Interactions in an ultracold gas of Rydberg atoms

We review our recent experiments on interact ion-induced effects in a gas of Rydberg atoms excited from a laser-cooled rubidium cloud. In density-dependent measurements we see a broadening of excitation lines and an on-resonance suppression of excitation, which can be explained by the van-der-Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii. Rapid ionization of the highly excited, initially stationary atoms is also observed. The initial ionization process is attributed to interaction-induced collisions of cold Rydberg atoms and subsequent Penning ionization. We present 3D degenerate Raman sideband cooling as a tool to reach ultralow temperatures and to provide structuring of the cold atom cloud for future studies on ultracold Rydberg gases.