High fidelity distribution of triggered polarization-entangled telecom photons via a 36km intra-city fiber network

Fiber-based distribution of triggered, entangled, single-photon pairs is a key requirement for the future development of terrestrial quantum networks. In this context, semiconductor quantum dots (QDs) are promising candidates for deterministic sources of on-demand polarization-entangled photon pairs. So far, the best QD polarization-entangled-pair sources emit in the near-infrared wavelength regime, where the transmission distance in deployed fibers is limited. Here, to be compatible with existing fiber network infrastructures, bi-directional polarization-conserving quantum frequency conversion (QFC) is employed to convert the QD emission from [780]nm to telecom wavelengths. We show the preservation of polarization entanglement after QFC (fidelity to Bell state F_ϕ^+, conv=0.972±0.003) of the biexciton transition. As a step towards real-world applicability, high entanglement fidelities (F_ϕ^+, loop=0.945±0.005) after the propagation of one photon of the entangled pair along a [35.8]km field installed standard single mode fiber link are reported. Furthermore, we successfully demonstrate a second polarization-conversing QFC step back to [780]nm preserving entanglement (F_ϕ^+, back=0.903±0.005). This further prepares the way for interfacing quantum light to various quantum memories.