Novel interactions in quantum gases
Prof. Dr. Tilman Pfau (5. Physikalisches Institut, Universität Stuttgart)

Interactions in atomic quantum gases make them a model system for many branches of physics including condensed matter and nonlinear dynamics. So far all the impressive phenomena in atomic Bose and Fermi gases (like superfluidity, soliton and vortex formation, Mott insulator transition, BEC-BCS crossover etc.) are caused by a contact interaction, originating from s-wave scattering off the van der Waals potential.

Here we report on our experiments on Bose Einstein condensation in a gas of chromium atoms [1] with a sizable magnetic dipolar interaction. By the use of Feshbach resonances [2] the contact interaction can be suppressed such that the dipolar interaction becomes dominant. We report on the investigation of the resulting quantum ferro fluid [3], the determination of the stability diagram for dipolar gases and the observation of the dipolar collapse.
We also report on our experiments on strongly interacting ultracold Rydberg atoms excited in a Rb Bose-Einstein condensate [4] or a thermal cloud. We observed a blockade of the Rydberg excitation due to the strong repulsive van der Waals interaction of Rydberg atoms in the 43 s state. We confirm the coherent collective nature of the excitation by the measurement of the scaling laws [4]. We confirm the coherence of excitation by reversing the excitation by a  phase-shift in the driving laser field, a so called rotary echo sequence [5]. Therefore coherent many body physics far from equilibrium can now be studied in this system.