Quan­tum mat­ter: Tun­ing den­sity waves

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

Density waves

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’; like a group of people with different colored shirts on standing in a line but in a pattern where no two people with the same color shirt stand next to each other.

“Density waves are observed in a variety of materials, including metals, insulators, and superconductors”, says theoretical physicist Farokh Mivehvar from the University of Innsbruck. “However, studying them is challenging, especially when this order (the alternating pattern of particles in the wave) concurs with other types of order such as superfluidity – a property of that allows particles to flow without resistance.”

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.

Tuning a Fermi gas with light

To explore this interplay, Brantut and his colleagues created a “unitary Fermi gas”, a thin gas of lithium atoms cooled to extremely low temperatures, 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 light particles, photons, to build up inside the cavity.

In the study, the researchers used the cavity to cause the particles in the Fermi gas to interact with one another at long distances: an atom emits a photon in some point in time that bounces back and forth between the cavity mirrors. It is later reabsorbed by another atom of the gas, regardless of how far it is located from the first atom. When enough photons are emitted and reabsorbed – easily tuned in the experiment – the atoms collectively organize into a density-wave pattern due to these photon-induced interactions. 

“The combination of atoms colliding directly with each other in the Fermi gas, while simultaneously exchanging photons over long distances, is a new type of matter where the interactions are extreme and tunable,” says Brantut. “We hope to see that this will improve our understanding of some of the most complex materials encountered in nature.”

The results have now been published in 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.