The dance of three atoms

Physicists from Austria and in the U.S. have built a system of three atoms leading to a new state of matter. In fact, these atoms behaved like three musketeers: all for one, one for all. This can open a new field in quantum mechanics, but these experiments need a temperature of just a billionth of a degree above absolute zero to be successful.

Physicists from Austria and in the U.S. have built a system of three atoms of cesium leading to a new state of matter. In their experiments, two of these atoms couldn't even be assembled as pairs because of their weak attraction. But by adding a third atom, they formed a new stable form of matter similar to Borromean rings. In fact, these atoms behaved like three musketeers: all for one, one for all. Even if the physicists think this can open a new field in quantum mechanics, please remember that these experiments need extremely low temperatures to be successful, a billionth of a degree above absolute zero to be precise.

This research work has been published by Nature under the title "Evidence for Efimov quantum states in an ultracold gas of caesium atoms" (Volume 440, Number 7082, Pages 315-318, March 16, 2006). Here is a link to the editor's summary, "Three Into One Does Go."

In the bizarre world of quantum physics, three interacting particles can form a loosely bound system even if the two-particle attraction is too weak to allow for the binding of a pair. This exotic trimer state was predicted 35 years ago by Russian physicist Vitali Efimov, who found a remarkable and counterintuitive solution to the notoriously difficult quantum-mechanical three-body problem. Efimov's well known result was a landmark in theoretical few-body physics, but until now these exotic states had not been demonstrated experimentally.

And here are other links to the first paragraph of the paper and to a pre-publication (PDF format, 18 pages, 181 KB).

Before continuing, you might want to know what Vitali Efimov proposed in 1970. His major contribution was published by Physics Letters B under the name "Energy levels arising from resonant two-body forces in a three-body system" (Volume 33, Issue 8, Pages 563-564, December 21, 1970). Here is a link to the abstract. But you also can read another paper also published in 1970, "Weakly bound states of three resonantly-interacting particles" (PDF format, 7 pages, 249 KB).

Now, what is this new state of matter? Here is the introduction of the University of Chicago news release.

An international team of physicists has converted three normal atoms into a special new state of matter whose existence was proposed by Russian scientist Vitaly Efimov in 1970.
In this new state of matter, any two of the three atoms--in this case cesium atoms-- repel one another in close proximity. "But when you put three of them together, it turns out that they attract and form a new state," said Cheng Chin, an Assistant Professor in Physics at the University of Chicago.

Cheng Chin was only one of the researchers working with Rudolf Grimm at the University of Innsbruck in Austria about this new state of matter. Here is a link to his web site about Ultracold Atoms and Quantum Gases, dubbed "the coolest place in Austria."

This new state behaves like the Borromean ring, a symbol of three interlocking circles that has historical significance in Italy. The Borromean concept also exists in physics, chemistry and mathematics.
"This ring means that three objects are entangled. If you pick up any one of them, the other two will follow. However, if you cut one of them off, the other two will fall apart," Chin said. "There is something magic about this number of three."

Below is a picture of Borromean rings, which according to Wikipedia "consist of three topological circles which are linked despite the fact that no two of them are linked." (Credit for picture: ORNL).

Borromean rings

For more information about this discovery, you also might want to read "Bosons form quantum threesome," published by PhysicsWeb on March 15, 2006.

But where this discovery will lead? Here is the answer from the University of Chicago.

The finding may lead to the establishment of a new research specialty devoted to understanding the quantum mechanical behavior of just a few interacting particles, Grimm said. Quantum mechanics governs the interactions of atoms and subatomic particles, but is best understood when applied to systems consisting of two particles or of many particles.
Now that the Efimov state has been achieved, scientists can aspire to engineer the very properties of matter, Chin said. The Innsbruck-Chicago team exerted total control over the atoms in the experiment, converting them into the Efimov state and back into normal atoms at will.

When the researchers say at will,' things need to be put in perspective. These experiments, including some done by nanotechnologists, need to be performed at extremely low temperatures. And previous experiments were not successful.

The 1999 Stanford experiment, led by physicists Vladan Vuletic and Steven Chu, was conducted at one millionth of a degree above absolute zero. "Now we know that their sample was too hot" to observe the Efimov state, Grimm said.

Too hot? These scientists live in a strange world!

Sources: University of Chicago news release, March 16, 2006; and various web sites

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