A sketch of the optical arrangement used to generate deterministic single photons using Rydberg interactions.

A quantum many-particle system in which interactions can be tuned over a vast range may enable profound changes in the way we understand and explore physics of the microscopic realm. For example, it may lead to previously unknown phases of matter and aid in the discovery of new phenomena. Strong coupling allows for fast, controllable many-body dynamics, whereas the weakly interactingmode can be used for precise external manipulations andmeasurements. In the context of quantum information science, strong interactions are required to implement fast quantum gates, and long-term storage is achieved in the noninteracting regime.

Cold atomic gases are a fruitful platform for such studies. They permit one to perform experiments under well-understood and controlled conditions. Resonant optical driving of an atomic gas between the ground level and a high-lying Rydberg level is a particularly promising setting for studies of many-body systems with strong, long-range interactions. Highly excited Rydberg atoms have many exaggerated properties. In particular, the interaction strength between such atoms can be varied over an enormous range. In a mesoscopic ensemble, such strong, long-range interactions can be used for fast preparation of desired many-particle states. In our laboratories we generate Rydberg excitations in an ultra-cold atomic gas and convert them into light. For principal quantum numbers n beyond ~70, we are able to realize conditions where no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. This approach holds promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks.

An important manifestation of Rydberg-level interactions is excitation blockade, in which an atom promoted to a Rydberg level shifts the energy levels of nearby atoms, suppressing their excitation. Recently we have observed, for the first time, coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom. The results pave the way towards quantum computation and simulation using ensembles of atoms.

The above photo features the vacuum cell in whcih the cold Rb atoms are confined in an optical lattice for Rydberg excitation.

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