It is the 30 th anniversary of the publication by C.K. Hong, Jeff Ou, and Leonard Mandel on the observation of two-photon interference – now called the Hong-Ou-Mandel (HOM) effect (see  for a report on this anniversary). In this simple and elegant experiment, two identical single-photon wavepackets are incident on a symmetric (50:50) beam splitter, and single-photon-counting detectors receive photons from the two output ports of the beam splitter. They observed that there are no coincidence counts recorded by the detectors, which they interpreted as a fusion of the photons at the beam splitter: The two photons leave the beam splitter together either from one output port or the other, while the chance of one photon leaving each output port is strongly suppressed. This effect arises from the destructive interference between the two quantum mechanical pathways for single photons leaving each output port. The HOM effect is interpreted as a purely quantum mechanical effect and has numerous applications in quantum optics and quantum information science. We have observed experimentally the HOM using what is thought to be highly classical wavepackets consisting of phase-randomized weak coherent states. In this seminar, I will review the HOM effect, discuss the motivation for using phase-randomized weak coherent states based on previous work from the quantum key distribution community, then point to several possible applications.
 I. Walmsley, ‘Quantum interference beyond the fringe,’ Science 358 , 1001 (2017).