Something strange happens when the force of interaction between two particles is gradually weakened: Just as the last two-body bound state is on the verge of becoming unbound, an infinite series of loosely bound three-body states suddenly appears. The particle trios are held together only by the intricacies of quantum mechanics—not by covalent or ionic bonding or any of the other mechanisms that stabilize more familiar multiparticle systems. And they spend most of their time in a classically forbidden region of the potential.
Forty-five years have passed since Vitaly Efimov predicted those trimers that now bear his name.1 Efimov trimers eluded experimentalists until 2006, when Rudolf Grimm and colleagues at the University of Innsbruck in Austria inferred their existence in an ultracold cesium gas by monitoring the loss of atoms from an optical trap.2 (See Physics Today, April 2006, page 18.) A spike in the loss...
