Observation of Rydberg blockade between two atoms

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Blockade interactions whereby a single particle prevents the flow or excitation of other particles provide a mechanism for control of quantum states, including entanglement of two or more particles. Blockade has been observed for electrons, photons and cold atoms. Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for 'Rydberg blockade', which might provide a novel approach to a number of quantum protocols. Dipolar interactions between Rydberg atoms were observed several decades ago and have been studied recently in a many-body regime using cold atoms. However, to harness Rydberg blockade for controlled quantum dynamics, it is necessary to achieve strong interactions between single pairs of atoms. Here, we demonstrate that a single Rydberg-excited rubidium atom blocks excitation of a second atom located more than 10 μm away. The observed probability of double excitation is less than 20%, consistent with a theoretical model of the Rydberg interaction augmented by Monte Carlo simulations that account for experimental imperfections.