Quantum phase expectation values of a mesoscopic Josephson junction from quantum current measurements

A simple way to acquire information on the mean values of the phase operato rs sin phi and cos phi of an ultrasmall Josephson junction prepared in an a rbitrary pure or not state is reported. Our proposal exploits the recently predicted occurrence of current spikes in the I-V characteristic of a mesoj unction irradiated by a quantum single-mode low-intensity coherent electrom agnetic field. A necessary condition for the validity of our treatment is p resented and discussed.