By adjusting knob to the quantum solid

Density waves In the study, the researchers used an optical resonator to get the
Density waves In the study, the researchers used an optical resonator to get the particles in the Fermi gas to interact with each other over long distances, forming density waves. University of Innsbruck/Harald Ritsch

Quantum physicists at the University of Innsbruck, together with colleagues at the ETH Lausanne, have found a new way to generate a crystalline structure that emerges as a "coherent matter density wave" in an atomic gas. The findings help to better understand the fascinating behavior of quantum matter near absolute zero.

"Ultracold gases were already known to have very good control over the interactions between atoms," says Jean-Philippe Brantut of EPF Lausanne. "Our experiment extends this capability even further." In collaboration with Helmut Ritsch’s group at the Institute of Theoretical Physics at the University of Innsbruck, the scientists have made a breakthrough that can advance not only quantum research but also the quantum physics-based technologies of the future.

Waves with high density

Scientists have long sought to understand how matter self-organizes into complex structures such as crystals. In the often mysterious world of quantum physics, this kind of self-organization of particles manifests itself in so-called "density waves" in which particles arrange themselves in a regular, repeating pattern; like a group of people with different colored shirts standing in a row but arranged so that no two shirts of the same color are ever seen next to each other.

"Density waves are observed in a variety of materials, including metals, insulators and superconductors," says theoretical physicist Farokh Mivehvar of the University of Innsbruck. "However, the study near absolute zero are difficult, especially when the ordering of particles is accompanied by other types of organization such as superfluidity." Importantly, superfluidity is not just a theoretical curiosity; it is of great interest for the development of new materials with unique properties, such as high-temperature superconductivity.

Tuning a Fermi gas with light

To study this interaction, the scientists at EPF Lausanne created a "degenerate Fermi gas," a thin gas of lithium atoms cooled to extremely low temperatures in which the atoms collide very frequently.

The researchers then placed this gas in an optical resonator, a device used to focus light in a small space for an extended period of time. Optical resonators consist of two mirrors facing each other that reflect the incident light back and forth hundreds of thousands of times, allowing light particles (photons) to accumulate in the cavity.

In the study, the researchers used the cavity to get the particles in the Fermi gas to interact with each other over long distances: A first atom emits a photon that bounces off the mirrors and is then reabsorbed by a second atom, regardless of how far away it is from the first atom. When enough photons are emitted and reabsorbed - which can be easily set in the experiment - the atoms organize together into a density wave pattern similar to a crystal.

"The combination of atoms colliding directly with each other in the Fermi gas, while at the same time exchanging photons over long distances, is a new kind of matter where the interactions are extreme," Brantut says. "We hope that what we will see there will improve our understanding of some of the most complex materials found in physics."

The results have now been published in the scientific journal Nature. The research was funded by the Austrian Science Fund (FWF), the Swiss National Science Foundation (SNF) and the European Research Council (ERC), among others.

Publication: Density-wave ordering in a unitary Fermi gas with photon-mediated interactions. Victor Helson, Timo Zwettler, Farokh Mivehvar, Elvia Colella, Kevin Roux, Hideki Konishi, Helmut Ritsch, Jean-Philippe Brantut. Nature 2023. doi: 10.1038/s41586’023 -06018-3 [arXiv: 2212.04402 ]