Observation of Rydberg blockade between two atoms

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(1-3), photons(4) and cold atoms(5). Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for 'Rydberg blockade'(6,7), which might provide a novel approach to a number of quantum protocols(8-11). Dipolar interactions between Rydberg atoms were observed several decades ago(12) and have been studied recently in a many-body regime using cold atoms(13-18). 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 mu m 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.