  A compact setup for two-way single-photon-level frequency conversion between
852 nm and 1560 nm has been implemented with the same periodically-poled
magnesium-oxide-doped lithium niobate (PPMgO:LN) bulk crystals for connecting
cesium D2 line (852 nm) to telecom C-band. By single-pass mixing a strong
continuous-wave pump laser at 1878 nm and the single-photon-level periodical
signal pulses in a 50-mm-long PPMgO:LN bulk crystal, the conversion efficiency
of ~1.7% ( ~1.9%) for 852-nm to 1560-nm down-conversion (1560-nm to 852-nm
up-conversion) have been achieved. We analyzed noise photons induced by the
strong pump laser beam, including the spontaneous Raman scattering (SRS) and
the spontaneous parametric down-conversion (SPDC) photons, and the photons
generated in the cascaded nonlinear processes. The signal-to-noise ratio (SNR)
has been improved remarkably by using the narrow-band filters and changing
polarization of the noise photons in the difference frequency generation (DFG)
process. With further improvement of the conversion efficiency by employing
PPMgO:LN waveguide, instead of bulk crystal, our study may provide the basics
for cyclic photon conversion in quantum network.
