  Interfacing the different building blocks of a future large scale quantum
network will demand efficient and noiseless frequency conversion of quantum
light. Nitrogen-vacancy (NV) centers in diamond are a leading candidate to form
the nodes of such a network. However, the performance of a suitable converter
remains a bottleneck, with existing demonstrations severely limited by
parasitic noise arising at the target telecom wavelength. Here, we demonstrate
a new platform for efficient low-noise quantum frequency conversion based on a
monolithic bulk ppKTP cavity and show its suitability for the conversion of 637
nm single photons from NV centers in diamond to telecommunication wavelengths.
By resonantly enhancing the power of an off-the-shelf pump laser, we achieve an
internal conversion efficiency of $(72.3\pm 0.4)\%$ while generating only
$(110\pm 4) \mbox{ kHz/nm}$ noise at the target wavelength without the need for
any active stabilization. This constitutes a 5-fold improvement in noise over
existing state-of-the-art single-step converters at this wavelengths. We verify
the almost ideal preservation of non-classical correlations by converting
photons from a spontaneous parametric down-conversion source and moreover show
the preservation of time-energy entanglement via Franson interferometry.

Quantum Physics