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Physics

A turbulent state

Brazilian researchers observe a new version of a quantum phenomenon

FERDINAND SCHMUTZER/WIKIMEDIA COMMONSEinstein: formulation of the fifth state of matter in the 1920’sFERDINAND SCHMUTZER/WIKIMEDIA COMMONS

From time to time, nature reveals some of its secrets to those who know how to look for them. Months ago, researchers from the University of São Paulo (USP) at São Carlos, inner-state São Paulo, witnessed one of these revelations, described in an article published in the July 24 issue of the journal Physical Review Letters. Manipulating a microscopic cloud of gas kept at an extremely low temperature, physicist Vanderlei Salvador Bagnato’s team detected in this gas an odd phenomenon that occurs in the world of particles: quantum turbulence, previously observed only in superfluid helium, a liquid with rather uncommon properties.

In the physicists’ experiment, they kept a cloud of 100 thousand to 200 thousand atoms of the chemical element rubidium trapped by magnetic fields in a space dozens of times smaller than a pinhead and chilled to a temperature very close to absolute zero (-273,15 degrees Celsius). Under such conditions, the rubidium atoms reached the lowest possible level of energy, practically stopped moving and started behaving as if they were a single super-atom with the cloud’s total dimensions – in this case, 150 micra (thousandths of a millimeter) long, equal to 150 thousand atoms arranged in a row.

This super-atom is called the Bose-Einstein Condensate, the fifth state of matter. Foreseen in 1924 by Albert Einstein based on the formulation of Satyendra Bose, an Indian physicist, the condensate was only produced experimentally in 1995 by two independent teams in the United States – that of Eric Cornell and of Carl Wierman, at the University of Colorado, and that of Wolfgang Ketterle, at the Massachusetts Institute of Technology, who won the Nobel Prize for the achievement. In the condensate, the atom particles stop behaving in accordance with the laws of classic physics, which governs the macroscopic world, and start to work according to the laws of quantum mechanics, with very different properties from the ones they have in other known states of matter (solid, liquid, gas or plasma).

By gently shaking the condensate, Bagnato’s team observed, in principle, the appearance of a few vortices, regions in which the atoms spiral like wind in a cyclone. As the level of shaking increased, however, the vortices, which at first were isolated and unconnected, became interlinked, generating what physicists call turbulence, which left the condensate looking like a Swiss cheese, each hole corresponding to a vortex.

New paths
“Before this experiment, we didn’t know whether there was turbulence in a Bose-Einstein condensate, nor how it would manifest itself,” tells us Bagnato, the coordinator of the Optics and Photonics Research Center, financed by FAPESP, and of the National Optics and Photonics Institute, which has the support of the Ministry of Science and Technology. According to Bagnato, this result unveils yet one more path for the study of quantum turbulence, previously detected only in liquid helium. By becoming liquid at just 2.17 degrees above absolute zero, i.e., around -271 degrees Celsius, helium acquires very uncommon characteristics, such as spreading in all directions with no physical resistance, a phenomenon known as superfluidity.

Bagnato and physicists Emanuel Henn, Jorge Seman and Kilvia Magalhães, from USP at São Carlos, and Giacomo Roati, from the University of Florence, in Italy, realized that they were witnessing something new when they submitted the condensate to a test called particle flight time. In this test, the physicists switch off, for extremely short periods of time, the magnetic fields that keep the quantum gas trapped, in order to observe how the particles spread. As a result, the gas expands. But not in the usual manner.

When a party balloon pops, the gas particles spread at the same speed in all directions. With a quantic gas such as the condensate, however, things are different: the expansion is faster in the direction in which the compression is greatest. This characteristic produces an effect that can easily be observed in a laboratory. Generally shaped like a cigar in three-dimensional space – elongated in width and more compressed regarding height and depth, for instance – the condensate starts to broaden faster along the dimension where it is narrower as soon as the magnetic forces are switched off. This property causes it to become, as it were, more elongated height-wise, and shorter in terms of width and depth, as if the cigar had undergone a 90 degree turn.

“Just 15 thousandths of a second are sufficient to witness this change,” tells us Kilvia. But this was not what the researchers observed in the condensate with turbulence, in which the expansion speed was the same across all dimensions. “This behavior is neither classical nor quantum-like,” exclaims Bagnato, a pioneer in the production of Bose-Einstein Condensate in Brazil. For its importance and its originality, the results presented in Physical Review Letters drew comments from physicists Natalia Berloff, from Cambridge University, in England, and Boris Svistunov, from the University of Massachusetts, United States, in the “Viewpoint” section of Physics, another journal of the American Physics Society (APS).

Understanding how and why turbulence arises in the world of quantum particles, according to the physicists, should allow one to also unveil the laws that govern turbulence in the macroscopic world. If this actually happens, it will be a major advance. The terror of aircraft pilots and ships captains, turbulence is seen by APS as one of the major challenges of modern physics.

Scientific article
HENN, E. A. et al. Emergence of Turbulence in an Oscillating Bose-Einstein Condensate. Physical Review Letters, v. 103, p. 45.301-1/45.304-1, July 2009.

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