Suppression of Excitation and Spectral Broadening Induced by   Interactions in a Cold Gas of Rydberg Atoms
  We report on the observation of ultralong range interactions in a gas of cold Rubidium Rydberg atoms. The van-der-Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: Spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S- and P- Rydberg states with main quantum numbers n ~ 60 and n ~ 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade. 
Kilian Singer, Markus Reetz-Lamour, Thomas Amthor, Luis Gustavo   Marcassa, Matthias Weidem\"uller