Let's consider a real source emitting a sequence of single-photons (like for example a N-V colour centre pumped with a pulsed laser with highly accurate frequency of the pulses). I want to characterize the source. In particular I want to directly verify experimentally that in each of the sequential time slots at most 1 single-photon is present, not more than one at the relevant wavelength range.

Ideally a detector that tells that the light is absorbed and how much energy was absorbed in that event is what I need. Is something like this existing today?