Physicists in Japan and the United States have used atoms about 3 billion times cooler than interstellar space to open the door to the unexplored realm of quantum magnetism.
“Unless an alien civilization is doing an experiment like this right now, just run this experiment at Kyoto University and it will make the coldest fermions in the universe,” said Rice University’s Kaden Hazzard, who is published today in physical physics. “Fermions are not rare particles. They include matter such as electrons, and are one of two kinds of particles that make up all matter.”
The Kyoto team, led by study author Yoshiro Takahashi, used a laser to cool its fermions (ytterbium atoms) to within one billionth of absolute zero, an unattainable temperature where all motion ceases. That’s about 3 billion times cooler than interstellar space, which is still heated by the afterglow of the Big Bang.
“The great thing about a cold is that the physics really change,” Hazzard said. “Physics is starting 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 behavior only becomes apparent when they are cooled to a fraction of absolute zero. Physicists have used laser cooling to study the quantum properties of ultracold atoms for more than 25 years. Lasers are used to cool atoms and confine their motion in optical lattices, 1D, 2D or 3D channels of light that can be used as quantum simulators capable of solving complex problems beyond the reach of conventional computers.
Takahashi’s lab uses an optical lattice to simulate the Hubbard model, a common quantum model created by theoretical physicist John Hubbard in 1963. Physicists use the Hubbard model to study the magnetic and superconducting behavior of materials, especially those in which the interaction of electrons produces collective behavior, a bit like cheering sports fans performing “waves” in a crowded stadium collective interaction.
“The thermometer they used in Kyoto is one of the important things our theory provides,” said Hazzard, associate professor of physics and astronomy and a member of the Rice Quantum Initiative. “Comparing their measurements with our calculations, we could determine the temperature. The record temperature was achieved due to interesting new physics related to the high symmetry of the system.”
The Hubbard model simulated in Kyoto has a special symmetry called SU(N), where SU stands for a special unitary group — a mathematical way of describing symmetry — and N represents the possible spin states of the particles in the model. The larger the value of N, the more complex the symmetry of the model and the magnetic behavior it describes. The ytterbium atom has six possible spin states, and the Kyoto simulator is the first to reveal magnetic correlations in the SU(6) Hubbard model, which cannot be calculated by computers.
“That’s the real reason to do this experiment,” Hazzard said. “Because we were interested in knowing 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 was designed to capture the smallest components to understand why solid materials become metals, insulators, magnets or superconductors.
“An interesting question that experiments can explore is the role of symmetry,” Ibarra-García-Padilla said. “Being able to engineer it in the lab is extraordinary. If we can understand this, it may guide us in making real materials with new, desired properties.”
Takahashi’s team showed that it can trap up to 300,000 atoms in its 3D lattice. Accurately calculating the behavior of a dozen particles in the SU(6) Hubbard model is beyond the reach of the most powerful supercomputers, Hazzard said. The Kyoto experiment offers physicists an opportunity to understand how these complex quantum systems work by watching them in action.
The results are an important step in that direction, Hazzard said, and include the first observations of particle coordination in the SU(6) Hubbard model.
“Currently this coordination is short-range, but as the particles cool further, more subtle and exotic phases of matter may emerge,” he said. “What’s interesting about some of these exotic phases is that they’re not ordered in an obvious pattern, and they’re not random. There are correlations, but if you look at two atoms and ask, ‘Are they related?’ you don’t see them … They’re much more subtle. You can’t see two or three or even 100 atoms. You have to see the whole system.”
In the Kyoto experiment, physicists have not yet had the tools to measure this behavior. But Hazzard said work is already underway to create these tools, and the success of the Kyoto team will fuel those efforts.
“These systems are very exotic and special, but hopefully by studying and understanding them, we can identify key ingredients that need to be present in real materials,” he said.
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Shintaro Taie, Observing antiferromagnetic correlations in the ultracold SU(N) Hubbard model, physical physics (2022). DOI: 10.1038/s41567-022-01725-6. www.nature.com/articles/s41567-022-01725-6
Provided by Rice University
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