Detection of low-intensity far-infrared coherent fields by mesoscopic Josephson junctions

Here we consider the effects of exposing ultrasmall dc voltage-biased Josephson junctions to a single-mode quantized far-infrared field prepared in a coherent state. We prove that the I-V characteristic of the mesojunction exhibits quantum Shapiro steps (QSS) whose amplitude and quantum noise reflect the intensity and the phase of the non classical coherent state. In this way, such a nanodevice may be exploited to probe the parameters of an external quantized e.m. field, detecting the amplitude and the quantum fluctuations of the first QSS only.