Strong Coupling of a Single Trapped Atom to a Whispering-Gallery-Mode Microresonator

The interaction between an atom and photons can be strongly enhanced by coupling the atom to an optical microresonator with high quality factor. Whispering-gallery-mode (WGM) resonators are particularly interesting for this purpose, as they offer chiral, i.e. propagation-direction-dependent, light-matter interaction [1], which enables novel protocols for processing light on the quantum level. However, coupling trapped atoms to WGM resonators has so far been an elusive goal.
In our experiment, we trap a single atom at a distance of 200 nm from the surface of a WGM bottle microresonator using a standing-wave optical dipole trap that is created by retroreflecting a focused light field from the resonator surface [2]. The concomitant position-dependent light shift detunes the atomic transition from the resonator. We counteract this light shift by superposing a compensation light with the dipole trap that shifts the atomic transition back into resonance [3]. Using this method, we observe a vacuum Rabi-splitting in the excitation spectrum of the coupled system, which demonstrates that we reach the strong coupling regime.

[1] C. Junge et al., PRL 110, 213604 (2013)
[2] E. Will et al., arXiv:2010.07267 (2020)
[3] A. P. Hilton et al., PRA 11, 024065 (2019)