Within the last few years, detectors such as superconducting transition edge sensors capable of spanning the single photon regime to millions of photons have become available. The issue is how to calibrate the response of these detectors. When individual photon number pulses can be resolved, the problem is relatively straightforward and can be regarded as solved. This covers the range from 1 to 20 photons per pulse. When the pulses have thousands of photons, the shot noise of the pulse is dominated by the noise of the detector and again the problem becomes straightforward. In between, the intrinsic noise of the detector is less than the shot noise of the pulses, yet individual detected numbers of photons cannot be distinguished. Characterizing such systems in an unambiguous way is not a solved problem.

A related problem occurs in the case of photon detection on a superconducting nanowire. Here, the intent is to make time-resolved measurements based on the arrival time of pulses at the two ends of the wire. Complications can arise such as when a second pulse arrives before the first is detected. Characterizing such detectors is at the heart of the project.