Mesoscopic coherent quantum dynamics in the strong blockade regime

Blockade effects on the single quantum level are at the heart of quantum devices like single-electron transistors. The blockade mechanisms are based on strong interactions like the Coulomb interaction in case of single electrons. Neutral atoms excited into a Rydberg state experience abnormally strong interactions that lead to the corresponding blockade effect for single Rydberg atoms. In this paper we report on measurements of a full van der Waals blockade, showing that only one out of several thousand atoms within a blockade volume can be excited. In addition, our experimental results clearly demonstrate the coherent nature of the excitation of magnetically trapped ultracold atoms into a Rydberg state, confirming for the first time the predicted dependency of the collective Rabi frequency on the square root of the mesoscopic system size. This collective coherent behaviour is generic for all mesoscopic systems which are able to carry only one single excitation quantum.