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).
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