The Efimov effect is an effect in the quantum mechanics of Few-body systems predicted by the Soviet theoretical physicist V. N. Efimov[1][2] in 1970. Efimov’s effect refers to a scenario in which three identical bosons interact, with the prediction of an infinite series of excited three-body energy levels when a two-body state is exactly at the dissociation threshold. One corollary is that there exist bound states (called Efimov states) of three bosons even if the two-particle attraction is too weak to allow two bosons to form a pair.
In 2005, for the first time the research group of Rudolf Grimm und Hanns-Christoph Nägerl from the Institute of experimental physicists (University of Innsbruck, Austria) experimentally confirmed such a state in an ultracold gas of caesium atoms. In 2006 they published their findings in the scientific journal Nature.[3] Further experimental proof for the existence of Efimov state has been given recently by independent groups.[4]. The characteristic universal scaling factor (22.7) of the states has also been confirmed[5][6] almost 40 years after Efimov's purely theoretical prediction.
The interest in the "universal phenomena" of cold atomic gases is still growing, especially because of the long waited experimental results.[7][8] The discipline of universality in cold atomic gases nearby the Efimov states are sometimes commonly referred to as "Efimov physics".
The unusual Efimov state has an infinite number of similar states. These states are completely identical except that their sizes and energy levels scale by a universal factor of 22.7 (in the case of three identical bosonic particles).
The Efimov states are independent of the underlying physical interaction, and can in principle be observed in all quantum mechanical systems (molecular, atomic, and nuclear). The states are very special because of their "non-classical" nature: The size of each three particle Efimov state is much larger than the force-range between the individual particle pairs. This means that the state is purely quantum mechanical. Similar phenomena are observed in two-neutron halo-nuclei, such as lithium-11. (Halo nuclei could be seen as special Efimov states, depending on the subtle definitions.)
A (three-particle) Efimov state where the (two-body) sub-systems are unbound, are often depicted symbolically by the Borromean rings. This means that if one of the particles are removed, the remaining two fall apart. In this case the Efimov state is also called a Borromean state.