Quantum atoms "dance to chaos"

SYDNEY: Chaotic behaviour has been observed for the first time in a quantum system of 'frozen' atoms. The controversial finding is an important step in applying classical physical laws to weird quantum systems and could have spin-off benefits in technologies such as electronics and optical fibres.

The research, led by Brian Saam from the University of Utah in Salt Lake City, U.S., is published in the journal Physical Review Letters.

His team examined the properties of atoms in four tubes of xenon exposed to first to a magnetic field, then to a laser beam and radio-wave pulses. This had the result of locking the samples into a crystal lattice that constrained the 'spin' of the atoms.

Nuclear spin is a measurement of the direction of spin of the atomic nucleus and its electrons. Spin state may be either 'up' or 'down'.

Chaos theory at work

Each of the four samples was initially set at a different spin configuration, however, after a few thousandths of a second, the spins of all four samples evolved so that they were in sync – an example of chaos theory at work, said the authors.

Chaos theory states that systems which evolve over time are highly sensitive to the initial conditions of those systems. A popular analogy is that of the butterfly that flaps its wings and alters the weather patterns to cause a storm elsewhere in the world.

Chaos theory is part of Newtonian or classical physics, which does not adequately describe how atoms behave at the quantum level (at extremely small scales).

"This type of common behaviour has been a signature of classically chaotic [Newtonian] systems, mostly studied using a computer, but it never had been observed in an experimental system that can only be described by quantum mechanics," Saam said.

"Somehow despite the fact these spins have very complicated interactions with each other and [they] started out in completely different orientations, they end up all moving in the same way after several milliseconds," he said. "That's never been seen before in a quantum mechanical system. These guys are dancing together."

Saam added that the experiment was based on a theory devised by one of the study authors which predicts that chaos plays a previously unacknowledged role in quantum mechanics.

Controversial implications

Applying chaotic behaviour to quantum systems could have spin-off benefits in "new technology and new solutions to problems not yet known", write the researchers.

Quantum mechanics underlies our understanding of how electronics work and how light behaves and underpins much of our understanding of materials at the atomic scale.

Nick Robins, a quantum physicist and joint head of the Atom Laser Group at the Australian National University in Canberra, said that the research was "thorough" but added that the application of chaos theory to quantum mechanics is problematic, which the researchers themselves admit in their paper.

“Chaos (classical physics) and quantum mechanics lie at extreme ends of the spectrum of physical theories. Quantum mechanics deals with probabilities and statistics, while chaos is a product of classical, deterministic mathematics," he said. "Quantum chaos straddles the boundary between these two difficult fields, and hence by its nature is controversial.”

“This research experimentally investigates a particular theoretical prediction of how chaos manifests itself in a statistical system. It's not clear to me, though, that this is a good example of truly quantum system,” said Robins, who is not one of the study authors.