Measurement of Subpicosecond Time Intervals between Two Photons by Interference by C. K. Hong and others (1987, 10.1103/PhysRevLett.59.2044)

Primary-paper scan hosted by Caltech: painterlab.caltech.edu/wp-content/uploads/2019/06/iqd_measurement_subpicosecond_time_two_photons.pdf

Physical Review Letters 59, 2044–2046. The experiment combines down-converted photon pairs at a beam splitter, scans relative path delay and records coincidences after accounting for accidental background; the ideal balanced, indistinguishable limit suppresses the coincidence output.