Ultracold lithium atoms reveal a hidden struggle between quantum magnetic forces

What happens when three different kinds of particles interact, and each pair behaves differently?

Physicists have now recreated this situation in the laboratory, building a three-component quantum system that displays competing magnetic correlations.

The achievement could help investigate some of physics’ hardest problems, including high-temperature superconductivity and the interactions governing particles inside atomic nuclei.

How researchers built a three-component quantum system

The traditional Hubbard model describes interacting electrons using two possible spin states: spin-up and spin-down. Despite its relatively simple mathematical structure, it remains notoriously difficult to solve.

Adding a third component could reveal new quantum behaviors, but three-body collisions can cause atoms to disappear from the system, making experiments difficult to sustain.

A team of international researchers overcame this challenge using ultracold lithium-6 atoms. The researchers placed them in an optical lattice, an artificial crystal created with laser light that provides regularly spaced positions for atoms.

The atoms can tunnel between neighboring sites, effectively moving from one position to another. Instead of using two, the team employed three different internal quantum states of lithium-6 to represent distinct fermionic components.

By adjusting the magnetic field, they could independently tune the interactions between each pair, giving them control over three different interaction strengths.

Watching competing quantum behaviors

To observe these interactions, the researchers developed a flavor-resolved quantum gas microscope that could distinguish the three components and locate individual atoms. This allowed them to examine how the particles occupied lattice sites and interacted.

One important target was a Mott insulating state, in which strong interactions suppress multiple occupancy of lattice sites and restrict particle movement. The researchers identified this behavior by measuring fluctuations in atomic occupancy and the system’s response to changes in density.

“It was particularly important to us to observe a so-called Mott state, in which the interactions suppress the double occupation of sites,” Peter Schauss, one of the study authors and a physics professor at the University of Virginia, said.

The experiment revealed competing quantum behaviors. Depending on interaction strengths, some pairs of components displayed ferromagnetic correlations, while others showed antiferromagnetic correlations.

The researchers also measured triply occupied sites, where all three components shared a location. These measurements agreed with numerical calculations performed by collaborators at Rice University.

A three-flavor system could bring quark physics into the lab

The findings could help physicists investigate questions that conventional calculations have struggled to answer. Kaden Hazzard, a Rice University associate professor and co-author, explained:

“Although the Hubbard model is important and simple to write down, it isn’t easy to solve. Theorists have been banging our heads on the two-component Hubbard model for decades, yet there are still major open questions. Adding a third tunable component lets us see how the two-component model reacts to it, potentially helping us answer these major questions,”

He also highlighted the relationship between experiments and theoretical research:

“The experiments are a wonderful testing ground for theoretical ideas. Sohail and I were able to compare sophisticated numerical calculations to the experiments, which are already pushed to the boundaries of our capabilities. These comparisons will feed back into helping us refine our theoretical tools. It’s an exciting and fast-developing time.”

Three-body collisions still cause atom losses, while heating makes it difficult to reach the low temperatures needed to explore more delicate quantum phases. Nevertheless, the system proved surprisingly stable\

“It turned out that the experimental realization is surprisingly stable,” Schauss said. “Usually, one fears that three-body collisions could quickly destroy the system. Despite this potential danger, we were able to successfully carry out the measurements and observe a Mott insulator,” Schauss said:

The researchers plan to refine the interactions, investigate a possible paired Mott state and explore triangular and kagome lattices. Ultimately, the system could help simulate aspects of quantum chromodynamics, which describes interactions between quarks inside protons and neutrons.

Its three components resemble the three color charges of quarks, while the platform could also shed light on interactions behind high-temperature superconductivity.

The study is published in the journal Science.

Source: https://interestingengineering.com/science/three-component-quantum-system