A new study confirms that as atoms are chilled and squeezed to extremes,
their ability to scatter light is suppressed.
An atom’s electrons are arranged in energy shells. Like concertgoers in an
arena, each electron occupies a single chair and cannot drop to a lower tier
if all its chairs are occupied. This fundamental property of atomic physics
is known as the Pauli exclusion principle, and it explains the shell
structure of atoms, the diversity of the periodic table of elements, and the
stability of the material universe.
Now, MIT physicists have observed the Pauli exclusion principle, or Pauli
blocking, in a completely new way: They’ve found that the effect can
suppress how a cloud of atoms scatters light.
Normally, when photons of light penetrate a cloud of atoms, the photons and
atoms can ping off each other like billiard balls, scattering light in every
direction to radiate light, and thus make the cloud visible. However, the
MIT team observed that when atoms are supercooled and ultrasqueezed, the
Pauli effect kicks in and the particles effectively have less room to
scatter light. The photons instead stream through, without being scattered.
In their experiments, the physicists observed this effect in a cloud of
lithium atoms. As they were made colder and more dense, the atoms scattered
less light and became progressively dimmer. The researchers suspect that if
they could push the conditions further, to temperatures of absolute zero,
the cloud would become entirely invisible.
The team’s results, reported today in Science, represent the first
observation of Pauli blocking’s effect on light-scattering by atoms. This
effect was predicted 30 years ago but not observed until now.
“Pauli blocking in general has been proven, and is absolutely essential for
the stability of the world around us,” says Wolfgang Ketterle, the John D.
Arthur Professor of Physics at MIT. “What we’ve observed is one very special
and simple form of Pauli blocking, which is that it prevents an atom from
what all atoms would naturally do: scatter light. This is the first clear
observation that this effect exists, and it shows a new phenomenon in
physics.”
Ketterle’s co-authors are lead author and former MIT postdoc Yair Margalit,
graduate student Yu-kun Lu, and Furkan Top PhD ’20. The team is affiliated
with the MIT Physics Department, the MIT-Harvard Center for Ultracold Atoms,
and MIT’s Research Laboratory of Electronics (RLE).
A light kick
When Ketterle came to MIT as a postdoc 30 years ago, his mentor, David
Pritchard, the Cecil, and Ida Green Professor of Physics, made a prediction
that Pauli blocking would suppress the way certain atoms known as fermions
scatter light.
His idea, broadly speaking, was that if atoms were frozen to a near
standstill and squeezed into a tight enough space, the atoms would behave
like electrons in packed energy shells, with no room to shift their
velocity, or position. If photons of light were to stream in, they wouldn’t
be able to scatter.
“An atom can only scatter a photon if it can absorb the force of its kick,
by moving to another chair,” explains Ketterle, invoking the arena seating
analogy. “If all other chairs are occupied, it no longer has the ability to
absorb the kick and scatter the photon. So, the atoms become transparent.”
“This phenomenon had never been observed before, because people were not
able to generate clouds that were cold and dense enough,” Ketterle adds.
“Controlling the atomic world”
In recent years, physicists including those in Ketterle’s group have
developed magnetic and laser-based techniques to bring atoms down to
ultracold temperatures. The limiting factor, he says, was density.
“If the density is not high enough, an atom can still scatter light by
jumping over a few chairs until it finds some room,” Ketterle says. “That
was the bottleneck.”
In their new study, he and his colleagues used techniques they developed
previously to first freeze a cloud of fermions — in this case, a special
isotope of lithium atom, which has three electrons, three protons, and three
neutrons. They froze a cloud of lithium atoms down to 20 microkelvins, which
is about 1/100,000 the temperature of interstellar space.
“We then used a tightly focused laser to squeeze the ultracold atoms to
record densities, which reached about a quadrillion atoms per cubic
centimeter,” Lu explains.
The researchers then shone another laser beam into the cloud, which they
carefully calibrated so that its photons would not heat up the ultracold
atoms or alter their density as the light passed through. Finally, they used
a lens and camera to capture and count the photons that managed to scatter
away.
“We’re actually counting a few hundred photons, which is really amazing,”
Margalit says. “A photon is such a little amount of light, but our equipment
is so sensitive that we can see them as a small blob of light on the
camera.”
At progressively colder temperatures and higher densities, the atoms
scattered less and less light, just as Pritchard’s theory predicted. At
their coldest, at around 20 microkelvin, the atoms were 38 percent dimmer,
meaning they scattered 38 percent less light than less cold, less dense
atoms.
“This regime of ultracold and very dense clouds has other effects that could
possibly deceive us,” Margalit says. “So, we spent a few good months sifting
through and putting aside these effects, to get the clearest measurement.”
Now that the team has observed Pauli blocking can indeed affect an atom’s
ability to scatter light, Ketterle says this fundamental knowledge may be
used to develop materials with suppressed light scattering, for instance to
preserve data in quantum computers.
“Whenever we control the quantum world, like in quantum computers, light
scattering is a problem, and means that information is leaking out of your
quantum computer,” he muses. “This is one way to suppress light scattering,
and we are contributing to the general theme of controlling the atomic
world.”
Reference:
Pauli blocking of light scattering in degenerate fermions by Yair Margalit,
Yu-Kun Lu, Furkan Çagri Top and Wolfgang Ketterle, 18 November 2021,
Science.
DOI: 10.1126/science.abi6153
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Physics