  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 Rydberg atoms.
In this paper we report on our measurements of a strong 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 demonstrate the coherent
nature of the excitation of magnetically trapped ultracold atoms into a Rydberg
state, confirming the predicted dependence 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.
