Cooper-pair qubit and Cooper-pair electrometer in one device

An all-superconductor charge qubit enabling a radio-frequency readout of its quantum state is described. The core element of the setup is a superconducting loop which includes the single-Cooper-pair (Bloch) transistor, This circuit has two functions: First. it operates as a charge qubit with magnetic control of Josephson coupling and electrostatic control of the charge on the transistor island. Secondly. it acts as the transducer of the rf electrometer. which probes the qubit state by measuring the Josephson inductance of the transistor. The evaluation of the basic parameters of this device shows its superiority over the rf-SET-based qubit setup. (C) 2001 Elsevier Science B.V. All rights reserved.