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H-C NaÈ gerl of Innsbruck University in Austria and his colleagues have for the first time observed three-particle bound quantum states whose existence was predicted theoretically by V I Efimov in 1970 already [Yadernaya Fizika 12 1080 (1970)] Ð in a rare example of the complex three-body problem allowing an exact analytical solution. A universal set of low-energy bound states appears for a system of three identical particles with a strong enough resonant two-body interaction. Interestingly, because of the quantum-mechanical nature of this effect, three-particle states can even exist in the absence of two-particle bound states. The size of the threeparticle system is much less than the absolute scattering length jaj, and the three-particle interactions are universal in character and independent of precisely how two particles interact when close together. The researchers studied in experiment a degenerate gas of caesium atoms held in an optical dipole trap at temperatures between 10 and 250 nK. The scattering length was controlled by a Feshbach resonance and could be varied over a wide range by tuning the external magnetic field. For a < 0, the Efimov states were identified by a sharp increase in atomic recombination losses in the trapÐ presumably due to the three-atom system rapidly decaying from the trimer state to the state involving a deeply bound dimer state plus a free atom. At a temperature of 10 nK, Efimov states appeared when the scattering length reached ÿ850 Bohr radii. Another type of Efimov state appeared for a > 0. In this case, recombination losses are much lower and exhibit in their spectrum a recombination minimum due to the destructive interference of the three-particle system's quantum decay channels. The position of the minimum agrees well with theoretical predictions. For a > 0, Efimov states arise due to Feshbach resonances in collisions between single atoms and dimers and can be interpreted as a threebody generalization to the Feshbach resonance concept. Source: Nature 440 315 (2006) http://arxiv.org/abs/cond-mat/0512394