A low-noise frequency down-conversion to the telecommunication band for   a quantum communication based on NV centers in diamond
  We demonstrate a low-noise frequency down-conversion of photons at 637 nm to the telecommunication band at 1587 nm by the difference frequency generation in a periodically-poled lithium niobate. An internal conversion efficiency of the converter is estimated to be 0.44 at the maximum which is achieved by a pump power of 430 mW, whereas a rate of internal background photons caused by the strong cw pump laser is estimated to be 9 kHz/mW within a bandwidth of about 1 nm. By using the experimental values related to the intrinsic property of the converter, and using the intensity correlation and the average photon number of a 637-nm input light pulse, we derive the intensity correlation of a converted telecom light pulse. Then we discuss feasibility of a single-photon frequency conversion to the telecommunication band for a long-distance quantum communication based on NV centers in diamond. 
Rikizo Ikuta, Toshiki Kobayashi, Shuto Yasui, Shigehito Miki, Taro   Yamashita, Hirotaka Terai, Mikio Fujiwara, Takashi Yama...
quant-ph