is the electronic charge). Coupling between charg e transfer and mechanical motion in nanometre-sized structures has also rec eived considerable attention(7-10). Here we demonstrate theoretically that a movable Cooper-pair box oscillating periodically between two remote super conducting electrodes can serve as a mediator of Josephson coupling, leadin g to coherent transfer of Cooper pairs between the electrodes. Both the mag nitude and the direction of the resulting Josephson current can be controll ed by externally applied electrostatic fields.