Unusual triumvirate

Institut für Experimentalphysik,
University of Innsbruck, and
Austrian Academy of Sciences,
Innsbruck, Austria

For the first time physicists observe Efimov states resulting from atom-molecule coupling

Three years ago experimental physicists from the University of Innsbruck attracted international attention when they demonstrated the existence of Efimov states. Now they found an alternative way to prove the existence of these elusive few-body quantum states and could thus validate a central aspect of the theory on Efimov states. They report on their results in the journal "Nature Physics".

In the beginning of the 1970s the Russian physicist Vitali Efimov proposed that three quantum particles could form a compound object even when pairwise binding was not possible. He had in fact found a comparatively simple quantum solution to the famous three-body problem, a solution for which there is no classical counterpart. Efimov's theory was frowned upon by theoretical physicists, and experiments for a long time could not show the existence of Efimov states. In 2006, a group led by the Innsbruck physicists Rudolf Grimm and Hanns-Christoph Nägerl could break the spell by finding experimental evidence for the existence of Efimov states. "Already in 2002 we had seen strange features in our data," says Grimm. "After having successfully completed a series of experiments with Bose-Einstein condensates we came back to these open questions, with a surprising result." A crucial part for the success of the Innsbruck experiments was the capability to control the strength of the atom interaction to optimize conditions for three-particle binding according to Efimov.

Efimov states resulting from atom-molecule coupling

"We have now, for the first time, been able to show that Efimov states can also result from the coupling of an atom to a dimer molecule" Francesca Ferlaino from Italy and Steven Knoop from the Netherlands explain. Both have been active as junior researchers in the Innsbruck group for three years now. In their experiments the Innsbruck scientists manipulate an ultracold gas of Cesium atoms at temperatures just a few billionths of a degree above absolute zero. In this gas, they associate a certain fraction of the atoms to dimer molecules. They then observe collisions between the atoms and the newly formed molecules. An Efimov trimer state manifests itself as enhanced particle loss, as a result of an atom-dimer collision, when an external magnetic field brings the trimer state into resonance with the free atom-dimer state.


Illustration by Jose D'Incao

Enhancing the understanding of complex systems

"In view of the previously observed three-atom resonance we now obtain complementary information from the atom-dimer resonance," Ferlaino and Knoop explain. It is now possible to evaluate the universal validity of Efimov's theory. "We find that Efimov's theory is qualitatively correct, but some details have to be corrected for realistic systems," Rudolf Grimm summarizes. "We thus expect that our findings will stimulate many more investigations into realistic systems that show Efimov's effect." A better understanding of Efimov states could pave the way to solutions for more complex few-body systems. In general, a full quantum treatment of a few-body system is considered to be very difficult, and simplifying assumptions are usually made. The Innsbruck experiments are expected to provide a new stimulus for this field of physics.


Observation of an Efimov-like trimer resonance in ultracold atom-dimer scattering
S. Knoop, F. Ferlaino, M. Mark, M. Berninger, H. Schöbel, H.-C. Nägerl, R. Grimm
Nature Physics 5, 227 (2009)


This work has been carried out in the LevT experiment.


from left to right: Michael Mark, Martin Berninger, Hanns-Christoph Nägerl,
Francesca Ferlaino, Rudi Grimm, Steven Knoop
(not in the picture: Harald Schöbel)

Further links

German version

Press release (in German)

Pictures to download


The experiment is supported by the Austrian Science Fund (FWF) within No. SFB 15 (project part 16).


last change: 24-02-2009 by SK