The Efimov Effect: Three’s Company, Two's a Crowd

At the April APS meeting in Jacksonville, physicists discussed the recent observations of the Efimov effect, a purely quantum phenomenon whereby two particles such as neutral atoms which ordinarily do not interact strongly with one another join together with a third atom under the right conditions. The trio can then form an infinite number of configurations, or put another way, an infinite number of "bound states" that hold the atoms together.

The effect was first predicted around 1970 by a physicist named Vitaly Efimov, then a Ph.D. candidate at the time, but was originally considered "too strange to be true," according to the University of Colorado’s Chris Greene, in part because the atoms would abruptly switch from being standoffish to becoming stuck-together Siamese Triplets at remarkably long distances from one another (approximately 500-10,000 times the size of a hydrogen atom in the case of neutral atoms). For decades, experimenters tried in vain to create these three-particle systems (which came to be known as "Efimov trimers").

In 1999, Greene and his collaborators Brett Esry and Jim Burke predicted that gases of ultracold atoms might provide the right conditions for creating the three-particle state. In 2005, a research team led by Rudi Grimm of the University of Innsbruck in Austria finally confirmed the Efimov state in an ultracold gas of cesium cooled to just 10 nanokelvin.

How do the neutral atoms attract one another in the first place? At small distances, ordinary chemical bonding mechanisms apply, but at the vast distances relevant to the Efimov effect, it is mainly through the van der Waals effect, in which rearrangements of electrical charge in one atom (forming an "electric dipole") create electric fields that can induce dipoles in, and thereby attract, neighboring atoms.

The observation of the Efimov effect is a coup for being able to study the rich quantum physics between three particles. The effect can conceivably occur in nucleons or molecules (and any object governed by quantum mechanics). However, it will likely be harder to observe in those systems because physicists cannot alter the strengths of interactions between the constituent particles as easily as they can in ultracold atom gases (through their "Feshbach resonances").

But the effect could provide insights on such systems as the triton, a nucleon with one proton and two neutrons, in addition to the BCS-BEC crossover, in which atoms switch from forming weakly bound Cooper pairs to entering a single collective quantum state. (See also article by Charles Day, Physics Today, April 2006, Esry et al., Phys. Rev. Lett, 30 August 1999, and Kraemer et al., Nature, 16 March 2006).