Colloquium: Cavity-enhanced quantum network nodes

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 nonlocal 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. This Colloquium describes how optical resonators facilitate quantum network nodes that achieve the aforementioned prerequisites in different physical systems (trapped atoms, defect centers in wide-band-gap semiconductors, and rare-earth dopants) by enabling high-fidelity qubit initialization and readout, efficient generation of qubit-photon and remote qubit-qubit entanglement, and quantum gates between stationary and flying qubits. These advances open a realistic perspective toward the implementation of global-scale quantum networks in the near future.