Anyone who has drained a bathtub or stirred cream into coffee has seen a
vortex, a ubiquitous formation that appears when fluid circulates. But unlike
water, fluids governed by the strange rules of quantum mechanics have a
special restriction: as was first predicted in 1945 by future Nobel winner
Lars Onsager, a vortex in a quantum fluid can only twist by whole-number
units.
These rotating structures are predicted to be widely useful for studying
everything from quantum systems to black holes. But while the smallest
possible quantum vortex, with a single unit of rotation, has been seen in
many systems, larger vortices are not stable. While scientists have
attempted to force larger vortices to hold themselves together, the results
have been mixed: when the vortices have been formed, the severity of the
methods used have generally destroyed their usefulness.
Now, Samuel Alperin and Professor Natalia Berloff from the University of
Cambridge have discovered a theoretical mechanism through which giant
quantum vortices are not only stable but form by themselves in otherwise
near-uniform fluids. The findings, published in the journal Optica, could
pave the way for experiments that might provide insight into the nature of
rotating black holes that have similarities with giant quantum vortices.
To do this, the researchers used a quantum hybrid of light and matter,
called a polariton. These particles are formed by shining laser light onto
specially layered materials. "When the light gets trapped in the layers, the
light and the matter become inseparable, and it becomes more practical to
look at the resulting substance as something that is distinct from either
light or matter, while inheriting properties of both," said Alperin, a Ph.D.
student at Cambridge's Department of Applied Mathematics and Theoretical
Physics.
One of the most significant properties of polaritons comes from the simple
fact that light can't be trapped forever. A fluid of polaritons, which
requires a high density of the exotic particles, is constantly expelling
light, and needs to be fed with fresh light from the laser to survive. "The
result," said Alperin, "is a fluid which is never allowed to settle, and
which doesn't need to obey what are usually basic restrictions in physics,
like the conservation of energy. Here the energy can change as a part of the
dynamics of the fluid."
It was exactly these constant flows of liquid light that the researchers
exploited to allow the elusive giant vortex to form. Instead of shining the
laser on the polariton fluid itself, the new proposal has the light shaped
like a ring, causing a constant inward flow similarly to how water flows to
a bathtub drain. According to the theory, this flow is enough to concentrate
any rotation into a single giant vortex.
"That the giant vortex really can exist under conditions that are amenable
to their study and technical use was quite surprising," Alperin said, "but
really it just goes to show how utterly distinct the hydrodynamics of
polaritons are from more well-studied quantum fluids. It's exciting
territory."
The researchers say that they are just at the beginning of their work on
giant quantum vortices. They were able to simulate the collision of several
quantum vortices as they dance around each other with ever increasing speed
until they collide to form a single giant vortex analogous to the collision
of black holes. They also explained the instabilities that limit the maximum
vortex size while exploring intricate physics of the vortex behavior.
"These structures have some interesting acoustic properties: they have
acoustic resonances that depend on their rotation, so they sort of sing
information about themselves," said Alperin. "Mathematically, it's quite
analogous to the way that rotating black holes radiate information about
their own properties."
The researchers hope that the similarity could lead to new insights into the
theory of quantum fluid dynamics, but they also say that polaritons might be
a useful tool to study the behavior of black holes.
Reference:
Samuel N. Alperin et al. Multiply charged vortex states of polariton
condensates, Optica (2021). DOI:
10.1364/OPTICA.418377
Tags:
Physics