Frequency down-conversion of non-classical light from visible wavelength to telecom wavelength using difference-frequency generation

We experimentally demonstrate frequency down-conversion of non-classical light by difference-frequency generation. We prepared a heralded single photon at wavelength of 780 nm, which corresponds to a resonant wavelength of a Rb atom for a quantum memory, and then converted the wavelength to a telecommunication wavelength of 1522 nm. The observed second-order correlation g
⁽²⁾
(0) of the converted light is 0.54 ±0.12, which clearly shows sub-Poissonian photon statistics of light.