Telecom Quantum Photonic Interface for a $^{40}$Ca$^+$ Single-Ion Quantum Memory

Entanglement-based quantum networks require quantum photonic interfaces between stationary quantum memories and photons, enabling entanglement distribution. Here we present such a photonic interface, designed for connecting a $^{40}$Ca$^+$ single-ion quantum memory to the telecom C-band. The interface combines a memory-resonant, cavity-enhanced spontaneous parametric down-conversion (SPDC) photon pair source with bi-directional polarization-conserving quantum frequency conversion (QFC). We demonstrate preservation of high-fidelity entanglement during conversion, fiber transmission over up to 40 km and back-conversion to the memory wavelength. Even for the longest distance and bi-directional conversion the entanglement fidelity remains larger than 95 % (98 %) without (with) background correction.