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 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 infor- mation processing, quantum communication, and precision measurement. While pioneering experiments have demonstrated the entanglement of two quantum nodes separated by up to 1 . 3km, 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 en- abling 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.