  Entanglement between stationary quantum memories and photonic channels is the
essential resource for future quantum networks. Together with entanglement
distillation it will enable for efficient distribution of quantum states. Here
we report on the generation and observation of entanglement between a Rb-87
atom and a photon at telecom wavelength over 20 km optical fiber. For this
purpose, we use polarization-preserving quantum frequency conversion to
transform the wavelength of a photon entangled with the atomic spin state from
780 nm to the telecom S-band at 1522 nm. We achieve an unprecedented external
device conversion efficiency of 57% and observe an entanglement fidelity
between the atom and telecom photon of $\geqslant$78.5$\pm$0.9% over 20 km
optical fiber, mainly limited by decoherence of the atomic state. This result
is an important milestone on the road to distribute quantum information on a
large scale.
