Predicted 30 years ago by Russian physicist Vitali Efimov, these quantum states consist of three atoms that are loosely bound together despite the absence of bound states of any two pairs of atoms in the system. This counterintuitive situation, Efimov argued, could occur for both bosons, that is, atoms that have integer values of intrinsic angular momentum or spin, or fermions -- atoms that have half-integer spin. (However, the Efimov effect would be absent for a system of three identical fermions if they were all in the same internal spin state). Despite numerous searches, no such states had been seen until now.

Hanns-Christoph Nägerl of Innsbruck University and colleagues first cooled a gas of caesium atoms to just 10 nK (Nature 440 315). They then used a magnetic field to carefully tune the "scattering length" -- a measure of the interaction strength -- between the atoms. Controlling the scattering length is crucial because Efimov states are only expected to form at lengths greater than the range of the interaction between just two atoms. The team observed an Efimov-like state resonance at a scattering length of about -850a0, where a0 is the Bohr radius (a standard unit of distance in atomic physics that measures 0.53 Angstroms).

"Efimov states present a particularly simple solution to the notoriously difficult three-body problem," explains Nägerl. "We hope that our measurements will trigger more refined theoretical and experimental work that will lead to a deeper understanding of this problem." The researchers now hope to confirm their results by measuring more than one Efimov resonance in their experiment, and also plan to study the properties of the three-body states in more detail.