**
Abstract:
**

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 [1] 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.

[1]
I. Walmsley, ‘Quantum interference beyond the fringe,’ Science
**
358
**
, 1001 (2017).