  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.
