Physicists in Japan, the United States, and China have broken through in the field of quantum magnetism using atoms that are about 3 billion times colder than interstellar space [1]. The quantum simulator developed at Kyoto University uses up to 300,000 atoms, enabling physicists to directly observe how particles interact in a quantum magnet with a complexity beyond the capabilities of even the most powerful supercomputers, realizing the quantum superiority of quantum magnet simulations.
In the quantum simulator, lasers are used to cool fermions (ytterbium atoms) to about one part per billion of absolute zero - a temperature that even all motion stops cannot reach.
The research results were published on September 1 in the journal Nature Physics under the title "Observation of antiferromagnetic correlations in the ultracold SU(N) Hubbard model" [2].
Co-authors of the study include Shintaro Taie, Naoki Nishizawa and Yosuke Takasu of Kyoto, Hao-Tian Wei of Rice University and Fudan University in Shanghai, Yoshihito Kuno of Tsukuba University in Ibaraki, Japan, and Richard Scalettar of the University of California, Davis.
01Simulating the quantum Hubbard model: first revelation of magnetic correlations in SU(6)
Fermions, including something like electrons, are one of the two classes of particles of which all matter is composed; atoms are also governed by the laws of quantum dynamics like electrons and photons, but their quantum behavior is only revealed when they are cooled to a fraction of a degree of absolute zero. Physicists have been using laser cooling to study the quantum properties of ultracold atoms for more than 25 years, and the cooling also limits their motion: making the atoms an optical lattice, a one-, two- or three-dimensional optical channel that can serve as a quantum simulator capable of solving complex problems that cannot be solved by conventional computers.
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An artistic interpretation of the phenomenon of "magnetic correlation" observed by physicists using a quantum simulator at Kyoto University, which uses ytterbium atoms that are about 3 billion times colder than deep space. The different colors represent the six possible spin states of each atom. The simulator uses up to 300,000 atoms, allowing physicists to directly observe how particles interact in a quantum magnet with a complexity that exceeds the capabilities of even the most powerful supercomputers.
The team used an optical lattice to simulate the Hubbard model - a quantum model created by theoretical physicist John Hubbard in 1963. They are often used to study the magnetic and superconducting behavior of materials, especially materials where the interaction between electrons produces collective behavior: it's a bit like the collective interaction of sports fans performing a "human wave" in a crowded stadium.
The Hubbard model simulated in Kyoto has a special symmetry called SU(N), where SU stands for Special Unitary Group (a mathematical method for describing symmetry) and N denotes the possible spin states of the particles in the model. the larger the value of N, the greater the complexity of the symmetry of the model and of the magnetic behavior it describes. The ytterbium atom has six possible spin states, and the Kyoto simulator is the first model to reveal the magnetic correlations in the SU(6) Hubbard model, which cannot be computed on a conventional computer.
The team showed that it can capture up to 300,000 atoms in its 3D lattice [3].
02Beyond classical computers, particle cooling goes further
"Any time this experiment at Kyoto University is running, it's making the coldest fermions in the universe. "At this point," says Kaden Hazzard of Rice University [4], "physics starts to become more quantum mechanical, and it allows us to see new phenomena. Accurately calculating the behavior of even a dozen particles in the SU(6) Hubbard model is beyond the reach of the most powerful supercomputers."
"The Kyoto experiment provides physicists with an opportunity to understand these complex quantum systems by observing them in action." Hazzard said the results are an important step in that direction, including the first observation of particle coordination in the SU(6) Hubbard model.
"Right now this coordination is short-range, but as particles are cooled further, more subtle and exotic phases of matter are expected to emerge in the future."
03In the future, it is expected to create high-temperature superconductors
For this experiment, the team used ytterbium atoms to create a magnet based on spin-like properties that had six options, each labeled with a color.
They used a laser to arrange the atoms in different configurations to produce the magnet. Some are one-dimensional like a wire, others are two-dimensional like a thin piece of material, or three-dimensional like a crystal. Atoms arranged in wires and sheets reach about 1.2 nanokelvin, more than 2 billion times colder than interstellar space. The situation is so complex for atoms arranged in three dimensions that researchers are still searching for the best way to measure the temperature.
Physicists say they have been interested in how atoms interact in such exotic magnets because they suspect that similar interactions occur in high-temperature superconductors (materials that conduct electricity perfectly). By understanding what happens, they could make better superconductors. Victor Gurarie of the University of Colorado at Boulder says the experiment is just cold enough: the atoms begin to "pay attention" to the quantum color states of their neighbors, a property that does not affect how they interact when they are warm.
Because the calculations are so difficult, he says, "future experiments like this may be the only way to study these quantum magnets."
Reference links:
[1]https://www.newscientist.com/article/2336247-quantum-magnet-is-billions-of-times-colder-than-interstellar-space/
[2]https://www.nature.com/articles/s41567-022-01725-6
[3]https://www.science20.com/news_staff/whats_3_billion_times_colder_than_deep_space_fermions_produced_by_special_unitary_spin_rates-256220
[4]https://newsupdate.uk/sun-matter-is-about-3-billion-times-colder-than-deep-space-universes-coldest-fermions-open-portal-to-high-symmetry-quantum-realm/
