Single-photon frequency down-conversion experiment

We report a single-photon frequency down-conversion experiment. Using the difference frequency generation process in a periodically poled lithium niobate waveguide, we successfully observed the phase-preserved frequency down-conversion of a coherent pulse train with an average photon number per pulse of <1, from the 0.7 mu m visible wavelength band to the 1.3 mu m telecom band. We expect this technology to become an important tool for flexible photonic quantum networking, including the realization of quantum repeater systems over optical fiber using atom-photon entanglement sources for the visible wavelength bands.