Vortices may conjure a mental image of whirlpools and tornadoes—spinning
bodies of water and air—but they can also exist on much smaller scales. In a
new study published in Science, researchers from the Weizmann Institute of
Science, together with collaborators from the Technion-Israel Institute of
Technology and Tel Aviv University, have created, for the first time,
vortices made of a single atom. These vortices could help answer fundamental
questions about the inner workings of the subatomic world and be used to
enhance a variety of technologies—for example, by providing new capabilities
for atomic microscopes.
Scientists have long been striving to produce various types of nano-scale
vortices in the lab, with recent focus on creating vortex beams—streams of
particles having spinning properties—where even their internal quantum
structure can be made to spin. Vortices made up of elementary particles,
electrons and photons, have been created experimentally in the past, but
until now vortex beams of atoms have existed only as a thought experiment.
"During a theoretical debate with Prof. Ido Kaminer from the Technion, we
came up with an idea for an experiment that would generate vortices of
single atoms," says Dr. Yair Segev, who has recently completed his Ph.D.
studies in the group of Prof. Edvardas Narevicius of Weizmann's Chemical and
Biological Physics Department.
In classical physics, spinning objects are often characterized by a property
known as angular momentum. Similar to linear momentum, it describes the
effort needed to stop a moving object in its tracks, or rather, to stop it
from spinning. Vortices—characterized by the circulation of flux around an
axis—embody this property perfectly in their relentless spin.
However, the very basic property of angular momentum, which characterizes
naturally occurring vortices both big and small, takes on a different twist
on the quantum scale. Unlike their classical physics equivalents, quantum
particles cannot take on any value of angular momentum; rather, they can
only take on values in discrete portions, or "quanta." Another difference is
the way in which a vortex particle carries its angular momentum—not as a
rigid, spinning propeller, but as a wave that flows and twists around its
own axis of motion.
These waves can be shaped and manipulated similarly to how breakwaters are
used to direct the flow of seawater close to shore, but on a much smaller
scale. "By placing physical obstacles in an atom's path, we can manipulate
the shape of its wave into various forms," says Alon Luski, a Ph.D. student
in Narevicius's group. Luski and Segev, who led the research along with Rea
David from their group, collaborated with colleagues from Tel Aviv
University to develop an innovative approach for directing the movement of
atoms. They created patterns of nanometric "breakwaters" called
gratings—tiny ceramic discs, several hundreds of nanometers in diameter,
with specific slit patterns. When the slits are arranged into a fork-like
shape, each atom that passes through them behaves like a wave that flows
through a physical obstacle, in this way acquiring angular momentum and
emerging as a spinning vortex. These "nano-forks" were produced through a
nano-fabrication process that was developed specifically for this experiment
by Dr. Ora Bitton and Hila Nadler, both of Weizmann's Chemical Research
Support Department.
To generate and observe atomic vortices, the researchers aim a supersonic
beam of helium atoms at these forked gratings. Before reaching the gratings,
the beam passes through a system of narrow slits that blocks some of the
atoms, transmitting only the atoms that behave more like large waves—those
that are better suited to being shaped by the gratings. When these "wavy"
atoms interact with the "forks," they are shaped into vortices, and their
intensity is recorded and photographed by a detector.
This results in a donut-shaped image constructed from millions of vortexed
helium atoms that collide with the detector. "When we saw the donut-shaped
image, we knew we had succeeded in creating vortices of these helium atoms,"
says Segev. Much like the "eye" of the storm, the center of these "donuts"
represents the space where each atomic vortex is calmest—the intensity of
the waves there is zero, so no atoms are found there. "The 'donuts' are the
fingerprint of a series of different vortex beams," explains Narevicius.
During the experiments, the researchers made an odd observation. "We saw
that next to the perfectly shaped donuts, there were two small spots of
'noise' as well," says Segev. "At first we thought this was a hardware
malfunction, but after extensive investigation we realized that what we're
looking at are actually unusual molecules, each made of two helium atoms,
that were joined together in our beams." In other words, they had generated
vortices of not only atoms but also of molecules.
Although the researchers used helium in their experiments, the experimental
setup may accommodate studies of other elements and molecules. It could also
be used to study hidden subatomic properties, such as the charge
distribution of protons or neutrons that may be revealed only when an atom
is spinning. Luski gives the example of a mechanical clock: "Mechanical
clocks are made of tiny gears and cogs, each moving at a certain frequency,
similarly to the internal structure of an atom. Now imagine taking that
clock and spinning it—this motion could change the internal frequency of the
gears, and the internal structure could be expressed in the properties of
the vortex as well."
In addition to offering a new way of studying the very basic properties of
matter, atomic vortex beams might find use in several technological
applications, such as in atomic microscopy. The interaction between spinning
atoms and any investigated material could lead to the discovery of novel
properties of that material, adding significant, previously inaccessible
data to many future experiments.
Reference:
Alon Luski et al, Vortex beams of atoms and molecules, Science (2021).
DOI: 10.1126/science.abj2451
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