Ultracold Atoms that Interact at a Distance

A research study has characterized the long-range interactions between erbium atoms bound to a few millionths of a degree above absolute zero. It is a new step towards the understanding of the bizarre quantum behavior of matter in ultracold conditions.

AUSTRIA – The ability to understand the bizarre quantum behavior of matter in ultracold conditions has made a big step forward thanks to a research work by Francesca Ferlaino and colleagues at the University of Innsbruck and published April 8 in the journal Science. The authors characterized the long-range dipolar interactions of a gas of erbium atoms.

The behavior of atoms in condensed matter is one of the most difficult to describe for quantum physics.  Especially in many processes, such as the folding of proteins from their linear structure or orientation of the liquid crystal, this behavior strongly depends on the dipolar interactions, that is, by the forces acting between atoms that behave as tiny magnetic bars: repel if they are oriented in the same way and are attracted if they are oriented in the opposite direction.

Now, take account of all possible dipolar interactions of a system formed by a large number of atoms, as in the case of condensed matter, it is a daunting task.  For this reason, the great physicist Richard Feynman in the eighties hypothesized the use of models, that is, samples of material made artificially in the laboratory.

This speculative hypothesis Feynman began to take shape about two decades ago thanks to the theoretical work of Ignacio Cirac and Peter Zoller, who formulated the idea to model the matter of exploiting gas atoms brought close to absolute zero, so that the atoms themselves were slowed down in their rotational and vibrational motions to a crawl, and arranged in a precise lattice structure, the same that characterizes microscopic metals and crystals.

This provision can be obtained by crossed beams of appropriate frequency laser that block the atoms in the sample into an ordered structure.  The relationship between atoms and laser is a bit ‘the same relationship that exists between eggs (atoms) and corrugated cartons (ordered structure) containing them.

One thing particularly interesting is that brought in ultracold conditions, ie, fractions of a degree of absolute zero, the gas atoms lose their individuality and behave as a whole, following the laws formulated statistics in the twenties by the Indian physicist Satyendra Nath Bose according to some works of Albert Einstein, and are therefore called Bose-Einstein condensates.

Research on these systems began a few years ago, and the early successes have been achieved with sodium and rubidium atoms. More recently, the Ferlaino group dedicated to the ambitious project to study, with the same much larger techniques, atoms such as those of erbium maintained at a temperature of a few millionths of a degree above absolute zero.  Since they are provided with a large number of valence electrons, or the outermost electrons of the atomic shell, of the erbium gas behavior is very complex, but for this also fascinating, especially for the strong magnetism that show and which leads them to interact even at relatively large distance.

In the experiment described in their published work, the authors studied the behavior of a three-dimensional lattice of erbium atoms in ultracold conditions, in which the distance between an atom and the other was of seven times the wavelength wave quantum associated with the atoms themselves, going well beyond studies of the same type that were focused on the short-range interactions.

The characterization of the quantum behavior of the sample was obtained due to magnetic fields applied from the outside which allowed changing the orientation of the individual atoms and thus verifying the mutual dipole interactions in different conditions.

“Our collaboration with the Zoller group was essential to understand fully the results,” said Ferlaino.  “Our work is an important step towards a better understanding of the quantum behavior of dipolar matter, much more complex than that of ultracold quantum gases in other experiments.”

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