  A future quantum network will consist of quantum processors that are
connected by quantum channels, just like conventional computers are wired up to
form the Internet. In contrast to classical devices, however, the entanglement
and non-local correlations available in a quantum-controlled system facilitate
novel fundamental tests of quantum theory. In addition, they enable numerous
applications in distributed quantum information processing, quantum
communication, and precision measurement.
  While pioneering experiments have demonstrated the entanglement of two
quantum nodes separated by up to 1.3 km, and three nodes in the same
laboratory, accessing the full potential of quantum networks requires scaling
of these prototypes to many more nodes and global distances. This is an
outstanding challenge, posing high demands on qubit control fidelity, qubit
coherence time, and coupling efficiency between stationary and flying qubits.
  In this work, I will describe how optical resonators facilitate quantum
network nodes that achieve the above-mentioned prerequisites in different
physical systems -- trapped atoms, defect centers in wide-bandgap
semiconductors, and rare-earth dopants -- by enabling high-fidelity qubit
initialization and readout, efficient generation of qubit-photon and remote
qubit-qubit entanglement, as well as quantum gates between stationary and
flying qubits. These advances open a realistic perspective towards the
implementation of global-scale quantum networks in the near future.
