  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 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 due to the Rydberg blockade, and a propagating light pulse
containing only a few photons. The strong light-matter 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. The demonstrated coupling strength
opens the way towards interfacing photonic and atomic qubits and preparation of
propagating non-classical states of light, two crucial building blocks in
future quantum networks.
