  Quantum internet will enable a number of revolutionary applications. It
relies on entanglement of remote quantum memories over long distances. Despite
enormous progresses so far, the maximal physical separation achieved between
two nodes is 1.3 km, and challenges for long distance remain. Here we make a
significant step forward by entangling two atomic ensembles in one lab via
photon transmission through metropolitan-scale fibers. We use cavity
enhancement to create bright atom-photon entanglement, and harness quantum
frequency conversion to shift the atomic wavelength to telecom. We realize
entanglement over 22 km field-deployed fibers via two-photon interference, and
entanglement over 50 km coiled fibers via single-photon interference. Our
experiment can be extended to physically separated nodes with similar distance
as a functional segment for atomic quantum networks, thus paving the way
towards establishing atomic entanglement over many nodes and over much longer
distance.
