Strong coupling between single atoms and non-transversal photons

We investigate the interaction between single quantum emitters and non-transversally polarised photons for which the electric field vector amplitude has a significant component in the direction of propagation. Even though this situation seems to be at odds with the description of light as a transverse wave, it regularly occurs when interfacing or manipulating emitters with non-paraxial, guided, or evanescent light. Here, we couple single rubidium atoms to single photons which are confined in an optical microresonator by continuous total internal reflection. We experimentally show that the non-transversal polarisation decisively alters the physics of strong light--matter interaction. This effect has far-reaching and highly advantageous implications, which, e.g., significantly advance the field of cavity quantum electrodynamics with whispering-gallery-mode resonators. In particular, our finding allows us to experimentally realise a paradigmatic quantum system, which is ideally suited for basic studies and technological applications.