No abstract

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 interaction between a single emitter and individual photons is a fundamental process in nature [1], underlying many phenomena such as vision and photosynthesis as well as applications including imaging, spectroscopy or optical information processing and communication. In the strong coupling limit, where the coherent interaction between a single photon and an individual emitter exceeds all decoherence and loss rates, a single emitter can function as interface between stationary and flying qubits, a central building block for future quantum networks [2,3]. Such a quantum optical node is able to mediate effective photon-photon interactions, thus enabling deterministic all-optical quantum gates [4][5][6].One groundbreaking scheme to achieve strong coupling is the use of electromagnetic (EM) cavities, where the photons are trapped within the finite volume of a high-finesse resonator. The physics of these systems is captured by the seminal Jaynes-Cummings model [7], which has been experimentally realized and extensively studied in atomic cavity quantum electrodynamics (QED) [8] and more recently in circuit QED systems combining on-chip microwave resonators with superconducting two-level systems [9,10]. Achieving a strong interaction between a propagating photon and a single emitter opens the possibility to realize novel quantumoptical devices where atoms process photonic qubits on the fly and facilitate the preparation of non-classical states of light [11]. However, mode matching between the input field and the dipolar emission pattern of the quantum emitter in free space is challenging and has so far limited the achievable coupling strength [12][13][14]. Waveguide QED systems seek to overcome this limitation by transversely confining the propagating EM mode coupled to one or more emitters [15][16][17][18][19][20][21].Here we report on the realization of coherent coupling between a propagating few-photon optical field and a single Rydberg superatom in free space. By exploiting the Rydberg blockade effect in an atomic ensemble [22][23][24][25], which allows at most a single excitation shared among all N constituents, we turn ∼ 10 4 individual ultra...