Scientists discover new method to create quantum density waves in atomic gas

Understanding density waves

Scientists have been captivated by the way certain materials, including crystals, organize themselves. When particles arrange themselves predictably, like people in a queue wearing various colored shirts, this is known as "density waves" in quantum physics. 

Although density waves are seen in a variety of materials, researching them can be difficult when they coexist with phenomena like superfluidity. The development of materials with special features, such as high-temperature superconductivity and enhanced quantum computing, is made possible by superfluidity, which is of significant interest.

Harnessing light to tune a Fermi gas

Professor Brantut and his associates developed a "unitary Fermi gas" to look deeper into the interaction between density waves and other phenomena. For this, a thin gas of lithium atoms had to be cooled to incredibly low temperatures, where atom-to-atom collisions are common. The scientists then put this gas inside an optical cavity, a tool made to keep light contained in a small area for a long time.

In order to allow photons to accumulate inside the cavity, optical cavities are made up of two opposing mirrors that reflect incoming light back and forth thousands of times. The optical cavity was used by the researchers in their investigation to create long-distance interactions between the Fermi gas particles. 

No matter how far away from the emitting atom it was, the process entailed one atom emitting a photon, which would bounce off the mirrors and be reabsorbed by another atom. The collective organization of the atoms into a density wave pattern was the result of this photon exchange.

Professor Brantut remarks, "The combination of direct atom collisions within the Fermi gas, coupled with the exchange of photons over long distances, gives rise to a new type of matter characterized by extreme interactions." He added, "We hope what we will see there will improve our understanding of some of the most complex materials encountered in physics."

Study Abstract: 

A density wave (DW) is a fundamental type of long-range order in quantum matter tied to self-organization into a crystalline structure. The interplay of DW order with superfluidity can lead to complex scenarios that pose a great challenge to theoretical analysis. In the past decades, tunable quantum Fermi gases have served as model systems for exploring the physics of strongly interacting fermions, including most notably magnetic ordering, pairing and superfluidity, and the crossover from a Bardeen–Cooper–Schrieffer superfluid to a Bose–Einstein condensate. Here, we realize a Fermi gas featuring both strong, tunable contact interactions and photon-mediated, spatially structured long-range interactions in a transversely driven high-finesse optical cavity. Above a critical long-range interaction strength, DW order is stabilized in the system, which we identify via its superradiant light-scattering properties. We quantitatively measure the variation of the onset of DW order as the contact interaction is varied across the Bardeen–Cooper–Schrieffer superfluid and Bose–Einstein condensate crossover, in qualitative agreement with a mean-field theory. The atomic DW susceptibility varies over an order of magnitude upon tuning the strength and the sign of the long-range interactions below the self-ordering threshold, demonstrating independent and simultaneous control over the contact and long-range interactions. Therefore, our experimental setup provides a fully tunable and microscopically controllable platform for the experimental study of the interplay of superfluidity and DW order.