  Distributed quantum networks will allow users to perform tasks and to
interact in ways which are not possible with present-day technology. Their
implementation is a key challenge for quantum science and requires the
development of stationary quantum nodes that can send and receive as well as
store and process quantum information locally. The nodes are connected by
quantum channels for flying information carriers, i.e. photons. These channels
serve both to directly exchange quantum information between nodes as well as to
distribute entanglement over the whole network. In order to scale such networks
to many particles and long distances, an efficient interface between the nodes
and the channels is required. This article describes the cavity-based approach
to this goal, with an emphasis on experimental systems in which single atoms
are trapped in and coupled to optical resonators. Besides being conceptually
appealing, this approach is promising for quantum networks on larger scales, as
it gives access to long qubit coherence times and high light-matter coupling
efficiencies. Thus, it allows one to generate entangled photons on the push of
a button, to reversibly map the quantum state of a photon onto an atom, to
transfer and teleport quantum states between remote atoms, to entangle distant
atoms, to detect optical photons nondestructively, to perform entangling
quantum gates between an atom and one or several photons, and even provides a
route towards efficient heralded quantum memories for future repeaters. The
presented general protocols and the identification of key parameters are
applicable to other experimental systems.
