title: Evidence for coherent collective Rydberg excitation in the strong blockade regime.
abstract: We report on strong van der Waals blockade in two-photon Rydberg excitation of ultracold magnetically trapped 87Rb atoms. The excitation dynamics was investigated for a large range of densities and laser intensities and shows a full saturation and a strong suppression with respect to single-atom behavior. The observed scaling of the initial increase with density and laser intensity provides evidence for coherent collective excitation. This coherent collective behavior, that was observed for up to several thousand atoms per blockade volume, is generic for all mesoscopic systems which are able to carry only one single quantum of excitation.
authors.affiliation: 5. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany. r.heidemann@physik.uni-stuttgart.de
authors.forename: Rolf
authors.initials: R
authors.lastname: Heidemann
authors.forename: Ulrich
authors.initials: U
authors.lastname: Raitzsch
authors.forename: Vera
authors.initials: V
authors.lastname: Bendkowsky
authors.forename: Björn
authors.initials: B
authors.lastname: Butscher
authors.forename: Robert
authors.initials: R
authors.lastname: Löw
authors.forename: Luis
authors.initials: L
authors.lastname: Santos
authors.forename: Tilman
authors.initials: T
authors.lastname: Pfau