  We report on the conversion to telecom wavelength of single photons emitted
by a nitrogen-vacancy (NV) defect in diamond. By means of difference frequency
generation, we convert spin-selective photons at 637 nm, associated with the
coherent NV zero-phonon-line, to the target wavelength of 1588 nm in the
L-telecom band. The successful conversion is evidenced by time-resolved
detection revealing a telecom photon lifetime identical to that of the original
637 nm photon. Furthermore, we show by second-order correlation measurements
that the single-photon statistics are preserved. The overall efficiency of this
one-step conversion reaches 17\% in our current setup, along with a
signal-to-noise ratio of $\approx$7 despite the low probability $(< 10^{-3})$
of an incident 637 nm photon. This result shows the potential for efficient
telecom photon - NV center interfaces and marks an important step towards
future long-range entanglement-based quantum networks.
