Large-scale quantum networks have the potential to revolutionize computation and information transfer. The frequency conversion of single photons has crucial roles to play in forming hybrid light-matter networks, primarily as a transducer linking shorter wavelengths typically used for processing and storage in atomic and solid-state systems with the fiber telecommunications bands, as well as enabling devices that operate at different wavelengths to share quantum information. In this paper, the authors report a sum-frequency generation (SFG) interface from 1560 nm in the telecoms C band to 795 nm, a convenient wavelength for storage in quantum memories. By splitting horizontal and vertical polarization components and up-converting each separately within a stabilized interferometer, the authors demonstrate the preservation of polarization entanglement upon conversion. Raman noise is suppressed with a free-space cavity filter and the majority of components are fiber-integrated, enabling the conversion of single photons with high signal-to-noise ratio and overall efficiency up to 4%, about two orders of magnitude higher than achieved in previous works.
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