Figure 1.

Schematic illustration of the Hong-Ou-Mandel (HOM) effect. Left: Two near-simultaneous photons with similar wavelength impinge on a beam splitter (BS), and the outputs are registered on two detectors, D1 and D2. When the photons’ arrival is simultaneous, making them indistinguishable, the HOM interference effect causes both photons to exit one side of the splitter or the other, inhibiting the outcome with one photon going to each detector. Right: The signature of the HOM effect is a drop in the rate of coincident detections at D1 and D2, as the photon arrival times become identical. This is typically observed as an “HOM dip” in coincidences as a function of some relative time delay at the BS input.