Japanese and U.S. physicists have used atoms about 3 billion times colder
than interstellar space to open a portal to an unexplored realm of quantum
magnetism.
"Unless an alien civilization is doing experiments like these right now,
anytime this experiment is running at Kyoto University it is making the
coldest fermions in the universe," said Rice University's Kaden Hazzard,
corresponding theory author of a study published today in Nature Physics.
"Fermions are not rare particles. They include things like electrons and are
one of two types of particles that all matter is made of."
A Kyoto team led by study author Yoshiro Takahashi used lasers to cool its
fermions, atoms of ytterbium, within about one-billionth of a degree of
absolute zero, the unattainable temperature where all motion stops. That's
about 3 billion times colder than interstellar space, which is still warmed
by the afterglow from the Big Bang.
"The payoff of getting this cold is that the physics really changes,"
Hazzard said. "The physics starts to become more quantum mechanical, and it
lets you see new phenomena."
Atoms are subject to the laws of quantum dynamics just like electrons and
photons, but their quantum behaviors only become evident when they are
cooled within a fraction of a degree of absolute zero. Physicists have used
laser cooling to study the quantum properties of ultracold atoms for more
than a quarter century. Lasers are used to both cool the atoms and restrict
their movements to optical lattices, 1D, 2D or 3D channels of light that can
serve as quantum simulators capable of solving complex problems beyond the
reach of conventional computers.
Takahashi's lab used optical lattices to simulate a Hubbard model, an
oft-used quantum model created in 1963 by theoretical physicist John
Hubbard. Physicists use Hubbard models to investigate the magnetic and
superconducting behavior of materials, especially those where interactions
between electrons produce collective behavior, somewhat like the collective
interactions of cheering sports fans who perform "the wave" in crowded
stadiums.
"The thermometer they use in Kyoto is one of the important things provided
by our theory," said Hazzard, associate professor of physics and astronomy
and a member of the Rice Quantum Initiative. "Comparing their measurements
to our calculations, we can determine the temperature. The record-setting
temperature is achieved thanks to fun new physics that has to do with the
very high symmetry of the system."
The Hubbard model simulated in Kyoto has special symmetry known as SU(N),
where SU stands for special unitary group—a mathematical way of describing
the symmetry—and N denotes the possible spin states of particles in the
model. The greater the value of N, the greater the model's symmetry and the
complexity of magnetic behaviors it describes. Ytterbium atoms have six
possible spin states, and the Kyoto simulator is the first to reveal
magnetic correlations in an SU(6) Hubbard model, which are impossible to
calculate on a computer.
"That's the real reason to do this experiment," Hazzard said. "Because we're
dying to know the physics of this SU(N) Hubbard model."
Study co-author Eduardo Ibarra-García-Padilla, a graduate student in
Hazzard's research group, said the Hubbard model aims to capture the minimal
ingredients to understand why solid materials become metals, insulators,
magnets or superconductors.
"One of the fascinating questions that experiments can explore is the role
of symmetry," Ibarra-García-Padilla said. "To have the capability to
engineer it in a laboratory is extraordinary. If we can understand this, it
may guide us to making real materials with new, desired properties."
Takahashi's team showed it could trap up to 300,000 atoms in its 3D lattice.
Hazzard said accurately calculating the behavior of even a dozen particles
in an SU(6) Hubbard model is beyond the reach of the most powerful
supercomputers. The Kyoto experiments offer physicists a chance to learn how
these complex quantum systems operate by watching them in action.
The results are a major step in this direction, and include the first
observations of particle coordination in an SU(6) Hubbard model, Hazzard
said.
"Right now this coordination is short-ranged, but as the particles are
cooled even further, subtler and more exotic phases of matter can appear,"
he said. "One of the interesting things about some of these exotic phases is
that they are not ordered in an obvious pattern, and they are also not
random. There are correlations, but if you look at two atoms and ask, 'Are
they correlated?' you won't see them. They are much more subtle. You can't
look at two or three or even 100 atoms. You kind of have to look at the
whole system."
Physicists don't yet have tools capable of measuring such behavior in the
Kyoto experiment. But Hazzard said work is already underway to create the
tools, and the Kyoto team's success will spur those efforts.
"These systems are pretty exotic and special, but the hope is that by
studying and understanding them, we can identify the key ingredients that
need to be there in real materials," he said.
Reference:
Shintaro Taie, Observation of antiferromagnetic correlations in an ultracold
SU(N) Hubbard model, Nature Physics (2022).
DOI: 10.1038/s41567-022-01725-6.
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Physics