An international research team from Germany, Thailand, India, China and the United States has realized a three-component Hubbard system in which interactions can be controlled. Using a quantum gas microscope, the researchers made individual atoms visible. The study, published in „Science,“ provides an important basis for future quantum simulations in nuclear physics and materials science (DOI: 10.1126/science.adv1247).
Ultracold atoms are often used to realize systems similar to those in solid-state physics, with fermionic atoms playing the role of electrons. The resulting system maps a Hubbard model – one of the most important models for describing interactions of fermions on a lattice and a promising approach to understanding high-temperature superconductivity. The standard Hubbard model usually has two spin states; a third component makes the system more versatile and allows richer quantum mechanical phases.
In an optical lattice, atoms in the lowest energy state form a quantum system in which they tunnel from one lattice site to the next. Identical fermions cannot occupy the same site; when two distinguishable fermions meet at the same site, an interaction energy arises that can be tuned via the magnetic field. The team led by Peter Schauss, who now researches at the University of Hamburg and in the Cluster of Excellence „CUI: Advanced Imaging of Matter,“ succeeded in realizing a Hubbard model with three components and tunable interactions, and in investigating fundamental states and phases.
„Particularly important to us was the observation of a so-called Mott state, in which interactions suppress double occupancy of sites,“ Schauss said. Strong interactions then prevent particles from moving freely between lattice sites, leading to insulating behavior. Notably, superconductivity can still occur near the Mott state. With the quantum gas microscope, each individual atom could be detected with site-specific resolution. Using atoms in different internal states, the team realized three types of fermions – a system that differs strongly from electrons on a lattice but resembles nuclear physics, where quarks can have three different colors.
„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, the measurements were carried out successfully and a Mott insulator was observed. The researchers also identified many further phases depending on the strength of interactions between the three states. For example, ferromagnetic correlations between two states and antiferromagnetic correlations between two others can occur in the same system. In addition, they developed a method to determine the number of triply occupied lattice sites; the measured values agreed with numerical calculations by collaborators at Rice University in the US. In the long term, the researchers plan to further adapt the interactions in their setup to study elements of lattice models of quantum chromodynamics in the laboratory.
