quantum chaos

In physics, chaos is something unpredictable. A butterfly flapping its wings somewhere in Guatemala might seem insignificant, but those flits and flutters might be the ultimate cause of a hurricane over the Indian Ocean. The butterfly effect captures what it means for something to behave chaotically: Two very similar starting points—a butterfly that either flaps its wings or doesn’t—could lead to two drastically different results, like a hurricane or calm winds.

But there's also a tamer, more subtle form of chaos in which similar starting points don’t cause drastically different results—at least not right away. This tamer chaos, known as ergodicity, is what allows a coffee cup to slowly cool down to room temperature or a piece of steak to heat up on a frying pan. It forms the basis of the field of statistical mechanics, which describes large collections of particles and how they exchange energy to arrive at a shared temperature. Chaos almost always grows out of ergodicity, forming its most eccentric variant.

Normally the word “chaos” evokes a lack of order: a hectic day, a teenager’s bedroom, tax season. And the physical understanding of chaos is not far off. It’s something that is extremely difficult to predict, like the weather. Chaos allows a small blip (the flutter of a butterfly wing) to grow into a big consequence (a typhoon halfway across the world), which explains why weather forecasts more than a few days into the future can be unreliable. Individual air molecules, which are constantly bouncing around, are also chaotic—it’s nearly impossible to pin down where any single molecule might be at any given moment.

Chaos is all around us, a fact that weather forecasters know all too well.Their job is notoriously difficult because small changes in air pressure or temperature, which ultimately drive winds and weather systems, can have huge consequences on a global scale. This sensitivity to tiny differences is commonly called the butterfly effect, and it makes weather patterns chaotic and hard to predict.Chaos pops up in many other places, too, and scientists have studied its role in physics for more than a century. But only since the 1980s have physicists investigated the connections between chaos and quantum mechanics—the most fundamental theory we have about the building blocks of the universe.One wrinkle in studying quantum chaos is that quantum physics itself seems to forbid chaotic behavior. The rules that govern the quantum world are actually too simple to give rise to the same kind of unpredictability as the weather. This prompted researchers to examine the differences between ordinary chaotic systems and their quantum counterparts more closely, a task that has been stalled because scientists lack the mathematical tools to quantify chaos in a quantum setting.Now, researchers from the Joint Quantum Institute (JQI) and the Condensed Matter Theory Center (CMTC) at the University of Maryland have used a promising diagnostic tool to characterize one of the simplest systems that physicists use to study chaos. This new diagnostic tracks the emergence of quantum interference effects and shows that they eventually destroy ordinary chaotic behavior. The work, performed by JQI and CMTC graduate student Efim Rozenbaum and two collaborators, was published online in Physical Review Letters on Feb. 21.