PULLMAN, Wash. – Physicists including Doerte Blume of Washington State University have found the long sought Efimov quantum state in a molecule of three helium atoms. Two atoms that typically repel each other become strongly attracted when a third atom is involved. Their work is in the journal Science.

“The Efimov state is not an exotic special case, but rather an example of a universal quantum effect that plays an essential role in many areas of physics,” said Maksim Kunitski, a researcher at Goethe University in Frankfurt, Germany, where the discovery was made. Examples of these areas are cold atoms, clusters, nuclear physics and, recently, solid-state physics. There are also first reports about its significance in biology.

Weakly bound, expansive system

In 1970, Vitaly Efimov analyzed a three-body quantum system in which the attraction between two bodies reduced such that they become unbound. His prediction was that instead of breaking up, the molecule consisting of three particles can support an infinite number of bound states with huge distances between the binding partners.

“Every classical notion as to why such a structure (of enormous spatial extent) remains stable fails here,” said Reinhard Dörner, head of the research group at the Institute for Nuclear Physics.

This counter-intuitive prediction led to the booming field of “Efimov physics.” It soon became apparent that a system consisting of three helium atoms would be the prime example of this quantum mechanical effect. But all experiments conducted to prove the existence of the gigantic, extremely weakly bound helium system failed.

Ultracold and vacuum applied

In 2006, physicists at the University of Innsbruck first found indirect indications of Efimov systems in cold quantum gases of caesium atoms. In the atom traps they used, the interaction between the particles can be externally controlled. Efimov systems, however, as soon as they appear, are ejected from the artificial environment of the trap and fall apart unseen.

But Kunitski produced a stable Efimov system consisting of three helium atoms by pressing gaseous helium at a temperature of only eight degrees above absolute zero through a tiny nozzle into a vacuum. In this ultracold molecular beam, helium molecules with two, three or more atoms are formed. By diffraction of the molecular beam at a super-fine transmission grating, the physicist was able to spatially separate the trimers (three-atom groups).

In collaboration with WSU theoretician Blume, the researchers determined that only one of the many possible Efimov states had in fact occurred naturally in the molecular beam. The distances between bonds in the huge molecule extend to more than 100 angstroms (compared to two angstroms in a water molecule). Thereby, the helium atoms do not form an isosceles triangle, but are arranged asymmetrically. That correlates well with the theoretical predictions that have existed for many years.

Substantial funding, modest lab

“This is the first stable Efimov system that has ever been discovered,” Dörner said. “The three-body system flies through the laboratory inside the vacuum chamber without further interaction and without the need for external fields.

“Maksim Kunitski has conducted this groundbreaking work in a laser laboratory at the Goethe University Frankfurt,” Dörner said. “He did not need a big machine to accomplish this.”

Dörner could afford to tackle a research project that was so risky with respect to its prospects of success because in 2009 the German Research Foundation (DFG) made 1.25 million Euros available as part of its Koselleck program.

“It was a rather bold plan,” said Dörner. “But now – at the end of the project and really only because the DFG provided me with this large amount for a risky project without detailed planning – the search was successful.”