Telecom Quantum Photonic Interface for a 40Ca+ Single-Ion Quantum Memory

Entanglement-based quantum networks require quantum photonic interfaces between station-
ary quantum memories and photons, enabling entanglement distribution. Here we present such
a photonic interface, designed for connecting a 40Ca+ single-ion quantum memory to the tele-
com 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 con-
version, 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.