  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 \unit[780]{nm} to telecom wavelengths.
We show the preservation of polarization entanglement after QFC (fidelity to
Bell state $F_{\phi^+, conv}=0.972\pm0.003$) of the biexciton transition. As a
step towards real-world applicability, high entanglement fidelities
($F_{\phi^+, loop}=0.945\pm0.005$) after the propagation of one photon of the
entangled pair along a \unit[35.8]{km} field installed standard single mode
fiber link are reported. Furthermore, we successfully demonstrate a second
polarization-conversing QFC step back to \unit[780]{nm} preserving entanglement
($F_{\phi^+, back}=0.903\pm0.005$). This further prepares the way for
interfacing quantum light to various quantum memories.
