  The silicon-vacancy center in diamond holds great promise as a qubit for
quantum communication networks. However, since the optical transitions are
located within the visible red spectral region, quantum frequency conversion to
low-loss telecommunication wavelengths becomes a necessity for its use in
long-range, fiber-linked networks. This work presents a highly efficient,
low-noise quantum frequency conversion device for photons emitted by a
silicon-vacancy (SiV) center in diamond to the telecom C-band. By using a
two-stage difference-frequency mixing scheme SPDC noise is circumvented and
Raman noise is minimized, resulting in a very low noise rate of $10.4 \pm 0.7$
photons per second as well as an overall device efficiency of $35.6\, \%$. By
converting single photons from SiV centers we demonstrate the preservation of
photon statistics upon conversion.
