  We show that irradiation of a voltage-biased superconducting quantum point
contact at frequencies of the order of the gap energy can remove the
suppression of subgap dc transport through Andreev levels. Quantum interference
among resonant scattering events involving photon absorption is furthermore
shown to make microwave spectroscopy of the Andreev levels feasible. We also
discuss how the same interference effect can be applied for detecting weak
electromagnetic signals up to the gap frequency, and how it is affected by
dephasing and relaxation.
