We report on the conversion to telecommunication wavelengths 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-telecommunication band. The successful conversion is evidenced by time-resolved detection revealing a telecommunication-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 approximately 7 despite the low probability (<10-3) of an incident 637-nm photon. This result shows the potential for efficient telecommunication-photon-NV-center interfaces and marks an important step towards future long-range entanglement-based quantum networks.

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We report on the fabrication and characterization of a Fabry-Perot microcavity enclosing a thin diamond membrane at cryogenic temperatures. The cavity is designed to enhance resonant emission of single nitrogen-vacancy centers by allowing spectral and spatial tuning while preserving the optical properties observed in bulk diamond. We demonstrate cavity finesse at cryogenic temperatures within the range of F ¼ 4000–12 000 and find a sub-nanometer cavity stability. Modeling shows that coupling nitrogen-vacancy centers to these cavities could lead to an increase in remote entanglement success rates by three orders of magnitude.@en