The distribution of entanglement between the nodes of a quantum network will allow new advances e.g. in long distance quantum communication, distributed quantum computing and quantum sensing. The realization of large-scale quantum networks will require quantum repeaters. The nodes of a quantum repeater are matter systems that should efficiently interact with quantum light, allow entanglement with photons (ideally at telecommunication wavelengths) and serve as a quantum memory allowing long-lived, faithful and multiplexed storage of (entangled) quantum bits. [more]