Dear Editor,

please find enclosed our manuscript on "Single atoms coupled to a near-concentric cavity".

While strong interaction between single atoms and single photons by means of
cavity quantum electrodynamics (cavity-QED) is well established and  has been
applied in many important proof-of-principle experiments over the last two
decades, such experiments stayed very demanding due to the use of very small
optical resonators of high finesse. Despite its wide promise for quantum
infomation processing purposes, this hindered widespread experimental adoption
in atomic physics.

In this work, we present an alternative approach to implement cavity-QED by
coupling the atom to a concentric cavity, providing the large electric field
for strong atom-light interaction in a tight focus. Such an approach has
recently gained renewed interest in the experimental community, and may even
help to make cavity-QED possible for species like ions and Rydberg atoms,
which are inherently difficult to place in cavities leading to strong
coupling.

The experimental work presented in this manuscript marks only the beginning of
this technique, as the parameters like cavity finesse (we have about 138)  are
still far away from what is technically possible, and could open many
atom-photon experiments in the strong coupling regime in the optical domain.

Therefore, we feel that the manuscript could be interesting to the audience of
Physical Review Letters. 


With Best Regards on behalf of all authors,

Christian Kurtsiefer