This experiment is widely used to ensure the indistinguishability of two bosonic particles in the experiment.
It should be emphasized that the signal and idler photons have no definite phase, and are therefore mutually incoherent, in the sense that they exhibit no second-order interference when brought together at detector D1 or D2. However, fourth-order interference effects occur, as demonstrated by the coincidence counting rate between D1 and D2.
Although the sum frequency is very well defined in the experiment, the individual down-shifted frequencies have large uncertainties, that, in practice, are largely determined by the pass bands of the in- terference filters IF inserted in the down-shifted beams, as shown in Fig. 1.