Mesoscopic superconductors under irradiation: microwave spectroscopy of Andreev states

We show that irradiation of a voltage-biased superconducting quantum point contact at frequencies of the order of the gap energy can remove the suppre ssion of subgap de transport through Andreev levels. Quantum interference a mong resonant scattering events involving photon absorption is furthermore shown to make microwave spectroscopy of the Andreev levels feasible. We als o discuss how the same interference effect can be applied for detecting wea k electromagnetic signals up to the gap frequency, and how it is affected b y dephasing and relaxation. (C) 1999 Academic Press.