  We propose and experimentally demonstrate a self-calibrating detector of
Cooper pair depairing in a superconductor based on a mesoscopic superconducting
island coupled to normal metal leads. On average, exactly one electron passes
through the device per broken Cooper pair, independent of the absorber volume,
device or material parameters. The device operation is explained by a simple
analytical model and verified with numerical simulations in quantitative
agreement with experiment. In a proof-of-concept experiment, we use such a
detector to measure the high-frequency phonons generated by another,
electrically decoupled superconducting island, with a measurable signal
resulting from less than 10 fW of dissipated power.
