In 1970, Vitaly Efimov, a Russian physicist, predicted a novel quantum-mechanical effect that three bosons can form an infinite number of long-range bound states even when the two-particle interaction is insufficient to support one quantum state [1]. Efimov states have attracted great interest in nuclear, few-body and helium physics, but the observation of Efimov quantum states has remained an elusive goal. We report the observation of an Efimov resonance in an ultracold gas of cesium atoms [2]. The resonance occurs in the range of large negative two-body scattering lengths, meaning two atoms are just shy of binding. This resonance arises from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as giant three-body recombination loss when the strength of the two-body interaction is controlled by magnetically-tuned Feshbach resonance. We also detect a minimum 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 an exciting starting point to explore the properties of few-body quantum systems.

[1] V. Efimov, "Energy levels arising from resonant two-body forces in a three-body system," Phys. Lett. B. 33, 563 (1970)
[2] T. Kraemer et al., "Evidence for Efimov quantum states in an ultracold gas of caesium atoms," Nature 440, 315 (2006)

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