Where atomic phenomena are concerned, three is not a crowd by any stretch, thanks to Efimov states. Postulated by Vitaly Efimov in 1970, the Efimov states are groups of three identical particles — they could be atoms or molecules — which form loosely bound entities known as resonances due to their short-lived nature.

This trio, or trimer, is reminiscent of Borromean rings, which are a set of three rings linked together in such a way that cutting any one of the rings and removing it causes the other two to fall apart. Similarly, Efimov states break up if any one atom or molecule is removed because the pair does not have the binding energy to stay together. Efimov states, in cesium gas cooled to 10 nanokelvin, were first observed by Rudolf Grimm and collaborators at Innsbruck, Austria, in 2006.

Efimov not only predicted the existence of these trimer states but also a scaling property of the corresponding excited states Thus, given a basic configuration of trimer, there would be a first excited state whose size is larger than the original by a factor of 22.7. It would have a binding energy that was smaller by a factor of 22.7 .

Researchers have now been able to see the evidence of the first order excited states of an Efimov triplet of cesium atoms in the lab. The results were published recently by Bo Huang et. al. in the journal Physical Review Letters. They observed a scale factor of 21.1 which is really close to the predicted one of 22.7.

These states were hard to construct in the lab. They required ultracold atoms, whose temperatures were in the order of a few nanokelvin. The authors cooled the gas to 7 nK and held them in an optical trap. By using a magnetic field they adjusted the interaction energy between the atoms and kept measuring the number of atoms escaping the trap.

The states were observed indirectly, fact by their decay. These states are fairly short-lived. When they decay, they send out highly energetic particles that escape from the trap at high speed.

At a certain value of the magnetic field, there was a significant peak in the number of atoms lost from the trap, and this was seen as a signature that the Efimov states had formed.

Apart from being conceptually important, as they offer a new way to form aggregates of particles, Efimov states may also form new types of superconducting matter.

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