Japanese and American physicists have used atoms about 3 billion times colder than interstellar space to open a portal to an untapped realm of quantum magnetism.
“Unless an alien civilization is doing experiments like this right now, every time this experiment is done at Kyoto University, it makes the coldest fermions in the universe,” said Rice University’s Kaden Hazzard, the corresponding author of a study published today. was published in Nature physics. “Fermions are not rare particles. They contain things like electrons and are one of the two types of particles from which all matter is made.”
A Kyoto team led by study author Yoshiro Takahashi used lasers to cool the fermions, atoms of ytterbium, to about a billionth of a degree from absolute zero, the unattainable temperature at which all motion stops. That’s about 3 billion times colder than interstellar space, which is still being warmed by the afterglow of the Big Bang.
“The payoff from getting this cold is that the physics really change,” Hazzard said. “Physics is starting to get more quantum mechanical, and it’s showing you new phenomena.”
Atoms are subject to the laws of quantum dynamics, just like electrons and photons, but their quantum behavior only becomes apparent when they are cooled to a fraction of a degree from absolute zero. Physicists have been using laser cooling for more than a quarter of a century to study the quantum properties of ultracold atoms. Lasers are used to both cool the atoms and confine their movements to optical gratings, 1D, 2D or 3D light channels that can serve as quantum simulators that can solve complex problems beyond the reach of conventional computers.
Takahashi’s lab used optical gratings to simulate a Hubbard model, a widely 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 running “the wave” in crowded stadiums.
“The thermometer they use in Kyoto is one of the most important things our theory provides,” said Hazzard, an associate professor of physics and astronomy and a member of the Rice Quantum Initiative. “If we compare their measurements with our calculations, we can determine the temperature. The record temperature is reached thanks to nice new physics related to the very high symmetry of the system.”
The Hubbard model simulated in Kyoto has a 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 symmetry of the model and the complexity of the magnetic behavior it describes. Ytterbium atoms have six possible spin states, and the Kyoto simulator is the first to reveal magnetic correlations in a SU(6) Hubbard model that are impossible to compute on a computer.
“That’s the real reason for doing this experiment,” Hazzard said. “Because we are eager to learn 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 investigate is the role of symmetry,” Ibarra-García-Padilla said. “It’s extraordinary to be able to develop it in a lab. If we can understand this, it could lead us to making real materials with new, desirable properties.”
Takahashi’s team showed that it could trap up to 300,000 atoms in its 3D lattice. Hazzard said accurately calculating the behavior of even a dozen particles in a SU(6) Hubbard model is beyond the reach of the most powerful supercomputers. The Kyoto experiments provide an opportunity for physicists to learn how these complex quantum systems work by seeing them in action.
The results are an important 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-lived, but as the particles are cooled even further, more subtle and exotic phases of matter may appear,” he said. “One of the interesting things about some of these exotic phases is that they’re not ordered in a clear pattern, nor are they random. There are correlations, but if you look at two atoms and ask, ‘Are they correlated?’ you won’t see them. They’re much more subtle. You can’t look at two or three or even 100 atoms. You really have to look at the whole system.”
Physicists do not yet have tools that can measure such behavior in the Kyoto experiment. But Hazzard said work is already underway to create the tools, and the Kyoto team’s success will drive those efforts.
“These systems are quite exotic and special, but the hope is that by studying and understanding them, we can identify the key ingredients that should be there in real materials,” he said.
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Shintaro Taie, Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model, Nature physics (2022). DOI: 10.1038/s41567-022-01725-6. www.nature.com/articles/s41567-022-01725-6
Provided by Rice University
Quote: SU(N) matter is about 3 billion times colder than deep space (2022, September 1,), retrieved September 1, 2022 from
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