This work was supported by the European Union's Horizon 2020 research and innovation program under grant agreement No. 820445 and project name "Quantum Internet Alliance," by Projects F 7109 and Y 849 of the Austrian Science Fund (FWF), and by the U.S. Army Research Laboratory's Center for Distributed Quantum Information under Cooperative Agreement Number W911NF-15-2-0060. We acknowledge funding for S.B. by an FWF Erwin Schrödinger fellowship (No. J 4229), for V. Krutyanskiy by the Erwin Schrödinger Center for Quantum Science & Technology (ESQ) Discovery Programme, for B.P.L. by the CIFAR Quantum Information Science Program of Canada, for A.M. by the U.S. National Science Foundation under Grant No. PHY- 1915130, for N.S. by the Commissariat a' l'Energie Atomique et aux Energies Alternatives (CEA), and for M.T. by the Early Stage Funding Program of the Vice-Rectorate for Research of the University of Innsbruck.

We report on an elementary quantum network of two atomic ions separated by 230 m. The ions are trapped in different buildings and connected with 520(2)m of optical fiber. At each network node, the electronic state of an ion is entangled with the polarization state of a single cavity photon; subsequent to interference of the photons at a beamsplitter, photon detection heralds entanglement between the two ions. Fidelities of up to (88.2+2.3−6.0)% are achieved with respect to a maximally entangled Bell state, with a success probability of 4×10−5. We analyze the routes to improve these metrics, paving the way for long-distance networks of entangled quantum processors.