A March 30 Nature paper by the Faraon group describes recent work toward a quantum network. They discuss one of the current challenges, developingFaraon optical cavity ytterbium atoms a system that is stable:

Distributing entanglement over long distances using optical networks is an intriguing macroscopic quantum phenomenon with applications in quantum systems for advanced computing and secure communication. Building quantum networks requires scalable quantum light–matter interfaces based on atoms, ions or other optically addressable qubits. Solid-state emitters, such as quantum dots and defects in diamond or silicon carbide , have emerged as promising candidates for such interfaces. So far, it has not been possible to scale up these systems, motivating the development of alternative platforms. A central challenge is identifying emitters that exhibit coherent optical and spin transitions while coupled to photonic cavities that enhance the light–matter interaction and channel emission into optical fibres.

Read the full paper  Jonathan M. Kindem and Andrei Ruskuc and John G. Bartholomew and Jake Rochman and Yan Qi Huan and Andrei Faraon (2020) Control and single-shot readout of an ion embedded in a nanophotonic cavity Nature (2020).

This work is also featured in the Caltech media article Tiny Optical Cavity Could Make Quantum Networks Possible

“This [nanophotonic cavity] checks most of the boxes. It’s a rare-earth ion that absorbs and emits photons in exactly the way we’d need to create a quantum network,” says Faraon, professor of applied physics and electrical engineering. “This could form the backbone technology for the quantum internet.”