Two-photon interference in an integrated fiber tritter

The schematic below shows the setup used for our measurements. The UV-pumped crystal produces correlated photon pairs, which are launched into singlemode optical fibers using microscope objectives. The photons leaving the tritter (3×3-coupler) are detected by silicon avalanche-photodiodes the signals of which are connected to the inputs to coincidence counters.

When the photons are indistinguishable at the tritter (they arrive at the same time) they will always take leave the tritter together in one output port, and therefore the measured coincidence rates drop. This effect is known as the "Hong-Ou-Mandel Dip". The picture below shows the coincidence rates between each pair of output ports: