A spinning fluid of quantum particles breaks up into a crystal formed from swirling, tornado-like structures.
(photo credit: Courtesy of the researchers via MIT News)
In a peer-reviewed study published
in Nature on Wednesday, the physicists managed to view
particles interacting purely due to quantum mechanics by observing a spinning
fluid of ultracold atoms. Researchers have predicted that such interactions
will dominate in a rotating fluid, causing the particles to exhibit
never-before-seen behaviors.
The researchers took a cloud of about 1 million sodium atoms
and cooled them to just above absolute zero at about 100 nanokelvins (-273.149
Celsius) in order to try and get them to behave like electrons in a magnetic
field. In the 1980s, physicists observed a new family of matter called quantum
Hall fluids, which consist of clouds of electrons floating in magnetic fields.
While classical physics predicts that these electrons would repel each other
and form a crystal, the particles instead adjust their behavior to what their
neighbors are doing in a correlated, quantum way.
“People discovered all kinds of amazing properties, and the reason was, in a magnetic field, electrons are (classically) frozen in place — all their kinetic energy is switched off, and what’s left is purely interactions,” said Richard Fletcher, assistant professor of physics at MIT, to MIT News. “So, this whole world emerged. But it was extremely hard to observe and understand.”
The MIT physicists took the cloud of ultracold sodium atoms
and used a system of electromagnets to confine them and then collectively spun
the cloud around at about 100 rotations per second while capturing footage of
the spinning cloud. The researchers found that after about 100 milliseconds,
the cloud spun into a long, needle-like structure. They then took matters one
step further, taking the cloud past the point when the effects of classical
physics should be suppressed, leaving only the interactions between the
particles and quantum laws.
Eventually, a quantum instability kicked in, causing the
needle to waver, then corkscrew and finally break apart into a crystalline string of rotating blobs similar to
miniature tornadoes, purely from the interplay of the rotation of the gas and
forces between the atoms.
“This evolution connects to the idea of how a butterfly in China can create a storm here, due to instabilities that set off turbulence,” explained Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “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.”
Zwierlein added that the effect was similar to the Coriolis
effect that explains how patterns such as spiral clouds emerge as an effect of
Earth's rotation. Biswaroop Mukherjee, Airlia Shaffer, Parth B. Patel, Zhenjie
Yan, Cedric Wilson, and Valentin Crépel, all affiliated with the MIT-Harvard
Center for Ultracold Atoms and MIT’s Research Laboratory of Electronics, were
co-authors on the study.
In June, physicists from two independent teams reported
in Nature the most direct experimental observations of
crystals made from electrons, known as Wigner crystals, yet. Wigner crystals
had been elusive because electrons don't act like the macro world does. Unlike
water which crystallizes when cooled down, electrons behave like waves due to
quantum mechanics and begin to slosh around and crash into their neighbors when
cooled down, instead of crystallizing, according to Quanta magazine.
Researchers in a group led by Hongkun Park at Harvard
University found that a "sandwich" of two very thin sheets of a
semiconductor cooled to below -230 degrees Celsius with a specific number of
electrons in each layer resulted in a Wigner crystal. Repulsive forces between
the electrons in each layer and between the layers worked together to arrange
the particles into a triangular grid, preventing the usual sloshing.