In quantum communications, vortex photons can encode higher-dimensional quantum states and build highdimensional communication networks (HDCNs). The interfaces that connect different wavelengths are significant in HDCNs. We construct a coherent orbital angular momentum (OAM) frequency bridge via difference frequency conversion in a nonlinear bulk crystal. Using a single resonant cavity, maximum quantum conversion efficiencies from visible to infrared are 36%, 15%, and 7.8% for topological charges of 0,1, and 2, respectively. The average fidelity obtained using quantum state tomography for the down-converted infrared OAM-state of topological charge 1 is 96.51%. We also prove that the OAM is conserved in this process by measuring visible and infrared interference patterns. This coherent OAM frequency-down conversion bridge represents a basis for an interface between two high-dimensional quantum systems operating with different spectra.

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Shi-long Liu, Zhi-yuan Zhou, Shi-kai Liu, Yin-hai Li, Shuai Shi, and Bao-sen Shi, "Coherent frequency bridge between visible and telecommunications band for vortex light," Proc. SPIE 10771, Quantum Communications and Quantum Imaging XVI, 107710N (Presented at SPIE Optical Engineering + Applications: August 20, 2018; Published: 18 September 2018); https://doi.org/10.1117/12.2320020.