  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 the 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.
