Photon-bunching measurement after two 25-km-long optical fibers

To show the feasibility of a long-distance partial Bell-state measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecommunication wavelength are created by parametric down-conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam splitter and transmitted via two fibers of 25 km. The wave packets are relatively delayed and recombined on a second beam splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the 25-km-long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a net visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long-distance Bell-state measurement with two independent sources, as, e.g., required in an entanglement swapping configuration in the scale of tens of kilometers.