1. Institut für Experimentalphysik, Universität Innsbruck, Technikerstrae 25, A–6020 Innsbruck, Austria
2. James Franck Institute, Physics Department of the University of Chicago, 5640 S. Ellis Avenue Chicago, Illinois 60637, USA
3. Institut für Quantenoptik und Quanteninformation der Österreichischen Akademie der Wissenschaften, Otto-Hittmair-Platz 1, A–6020 Innsbruck, Austria

Correspondence to: H.-C. Nägerl¹ Correspondence and requests for materials should be addressed to H.-C.N. (Email: christoph.naegerl@ultracold.at).

Abstract

Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov's prediction^{1, 2} of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov's problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics^{3, 4, 5, 6, 7, 8}. However, the observation of Efimov quantum states has remained an elusive goal^{3, 5}. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss^{9, 10} when the strength of the two-body interaction is varied. We also detect a minimum^{9, 11, 12} in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems⁷. While Feshbach resonances^{13, 14} have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter¹⁵ to the world of few-body quantum phenomena.

1. Institut für Experimentalphysik, Universität Innsbruck, Technikerstrae 25, A–6020 Innsbruck, Austria
2. James Franck Institute, Physics Department of the University of Chicago, 5640 S. Ellis Avenue Chicago, Illinois 60637, USA
3. Institut für Quantenoptik und Quanteninformation der Österreichischen Akademie der Wissenschaften, Otto-Hittmair-Platz 1, A–6020 Innsbruck, Austria

Correspondence to: H.-C. Nägerl¹ Correspondence and requests for materials should be addressed to H.-C.N. (Email: christoph.naegerl@ultracold.at).

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