Highly Sensitive Electrometers Based on Single Cooper Pair Tunneling

A superconducting transistor structure comprising two Josephson tunnel junctions connected in series and a small island in between, which is capacitively coupled to a gate, is considered. When self-capacitances of the junctions are sufficiently small that the corresponding charging energy Ec is of the order of magnitude of the Josephson coupling strength EJ (and both are ≫kBT), the interplay of the charging and Josephson effects in the circuit becomes essential. This leads to a characteristic IV curve which can be effectively modulated by the gate in two limit cases of external electrodynamic impedance Zs(ω): (a) Zs = Rs ≫ RQ ≈ 6.5 kΩ and (b) Rs ≪ RQ. Both circuits can serve as electrometers which are competitive with traditional single-electron devices. Preliminary experimental results are discussed.