SU(N) Matter Is About Three Billion Times Colder Than Deep Space

Japanese and United States physicists have utilized atoms about 3 billion times colder than interstellar space to open one 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 College, it is making the coldest fermions in the universe,” said Rice College’s Kaden Hazzard, corresponding concept author of a study published today in Nature Physics. “Fermions are not unusual particles. They consist of things like electrons and are one of two types of particles that all matter is made of.”

A Kyoto group led by study author Yoshiro Takahashi utilized lasers to cool its fermions, atoms of ytterbium, within regarding one-billionth of a degree of absolute zero, the unattainable temperature where all motion quits. That is about three billion times colder than interstellar space, which is yet warmed by the afterglow from the Big Bang.

“The payoff of getting this cold is that the physics actually changes,” Hazzard stated. “The physics begins to become more quantum mechanical, and it lets you observe new phenomena.”

Atoms are subject to the laws of quantum dynamics, just like electrons and photons. However, their quantum behaviors only become evident when they are cooled within a fraction of a level of absolute zero. Physicists have utilized laser cooling to study the quantum properties of ultracold atoms for more than a quarter century. Lasers are utilized to both cool the atoms and restrict their movements to optical lattices, 1D, 2D or 3D channels of light that could serve as quantum simulators capable of solving complex issues beyond the reach of conventional computers.

Takahashi’s lab utilized optical lattices to simulate a Hubbard design, an oft-used quantum model produced in 1963 by theoretical physicist John Hubbard. Physicists utilize Hubbard models to investigate the magnetic and superconducting behavior of products, especially those where interactions between electrons create collective behavior, somewhat like the collective interactions of cheering sports fans that perform “the wave” in crowded stadiums.

“The thermometer they utilize in Kyoto is one of the essential things provided by our theory,” stated Hazzard, associate professor of physics and astronomy and a member of the Rice Quantum Initiative. “Comparing their measurements to our calculations, we could determine the temperature. The record-setting temperature is achieved thanks to exciting new physics that has to do with the very high symmetry of the system.”

The Hubbard model simulated in Kyoto has unique symmetry known as SU( N), where SU stands for special unitary group– a mathematical method of describing the symmetry–, and N denotes the feasible spin states of particles in the model. The higher 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. The Kyoto simulator is the first to expose magnetic correlations in an SU( six) Hubbard design, which are difficult to calculate on a computer.

“That is the real factor to do this experiment,” Hazzard said. “Because we are dying to know the physics of this SU( N) Hubbard design.”

Study co-author Eduardo Ibarra-García-Padilla, one graduate student in Hazzard’s research team, said the Hubbard model aims to capture the minimal ingredients to comprehend why strong products become steels, insulators, magnets, or superconductors.

“One of the exciting questions that experiments can explore is the function of symmetry,” Ibarra-García-Padilla said. “To have the ability to engineer it in a laboratory is extraordinary. If we can comprehend this, it may guide us to making real materials with new, wanted properties.”

Takahashi’s team revealed it could trap up to 300,000 atoms in its 3D lattice. Hazzard stated accurately calculating the behavior of even a dozen fragments in an SU( six) Hubbard model is beyond the reach of the most potent supercomputers. The Kyoto experiments provide physicists a chance to learn how these complex quantum systems run by watching them in action.

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

“Right now, this coordination is short-ranged; however, as the particles are cooled even further, subtler and more exotic stages of matter can appear,” he stated. “One of the interesting points about some of these unique phases is that they are not ordered in an evident pattern, and they are also not random. There are correlations; however, if you look at 2 atoms and ask, ‘Are they associated?’ you will not see them. They are much more subtle. You can not look at two or three or perhaps 100 atoms. You kind of need to look at the whole system.”

Physicists do not yet have tools capable of measuring such behavior in the Kyoto experiment. However, Hazzard stated work is currently underway to produce the devices, and the Kyoto team’s success will stimulate those efforts.

“These systems are quite unique and special, but the hope is that by studying and understanding them, we can recognize the key ingredients that require 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. www.nature.com/articles/s41567-022-01725-6

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