  In quantum communications, vortex photons can encode higher-dimensional
quantum states and build high-dimensional 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 for HDCNs. 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.
