Groundbreaking quantum simulator of complex magnetic correlations

SU(N) matter is about 3 billion times cooler than deep space – opening a gateway to the quantum realm of high symmetry

An artist’s conception of the complex magnetic correlations observed by physicists in Kyoto University’s groundbreaking quantum simulator using ytterbium atoms about 3 billion times cooler than deep space. The different colors represent the six possible spin states of each atom. Using up to 300,000 atoms, the simulator enables physicists to directly observe how particles interact in quantum magnets with a complexity beyond the capabilities of even the most powerful supercomputers.Photo credit: Courtesy of Ella Maru Studio/K. Hazzard/Rice University

The coldest fermions in the universe open the door to the quantum realm of high symmetry.

Using atoms about 3 billion times cooler than interstellar space, physicists from Japan and the United States have opened the door to the unexplored realm of quantum magnetism.

Kaden Hazzard of Rice University, corresponding author of a study published on September 1, 2022, said: “Unless an alien civilization is doing such an experiment right now, just doing this experiment at Kyoto University would make The coldest fermions.” , in the journal

A research team from Kyoto University, led by study author Yoshiro Takahashi, used lasers to cool its fermions, atoms of ytterbium, within about one-billionth of a degree of

Eduardo Ibarra-García-Padilla, Kaden Hazzard and Hao-Tian Wei

Rice University theoretical physicists (from left) Eduardo Ibarra-García-Padilla, Kaden Hazzard and Hao-Tian Wei are collaborating with experimental physicists at Kyoto University in Japan to study unexplored quantum magnets using the uni­­verse’s coldest fermions. Credit: Photo by Jeff Fitlow/Rice University

Takahashi’s lab used optical lattices to simulate a Hubbard model, an often-used quantum model created by theoretical physicist John Hubbard in 1963. Physicists use Hubbard models to study 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.”

“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.” — Kaden Hazzard

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 is a graduate student in Hazzard’s research group. He 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. Accurately calculating the behavior of even a dozen particles in an SU(6) Hubbard model is beyond the reach of the most powerful supercomputers according to Hazzard. The Kyoto experiments offer physicists a chance to learn how these complex quantum systems operate by watching them in action.

Hazzard said the results are a major step in this direction, and include the first observations of particle coordination in an SU(6) Hubbard model.

“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. However, according to Hazzard 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: “Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model” by Shintaro Taie, Eduardo Ibarra-García-Padilla, Naoki Nishizawa, Yosuke Takasu, Yoshihito Kuno, Hao-Tian Wei, Richard T. Scalettar, Kaden R. A. Hazzard and Yoshiro Takahashi, 1 September 2022, Nature Physics.
DOI: 10.1038/s41567-022-01725-6

Study co-authors include Shintaro Taie, Naoki Nishizawa and Yosuke Takasu of Kyoto, Hao-Tian Wei of both Rice and Fudan University in Shanghai, Yoshihito Kuno of the University of Tsukuba in Ibaraki, Japan, and Richard Scalettar of the University of California, Davis.

The research at Rice was supported by the Welch Foundation (C-1872) and the National Science Foundation (1848304).

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