Hong-Ou-Mandel dip measurements of polarization-entangled photon pairs at 1550 nm.

We performed a quantum interference experiment using two polarization-entangled photon pairs at 1550 nm, created in periodically poled lithium niobate waveguides. Using four-fold coincidences, a Hong-Ou-Mandel dip at diagonal polarization was observed with a visibility of 74.5% before subtracting accidental coincidences. This experiment lays a foundation for demonstrating polarization-based entanglement swapping and for realizing a quantum relay.