The world we experience is governed by classical physics. How we move, where
we are, and how fast we're going are all determined by the classical
assumption that we can only exist in one place at any one moment in time.
But in the quantum world, the behavior of individual atoms is governed by
the eerie principle that a particle's location is a probability. An atom,
for instance, has a certain chance of being in one location and another
chance of being at another location, at the same exact time.
When particles interact, purely as a consequence of these quantum effects, a
host of odd phenomena should ensue. But observing such purely quantum
mechanical behavior of interacting particles amid the overwhelming noise of
the classical world is a tricky undertaking.
Now, MIT physicists have directly observed the interplay of interactions and
quantum mechanics in a particular state of matter: a spinning fluid of
ultracold atoms. Researchers have predicted that, in a rotating fluid,
interactions will dominate and drive the particles to exhibit exotic,
never-before-seen behaviors.
In a study published today in Nature, the MIT team has rapidly rotated a
quantum fluid of ultracold atoms. They watched as the initially round cloud
of atoms first deformed into a thin, needle-like structure. Then, at the
point when classical effects should be suppressed, leaving solely
interactions and quantum laws to dominate the atoms' behavior, the needle
spontaneously broke into a crystalline pattern, resembling a string of
miniature, quantum tornadoes.
"This crystallization is driven purely by interactions, and tells us we're
going from the classical world to the quantum world," says Richard Fletcher,
assistant professor of physics at MIT.
The results are the first direct, in-situ documentation of the evolution of
a rapidly-rotating quantum gas. Martin Zwierlein, the Thomas A. Frank
Professor of Physics at MIT, says the evolution of the spinning atoms is
broadly similar to how Earth's rotation spins up large-scale weather
patterns.
"The Coriolis effect that explains Earth's rotational effect is similar to
the Lorentz force that explains how charged particles behave in a magnetic
field," Zwierlein notes. "Even in classical physics, this gives rise to
intriguing pattern formation, like clouds wrapping around the Earth in
beautiful spiral motions. And now we can study this in the quantum world."
The study's coauthors include Biswaroop Mukherjee, Airlia Shaffer, Parth B.
Patel, Zhenjie Yan, Cedric Wilson, and Valentin Crépel, who are all
affiliated with the MIT-Harvard Center for Ultracold Atoms and MIT's
Research Laboratory of Electronics.
Spinning stand-ins
In the 1980s, physicists began observing a new family of matter known as
quantum Hall fluids, which consists of clouds of electrons floating in
magnetic fields. Instead of repelling each other and forming a crystal, as
classical physics would predict, the particles adjusted their behavior to
what their neighbors were 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,"
Fletcher says. "So, this whole world emerged. But it was extremely hard to
observe and understand."
In particular, electrons in a magnetic field move in very small motions that
are hard to see. Zwierlein and his colleagues reasoned that, as the motion
of atoms under rotation occurs at much larger length scales, they might be
able to use utracold atoms as stand-ins for electrons, and be able to watch
identical physics.
"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," Zwierlein says.
"Then we can visualize what individual atoms are doing, and see if they obey
the same quantum mechanical physics."
Weather in a carousel
In their new study, the physicists used lasers to trap a cloud of about 1
million sodium atoms, and cooled the atoms to temperatures of about 100
nanokelvins. They then used a system of electromagnets to generate a trap to
confine the atoms, and collectively spun the atoms around, like marbles in a
bowl, at about 100 rotations per second.
The team imaged the cloud with a camera, capturing a perspective similar to
a child's when facing towards the center on a playground carousel. After
about 100 milliseconds, the researchers observed that the atoms spun into a
long, needle-like structure, which reached a critical, quantum thinness.
"In a classical fluid, like cigarette smoke, it would just keep getting
thinner," Zwierlein says. "But in the quantum world, a fluid reaches a limit
to how thin it can get."
"When we saw it had reached this limit, we had good reason to think we were
knocking on the door of interesting, quantum physics," adds Fletcher, who
with Zwierlein, published the results up to this point in a previous Science
paper. "Then the question was, what would this needle-thin fluid do under
the influence of purely rotation and interactions?"
In their new paper, the team took their experiment a crucial step further,
to see how the needle-like fluid would evolve. As the fluid continued to
spin, they observed a quantum instability starting to kick in: The needle
began to waver, then corkscrew, and finally broke into a string of rotating
blobs, or miniature tornadoes—a quantum crystal, arising 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," Zwierlein
explains. "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."
Reference:
Martin Zwierlein, Crystallization of bosonic quantum Hall states in a
rotating quantum gas, Nature (2022).
DOI: 10.1038/s41586-021-04170-2.
Richard J. Fletcher et al, Geometric squeezing into the lowest Landau level,
Science (2021).
DOI: 10.1126/science.aba7202
Tags:
Physics