Japanese and American physicists utilized atoms 3 billion times colder than interstellar space to build a portal to a hitherto unknown domain of quantum magnetism.
An artist’s conception of the complex magnetic correlations physicists have observed with a groundbreaking quantum simulator at Kyoto University that uses ytterbium atoms about 3 billion times colder than deep space. 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 quantum magnets whose complexity is beyond the reach of even the most powerful supercomputer. Image Credit: Image by Ella Maru Studio /Courtesy of K. Hazzard/Rice University
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, Study Corresponding Theory Author and Professor, Rice University
The study was published in Nature Physics. “Fermions are not rare particles. They include things like electrons and are one of two types of particles that all matter is made of,” said Hazzard.
A Kyoto team headed by research author Yoshiro Takahashi utilized lasers to cool its fermions, ytterbium atoms, to be within one billionth of a degree of absolute zero, the unachievable temperature at which all motion ceases. That is around 3 billion times cooler than interstellar space, which is still warmed by the Big Bang’s afterglow.
The payoff of getting this cold is that the physics changes. The physics starts to become more quantum mechanical, and it lets you see new phenomena,” Hazzard said.
Atoms, like electrons and photons, are subject to quantum dynamics principles, but their quantum behaviors are only visible when they are cooled to a fraction of a degree below absolute zero. For more than a quarter-century, researchers have utilized laser cooling to investigate the quantum features of ultracold atoms.
Lasers are used to both chill the atoms and confine their motions to optical lattices, which are 1D, 2D, or 3D channels of light that can function as quantum simulators that can solve complex situations beyond the capabilities of ordinary computers.
Takahashi’s lab employed optical lattices to recreate the Hubbard model, a popular quantum model developed by theoretical physicist John Hubbard in 1963.
Physicists to examine the magnetic and superconducting properties of materials, particularly those where interactions between electrons cause collective behavior, similar to how cheering sports fans execute “the wave” in packed stadiums, use Hubbard models.
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.
Kaden Hazzard, Study Corresponding Theory Author and Professor, Rice University
The special symmetry defined as SU(N) exists in the Hubbard model simulated in Kyoto, where SU stands for the special unitary group—a mathematical means of characterizing the symmetry—and N signifies the potential spin states of particles in the model.
The bigger the amount of N, the more symmetric the model is and the more complicated the magnetic behaviors it explains. The Kyoto simulator is the first to discover magnetic correlations in an SU(6) Hubbard model, which are difficult to calculate on a computer.
That’s the real reason to do this experiment. Because we’re dying to know the physics of this SU(N) Hubbard model,” Hazzard said.
The Hubbard model, according to study co-author Eduardo Ibarra-Garca-Padilla, a graduate student in Hazzard’s research group, tries to capture the minimal components to comprehend why solid materials develop metals, insulators, magnets, or superconductors.
One of the fascinating questions that experiments can explore is the role of symmetry. 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,” Ibarra-García-Padilla said.
Takahashi’s team demonstrated that its 3D lattice could trap up to 300,000 atoms. According to Hazzard, even the most sophisticated supercomputers are unable to precisely compute the behavior of a dozen particles in an SU(6) Hubbard model. The Kyoto experiments allow scientists to observe how these complicated quantum systems work in real time.
The findings are a significant step forward, with the first evidence of particle cooperation in an SU(6) Hubbard model, according to Hazzard.
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 cannot look at two, three, or even 100 atoms. You kind of have to look at the whole system,” Hazzard said.
In the Kyoto experiment, researchers do not yet have methods to measure such behavior. However, Hazzard stated that work on the tools is already beginning, and the achievement of the Kyoto team will fuel 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.
The Welch Foundation (C-1872) and the National Science Foundation (1848304) funded Rice University’s research.
Journal Reference:
Taie, S., et al. (2022) Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model. Nature Physics. doi.org/10.1038/s41567-022-01725-6.