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We report on strong van der Waals blockade in two-photon Rydberg excitation of ultracold magnetically trapped atoms and BECs. The excitation dynamics was investigated in detail and shows a full saturation and a strong suppression with respect to single atom behaviour. The observed scaling of the initial increase with density and laser intensity provides evidence for coherent collective excitation. Recently we studied in more detail the coherence of the excitation dynamics with a spin echo technique.
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