Coincidence counting

Count detections in two channels within a specified time window to estimate pair correlations. Delayed windows or independent singles rates estimate accidental coincidences. Finite efficiency, timing jitter, dark counts and multiple pairs must be included before interpreting a coincidence fringe as a property of the prepared state.

Retaining events with a detection in each of four channels within a specified timing window. For exactly four incident photons and ideal photodetectors, a click in every channel requires one photon per channel. More photons, losses and detector timing introduce additional qualifications.