What happens when three different types of particles interact and each pair behaves differently?
Physicists have now recreated this situation in the laboratory and built a three-component quantum system that has competing magnetic correlations.
The achievement could help probe some of the most difficult problems in physics, including high-temperature superconductivity and the interactions that govern particles within 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 conduct.
A team of international researchers has mastered 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 locations, effectively moving from one position to another. Instead of using two, the team used three different internal quantum states of lithium-6 to represent different fermionic components.
By adjusting the magnetic field, they were able to tune the interactions between each pair independently, giving them control over three different interaction strengths.
Observe competing quantum behavior
To observe these interactions, the researchers developed a taste-resolving quantum gas microscope that could distinguish the three components and locate individual atoms. This allowed them to examine how the particles occupied and interacted with lattice sites.
An important goal was a Mott isolation state in which strong interactions suppress multiple occupancy of lattice sites and restrict particle motion. The researchers identified this behavior by measuring fluctuations in atomic occupancy and the system’s response to density changes.
“It was particularly important to us to observe a so-called Mott state, in which the interactions suppress the double occupancy of sites,” said Peter Schauss, one of the study authors and a physics professor at the University of Virginia.
The experiment revealed competing quantum behavior. Depending on the strength of the interaction, some component pairs showed ferromagnetic correlations while others showed antiferromagnetic correlations.
The researchers also measured triplicate locations, where all three components shared a common location. These measurements agreed with numerical calculations carried out by employees at Rice University.
A three-flavor system could bring quark physics to the lab
The results could help physicists investigate questions that are difficult to answer using traditional calculations. Rice University associate professor and co-author Kaden Hazzard explained:
“Although the Hubbard model is important and easy to write down, it is not easy to solve. Theorists have been studying the two-component Hubbard model for decades, but there are still major unanswered questions. By adding a third tunable component, we can see how the two-component model responds, which may help us answer these important questions.”
He also highlighted the connection 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 with the experiments that are already pushing the limits of our capabilities. These comparisons will help us refine our theoretical tools. It is an exciting and rapidly evolving time.”
Three-body collisions still result in atomic losses, while heating makes it difficult to reach the low temperatures needed to explore more sensitive quantum phases. Nevertheless, the system proved to be surprisingly stable.
“It turned out that the experimental implementation is surprisingly stable,” said Schauss. “Normally 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,” said Schauss:
The researchers plan to refine the interactions, investigate a possible paired Mott state, and explore triangle and Kagome lattices. Ultimately, the system could help simulate aspects of quantum chromodynamics, which describes interactions between quarks within protons and neutrons.
Its three components resemble the three color charges of quarks, while the platform could also shed light on the interactions behind high-temperature superconductivity.
The study is published in the journal Science.
