Highly sensitive electrometers based on single Cooper pair tunneling

A superconducting transistor structure comprising two Josephson tunnel junc tions connected in series and a small island in between, which is capacitiv ely coupled to a gate, is considered. When self-capacitances of the junctio ns are sufficiently small that the corresponding charging energy E-c is of the order of magnitude of the Josephson coupling strength E-J (and both are much greater than k(B)T), the interplay of the charging and Josephson effe cts in the circuit becomes essential. This leads to a characteristic I-V cu rve which can be effectively modulated by the gate in two limit cases of ex ternal electrodynamic impedance Z(s)(omega): (a) Z(s) = R-s much greater th an R-Q much greater than 6.5 k Omega and (b) R-s much less than R-Q. Both c ircuits can serve as electrometers which are competitive with traditional s ingle-electron devices. Preliminary experimental results are discussed.