Quantum capacitive phase detector

We discuss how a single Cooper-pair transistor may be used to detect the superconducting phase difference by using the phase dependence of the input capacitance from gate to ground. The proposed device has a low power dissipation because its operation is in principle free from quasiparticle generation. According to the sensitivity estimates, the device may be used for efficient qubit readout in a galvanically isolated and symmetrized circuit.