Strong interaction between single atoms and photons via cavity quantum
electrodynamics has been applied many times, including first
demonstrations of quantum networks. However, the small optical resonators of
high finesse used there make scaling of quantum networks very challenging.

Here, we present experimental results on an alternative approach using a
concentric cavity. The tightly focused mode leads to strong atom-photon
interaction at a macroscopic cavity length, while relaxing demands for a high
finesse.

This is also relevant for many quantum systems, in particular for ions and
Rydberg atoms which are difficult to incorporate in small cavities.