Three interacting particles form a system which is well known for its complex physical behavior. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of weakly bound trimer states appearing for three identical bosons with a resonant pairwise interaction [1]. While Efimov's scenario has attracted great interest in many areas of physics, an experimental evidence was missing so far. In our talk we report the observation of an Efimov resonance in an ultracold thermal gas of Cs atoms [2]. For Cs atoms the binding energy of a weakly bound dimer state can be magnetically tuned near a Feshbach resonance, giving access to the regime of resonant two-body interaction which can be either attractive or repulsive. The Efimov resonance occurs in the range of large attractive two-body interaction and arises from the coupling of three free atoms to an Efimov trimer. We observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is tuned. In addition, we find a scattering resonance in ultracold collisions between atoms and weakly bound dimer molecules as the dimer binding energy is tuned. It is likely that this resonance arises from the coupling of the dimer and an atom to an Efimov trimer, thus providing a direct link between the weakly bound dimer state and the appearance of Efimov states.

[1] V. Efimov, Phys. Lett. 33B, 563 (1970).
[2] T. Kraemer, M. Mark, P. Waldburger, J. G. Danzl, C. Chin, B. Engeser, A. D. Lange, K. Pilch, A. Jaakkola, H.-C. Nägerl, R. Grimm, Nature 440, 315 ( 2006).