Quantum tornadoes

 

"Quantum tornadoes" mark crossover from classical to quantum physics

The universe is governed by two sets of seemingly incompatible laws of physics – there’s the classical physics we’re used to on our scale, and the spooky world of quantum physics on the atomic scale. MIT physicists have now observed the moment atoms switch from one to the other, as they form intriguing “quantum tornadoes.”

Things that seem impossible to our everyday understanding of the world are perfectly possible in quantum physics. Particles can essentially exist in multiple places at once, for instance, or tunnel through barriers, or share information across vast distances instantly.

These and other odd phenomena can arise as particles interact with each other, but frustratingly the overarching world of classical physics can interfere and make it hard to study these fragile interactions. 

One way to amplify quantum effects is to cool atoms right down to a fraction above absolute zero, creating a state of matter called a Bose-Einstein condensate (BEC) that can exhibit quantum properties on a larger, visible scale.

For the new study the MIT team did just that, to investigate what’s known as a quantum Hall fluid. This strange type of matter is made up of clouds of electrons trapped in magnetic fields, which begin to interact with each other in unusual ways to produce quantum effects. Rather than electrons, which are too hard to see clearly in this system, the researchers made a BEC out of about a million ultracold sodium atoms.

“We thought, let’s get these cold atoms to behave as if they were electrons in a magnetic field, but that we could control precisely,” says Martin Zwierlein, corresponding author of the study. “Then we can visualize what individual atoms are doing, and see if they obey the same quantum mechanical physics.”

The team placed this cloud of atoms in an electromagnetic trap, then spun them around at 100 rotations per second. The cloud stretched out into a long needle shape that got thinner and thinner – and that’s when the atoms switched over into quantum behavior.

The needle structure first started to bend back and forth like a snake in motion, then it broke into discrete segments. Still spinning, these segments formed a strange crystalline pattern that the team described as a string of quantum tornadoes. This behavior is governed entirely by the interactions between the atoms, and could have some intriguing implications for quantum and classical mechanics.

"This evolution connects to the idea of how a butterfly in China can create a storm here, due to instabilities that set off turbulence,” says Zwierlein. “Here, we have quantum weather: The fluid, just from its quantum instabilities, fragments into this crystalline structure of smaller clouds and vortices. And it’s a breakthrough to be able to see these quantum effects directly.”