Superconducting detector dynamics studied by quantum pump-probe spectroscopy

We explore the dynamics of superconducting single-photon detectors (SSPDs) on the picosecond time-scale using a correlated photon-pair source based on spontaneous parametric downconversion (SPDC), corresponding to a pump-probe experiment at the single-photon level. We show that the detector can operate in a regime where the two-photon detection probability is orders of magnitude larger than the single-photon detection probability. The characteristic relaxation time-scale of the out-of-equilibrium hot-spot is found to be ? 15?ps. Our measurement technique is an effective tool to study fast two-photon processes, without requiring a power-dependence measurements to determine the number of photons involved.