“Cold atomic gases were well known in the past for the ability to program the interactions between atoms,” says Jean-Philippe Brantut at EPFL in Lausanne. “Our experiment doubles this ability!” Working with the group of Helmut Ritsch at the univer­sity of Innsbruck, they have made a breakthrough that can impact not only quantum research but quantum-based technologies in the future.

Scientists have long been interested in understanding how materials self-organize into complex structures, such as crystals. In the often-arcane world of quantum physics, this sort of self-organi­zation of particles is seen in density waves, where particles arrange themselves into a regular, repeating pattern or order. Density waves are observed in a variety of materials, including metals, insulators, and super­conductors. However, studying them has been difficult, especially when this order occurs with other types of organi­zation such as superfluidity.

It's worth noting that superfluidity is not just a theoretical curiosity; it is of immense interest for developing materials with unique properties, such as high-temperature super­conductivity, which could lead to more efficient energy transfer and storage, or for building quantum computers. To explore this interplay, Brantut and his colleagues, the researchers created a unitary Fermi gas, a thin gas of lithium atoms cooled to extremely low temperatures, and where atoms collide with each other very often.

The researchers then placed this gas in an optical cavity, a device used to confine light in a small space for an extended period of time. Optical cavities are made of two facing mirrors that reflect incoming light back and forth between them thousands of times, allowing photons to build up inside the cavity. In their study, the researchers used the cavity to cause the particles in the Fermi gas to interact at long distance: a first atom would emit a photon that bounces onto the mirrors, which is then reabsorbed by second atom of the gas, regardless how far it is from the first. When enough photons are emitted and reabsorbed – easily tuned in the experiment – the atoms collec­tively organize into a density wave pattern.

“The combi­nation of atoms colliding directly with each other in the Fermi gas, while simultaneously exchanging photons over long distance, is a new type of matter where the inter­actions are extreme,” says Brantut. “We hope what we will see there will improve our under­standing of some of the most complex materials encountered in physics.” (Source: EPFL)

Reference: V. Helson et al.: Density-wave ordering in a unitary Fermi gas with photon-mediated interactions, Nature, online 24 May 2023; DOI: 10.1038/s41586-023-06018-3

Link: Center for Quantum Science and Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland