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Observation of an Efimov spectrum in an atomic system

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In 1970, Vitaly Efimov predicted that three quantum particles subjected to a resonant pair-wise interaction can join into an infinite number of loosely bound states, even if each pair of particles cannot bind. The properties of these aggregates, such as the peculiar geometric scaling of their energy spectrum, are universal, that is, independent of the microscopic details of their components. Despite an extensive search in many different physical systems, including atoms, molecules and nuclei, the characteristic spectrum of Efimov trimer states has not been observed so far. Here, we report on the discovery of two bound trimer states of potassium atoms very close to the Efimov scenario, which we reveal by studying three-particle collisions in an ultracold gas. Our observation provides the first evidence of an Efimov spectrum and enables a direct test of its scaling behaviour, providing potentially general insights into the physics of few-body systems.

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Figure 1: Efimov spectrum.
Figure 2: Atom–dimer resonances at a>0.

Change history

  • 16 July 2009

    In the version of this article originally published online, a correction was needed to Fig. 1a, where E2 should have read E2 . This correction has now been made in all versions of the article.


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We thank P. Massignan and H. Stoof for motivating this work and for discussions. We acknowledge also B. D. Esry, J. P. D’Incao, R. Grimm and co-workers, and A. Simoni for discussions. We are grateful to S. Bartalini and G. Thalhammer for technical help. This work has been supported by CNR within the EUROCORES Programme EuroQUAM of the European Science Foundation, by the European Research Council through the Starting Grant ‘QUPOL’ and by the Italian MIUR (PRIN2006 programme).

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Zaccanti, M., Deissler, B., D’Errico, C. et al. Observation of an Efimov spectrum in an atomic system. Nature Phys 5, 586–591 (2009).

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