Free-Space Quantum Electrodynamics with a Single Rydberg Superatom

The interaction of a single photon with an individual two-level system is the textbook example of quantum electrodynamics. Achieving strong coupling in this system has so far required confinement of the light field inside resonators or waveguides. Here, we demonstrate strong coherent coupling between a single Rydberg superatom, consisting of thousands of atoms behaving as a single two-level system because of the Rydberg blockade, and a propagating light pulse containing only a few photons. The strong lightmatter coupling, in combination with the direct access to the outgoing field, allows us to observe, for the first time, the effect of the interactions on the driving field at the single-photon level. We find that all our results are in quantitative agreement with the predictions of the theory of a single two-level system strongly coupled to a single quantized propagating light mode.