Mapping the strong interaction between Rydberg atoms onto single photons via electromagnetically induced transparency enables manipulation of light at the single-photon level and few-photon devices such as all-optical switches and transistors operated by individual photons. Here we demonstrate experimentally that Stark-tuned Förster resonances can substantially increase this effective interaction between individual photons. This technique boosts the gain of a single-photon transistor to over 100, enhances the non-destructive detection of single Rydberg atoms to a fidelity beyond 0.8, and enables high-precision spectroscopy on Rydberg pair states. On top, we achieve a gain larger than 2 with gate photon read-out after the transistor operation. Theory models for Rydberg polariton propagation on Förster resonance and for the projection of the stored spin-wave yield excellent agreement to our data and successfully identify the main decoherence mechanism of the Rydberg transistor, paving the way towards photonic quantum gates.

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...

We report on the realization of a free-space single-photon absorber, which deterministically absorbs exactly one photon from an input pulse. Our scheme is based on the saturation of an optically thick medium by a single photon due to Rydberg blockade. By converting one absorbed input photon into a stationary Rydberg excitation, decoupled from the light fields through fast engineered dephasing, we blockade the full atomic cloud and change our optical medium from opaque to transparent. We show that this results in the subtraction of one photon from the input pulse over a wide range of input photon numbers. We investigate the change of the pulse shape and temporal photon statistics of the transmitted light pulses for different input photon numbers and compare the results to simulations. Based on the experimental results, we discuss the applicability of our single-photon absorber for number resolved photon detection schemes or quantum gate operations.The elementary operation of subtracting exactly one photon from an arbitrary light pulse is of great interest for testing fundamental concepts of quantum optics [1,2] as well as for the preparation of non-classical states of light for quantum information [3][4][5][6], simulation [7][8][9], and metrology protocols [10]. Heralded single-photon subtraction has been realized by monitoring the weak reflection of a highly imbalanced beam splitter, where a single detection event corresponds to subtraction of a photon from the transmitted pulse [1,11]. For sufficiently low reflectivity such that the subtraction of two or more photons becomes negligible, this procedure implements the photon annihilation operatorα [1]. This operation is inherently probabilistic, with the success rate depending on the number of incoming photons. In contrast, deterministic single-photon subtraction, where always exactly one photon is removed independent of the input photon state, can be implemented by sending the light through a medium saturable by a single absorption event. One realization of such a single-photon absorber is a single 3-level quantum emitter strongly coupled to an optical resonator [12,13], as recently demonstrated by Rosenblum et al. using a single atom coupled to a microsphere resonator [14].Here we demonstrate the experimental realiziation of a deterministic free-space single-photon absorber [15], which is based on the saturation of an optically thick free-space medium by a single photon due to Rydberg blockade [16]. Single-photon subtraction adds a new component to the growing Rydberg quantum optics toolbox [17][18][19][20], which already contains photonic logic building-blocks such as single-photon sources [21], switches [22], transistors [23][24][25], and conditional π-phase shifts [26]. Our approach is scalable to multiple cascaded absorbers, essential for preparation of non-classical light states for quantum information and metrology applications [5,11,27], and, in combination with the singlephoton transistor, high-fidelity number-resolved photon detection [15,28,29].Any pr...