Observation of an Efimov spectrum in an atomic system

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.