Researchers observe controversial quantum flyers

For the first time Efimov effect "live" seen

Schematic representation of the Heidelberg Efimov trimer consisting of three distinguishable lithium atoms (small spheres). The trimer itself is as symmetrical as a quantum object like a sphere (gray). © MPI for Nuclear Physics
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As early as 1970, Russian physicist Vitaly Efimov predicted a special quantum triple effect. Thus, three quantum particles can still bond with each other even if the force between two of these particles has long since become too weak. In physics, the existence of this seemingly paradoxical behavior has long been controversial. Now researchers have for the first time observed it directly.

The Efimov effect hides a universal quantum property. This could play an important role in molecules and in atomic nuclei as well as for neutrons, according to the physicists in the scientific journal "Science".

Quantum mechanics "saves" the world

Quantum mechanics is sometimes radically contrary to our everyday intuition. But then it can save our world - at least from the scientific perspective. Only thanks to it, atoms remain stable, otherwise the electrons would have to plunge into the atomic nuclei. In addition, the forces of the quantum world allow chemical bonds between atoms and molecules, even though the electrons involved in them repel each other violently. So we owe the forces of the quantum world the existence of the various molecules - and thus our diverse, colorful world.

So far only indirect circumstantial proof

The often strange quantum properties, however, occasionally cause professional disputes even among established physicists. An example of this is the Efimov effect. Its prediction was disputed for decades until physicists at the University of Innsbruck were able to prove it for the first time in 2006. However, with their experiment at the time, they succeeded only as indirect circumstantial evidence. Selim Jochim, a professor at the University of Heidelberg and a researcher at the Max Planck Institute for Nuclear Physics, was able to follow suit with his team. The physicists observed the Efimov effect for the first time directly.

They cleverly manipulated a lithium-6-atom gas that they had previously cooled to an extremely low temperature of about one millionth of a degree above the absolute temperature zero. This ultra-cold is necessary because the Efimov effect is "quite, very weak, " according to Jochim: "It is much weaker than the typical interactions between atoms that provide chemical bonds." Higher temperatures mean more violent movements of the atoms in the atom gas cloud. They would collide more often and stronger. This would make it impossible to observe the sensitive Efimov states. display

Atomic fidget

For this reason, the people of Heidelberg must first freeze the nuclear fidget for the time being. What they make then is a kind of molecule of three lithium-6 atoms. In general, molecules of three identical atoms are called trimers. However, Efimov's trimer-a spherical quantum object-is much larger than normal trimers, as chemistry knows it. "The binding of the Efimov effect goes so far that the atoms can no longer 'see' one another, explains Jochim. It is as if three atomic musketeers are roaming through a microscopic energy landscape where they have lost sight of each other. Nevertheless, they keep each other - albeit weak - triple connection.

Fascinating quantum triad

This threefold connection allows the atoms to bind in threes, even if the force between them is too weak. In physics, such a compound is called borromian. This alludes to three rings that Italian filmmaker Filippo Borromeo incorporated into his family crest in the 15th century to symbolize his attachment to his supporters: when you remove a ring from the three nested rings, you fall also the other two apart. However, a force that only binds in the presence of three particles is, from a physical point of view, something utterly peculiar.

You can imagine the entire physics in such a way that interactions, for example the electric force, always take place between two particles, J Jochim explains: This is also true if you consider the two very It is no wonder that experimental physicists have been trying for many years to find this fascinating quantum triad. They studied different physical systems, but failed until a few years ago.

Ultra-cold gas with special properties

The basis of the Heidelberg experiment is the production of an ultracold gas with special properties. For this purpose, the physicists brake their lithium atoms with laser light and capture them in a so-called magneto-optical trap. With this procedure, which is routine today, a Bose-Einstein condensate was first produced in 1995. Bosons are one of two basic species of particles that the quantum box knows. They are extremely sociable and tend to collapse at high temperatures in the quantum mechanical ground state. This ground state is the lowest rung of the quantum energy ladder. So there the ultra-cold Bose-Einstein condensate gathers.

However, it is one of the further peculiarities of quantum physics that the bosons in the Bose-Einstein condensate no longer possess any individuality. They are identical and therefore indistinguishable. "This would make it very difficult to directly observe the Efimov effect for experimental reasons, " says Jochim. Therefore, the Heidelberg chose lithium-6 atoms. These atoms belong to the so-called fermions and thus to the second particle type of quantum mechanics. Fermions are extreme individualists: each of them claims its own quantum state in the common system on its own. So they can not cover together in a quantum state, as happens with the Bose-Einstein condensate.

Cold-thrown lithium atoms

However, Jochim's team could not just pack three of these cold-thinned lithium atoms with a kind of super tweezers and put them together at the right distance. They had to find another way. Her experiment is complex, but in principle it uses two tricks. Trick one is easy - wait. Even if it is ultracold in the gas, the lithium atoms are still moving. Consequently, at some point, three of them will have just the right distance from each other to form an Efimov trimer.

But they do not do this voluntarily. Only trick two gives them the push. To understand it, one needs the quantum individualism of the fermions. The three atoms can only shape the Efimov triumvirate if they adapt their atomic inner life to the partnership. This is what the strict rules for this common quantum state demand. This interior consists of a tiny magnet that forms the core of the Lithium-6. Its properties are described by the so-called nuclear spin. It is now one of the peculiarities of the quantum world that the nuclear spins must be oriented exactly to each other, so that the three atoms can come together.

Radio field provides Anschubser

To do this, one of the spins has to be folded in the Heidelberg experiment. A radio field, which the researchers inject into their atomic cloud, provides for the push-pull at exactly the right moment. The atomic triad can actually be thought of as a radio. "Free atoms have their reception set to a different frequency, " Jochim explains: "That's why they do not notice our radio field." This changes as soon as three of them are at the right distance. "It's like changing the frequency on the radio to the neighboring transmitter, " explains the physicist. Now the three atoms receive the radio program of the Heidelberger, which commands them, so to speak, the spin-folding. The result is the Efimov trimer.

Goal: long-lasting Efimov states

As the trimer continues to receive, the radio field unfortunately destroys the sensitive Efimov triad again. But his short lifetime of less than a thousandth of a second was enough for the researchers to study the Efimov state. For example, they applied an additional magnetic field whose strength varied. So they could prove that the trimers actually behave as predicted by the theory of Efimov.

In the future, Jochim would like to produce Efimov states that are more durable. Then he could examine their universal properties more closely. "Universality means that it is no longer important which concrete physical system and which force we are considering, " explains Jochim. Efimov states can play a role in the electron shells of molecules and atoms as well as in the atomic nuclei.

(MPG, 16.11.2010 - DLO)