Polarization-independent frequency conversion from the infrared into the ultraviolet

In order to connect different quantum nodes as part of a quantum network, it is often necessary to bridge large spectral gaps through nonlinear frequency conversion as the individual nodes frequently operate at distinct wavelengths. Here, we demonstrate polarization-independent frequency conversion from 863 to 370 nm using a sum frequency generation process (SFG) in a periodically poled lithium niobate (PPLN) crystal. Our setup implements a Sagnac interferometer, which is modified to allow operation across a large wavelength range. We characterize the performance of the system in terms of conversion efficiency, spectral acceptance, and polarization preservation.