|
An NMR study of xenon atoms has demonstrated a fundamental new property - what appears to be chaotic behaviour in a quantum system - in the magnetic spin of these frozen atoms. The work could lead to improvements in our understanding of matter as well as in magnetic resonance imaging, where xenon is being developed as a contrast agent. Physicist Brian Saam and colleagues at the University of Utah's College of Science, USA and the University of Heidelberg, in Germany, hoped to reconcile chaos theory, which is based on 300-year old Newtonian mechanics with the modern theory of quantum mechanics on the basis of earlier predictions. Chaos theory describes how weather, certain chemical reactions, planetary orbits, and other dynamic systems change over time, with the changes often highly sensitive to starting conditions. Although chaotic by name this does not imply disorder. Indeed, the foundations of chaos theory lie in the clockwork world of Newtonian mechanics. Quantum mechanics, on the other hand, explains Saam, helps describes the behaviour of molecules, atoms electrons and other subatomic particles using probability and the uncertainty principle rather than Newton's laws of motion. "[Quantum mechanics] plays a key role in understanding how electronics work, how all sorts of interesting materials behave, how light behaves during communication by optical fibres," he says. "When you look at all the technology governed by quantum physics, it's not unreasonable to assume that if one can apply chaos theory in a meaningful way to quantum systems, that will provide new insights, new technology, new solutions to problems not yet known." The research team, including graduate student Steven Morgan, turned to NMR to help them study the interface of chaos theory with the quantum world. The researcers created the hyperpolarized xenon samples in the gas phase with a laser, by spin-exchange optical pumping, then condensed the xenon to a solid, via exposure to liquid nitrogen, and point out the hyperpolarization survives the phase transition. They then used conventional NMR to look at how the nuclear spins arealigned in four different configurations in four samples of frozen xenon, each containing about 100 billion, billion atoms. Despite differing initial configurations, the researchers found that the dancing nuclear spins of the xenon atoms ultimately synchronised with each other on the relatively long relaxation timescale of a few thousandths of a second. "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 only can be described by quantum mechanics," Saam adds. Heidelberg's Boris Fine had predicted this synchronised behaviour for nuclear spins in 2005 by adapting chaos theory to quantum theory. The evolution of disorder into order by the xenon nuclear spins is, the experiments hint, a signature of chaos theory. "I would hesitate to say that we have proven that the xenon spin behavior is chaotic", Saam told SpectroscopyNOW, "We are very careful about this in the actual paper. It is true that there is no explanation for this behaviour within the current framework of statistical physics and that the timescale and the particular way in which these four systems evolve into a universal long-time behaviour is highly suggestive (reminiscent) of chaotic behavior seen in classical systems." Saam adds that, "When you have a [chaotic] system that is characterized by extreme randomness, it paradoxically can produce ordered behaviour after a certain amount of time," says Saam. "There is strong evidence that is happening here in our experiment." Of course, while chaos does not necessarily mean disordered, and in this case it means the precise opposite, its relationship to Newtonian mechanics does not imply predictability, something it shares in some sense with the quantum world. Indeed, chaotic systems are highly sensitive to the starting conditions, a phenomenon known colloquially as "the butterfly effect". Despite the apparent unpredictability of systems so sensitive to their starting conditions, Saam explains that chaos theory can nevertheless make predictions about extremely complex motions of many particles that are interacting with each other. "Although they are held in place in the crystal structure, the xenon nuclear spins can interact with each other and change the direction in which they're pointed in much the same way that magnets interact with each other when brought close together," Saam explains. The initial spin configurations of each sample evolve in an extremely complicated way and each sample rapidly loses its memory of the initial state, this much was well known. What was most surprising about Fine's predictions and Saam's experiments is that while each sample's initial NMR signal was very different from the others, they displayed "identical long-time behaviour," Saam adds. "Somehow despite the fact these spins have very complicated interactions with each other and started out in completely different orientations, they end up all moving in the same way after several milliseconds," he says. "That's never been seen before in a quantum mechanical system. These guys are dancing together." Related links: The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
|
 Saam, freezing xenon fundamentally
|
