The experiment with detectors described here was carried out as a real experiment in 1986 by the French physicists Grangier, Roger and Aspect1. The problem was to produce individual photons in a controlled way. As a source of individual photons, this experiment used calcium atoms which, following laser excitation into a higher-lying s state, could only return to the ground state by emitting two photons, for reasons of angular-momentum conservation. One of the two photons fell on the beam splitter; the other was detected separately and indicated that a photon was indeed present in the apparatus. In this way, individual photons could be handled in a controlled manner.
1 Grangier, Philippe; Roger, Gérard; Aspect, Alain: Experimental evidence for a photon anticorrelation effect on a beamsplitter,
Europhys. Lett. 1 (1986), p. 173.