Coherence and entanglement preservation of frequency-converted heralded single photons.

We report on quantum frequency conversion of near-infrared photons from a wave-length of 854 nm to the telecommunication O-band at 1310 nm with 8 % overall conversion efficiency. Entangled photon pairs at 854 nm are generated via type-II spontaneous parametric down conversion. One photon is mixed with a strong pump field in a nonlinear ridge waveguide for its conversion to 1310 nm. We demonstrate preservation of first and second order coherence of the photons in the conversion process. Based on this we infer the coherence function of the two-photon state and compare it with the actual measured one. This measurement demonstrates preservation of time-energy entanglement of the pair. With 88 % visibility we violate a Bell inequality.