Physicists Discover Exotic “Fractional Fermi Sea” Quantum State

Physicists engineer a fractional Fermi sea by repeatedly cycling the interactions of ultracold cesium atoms between extreme attraction and repulsion. Published in Physical Review Letters, the theoretical framework from researchers from the group led by Hanns-Christoph Nägerl demonstrates how quantum particles maintain hidden order rather than dissolving into thermal disorder when driven far from equilibrium.

Pushing Ultracold Cesium Atoms Beyond Equilibrium

Under ordinary conditions, fermions follow strict rules regarding energy states. Electrons and similar particles cannot occupy identical quantum states. At extremely low temperatures, they stack neatly to form what physics defines as a Fermi sea.

Hanns-Christoph Nägerl led a research group investigating what happens when this stable baseline is deliberately disrupted. Working alongside theoretical physicist Alvise Bastianello of the CNRS and Université Paris-Dauphine, the team examined one-dimensional systems of ultracold cesium atoms. These atoms were restricted to a single direction of movement, allowing precise control over quantum interactions.

Rather than maintaining a steady state, the researchers continuously cycled the interactions between the atoms. They shifted the particles smoothly from powerful repulsion to strong attraction.

Yi Zeng, the leading author of the study, explains the mechanics of the transition. “Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state,” Zeng notes. “This gives us a controlled way to explore quantum matter beyond the usual equilibrium paradigms.”

Hidden Order and Friedel Oscillations in Excited States

Systems driven far from equilibrium typically yield chaotic results. Yet the fractional Fermi sea defies standard expectations by retaining distinct mathematical structures.

The particles do not behave randomly. The system displays pronounced Friedel oscillations, which are wave-like variations tracking how quantum particles influence one another. Furthermore, correlations between particles weaken with distance in a manner distinct from traditional predictions.

These signatures challenge Tomonaga-Luttinger liquid theory. That framework has long governed how interacting quantum particles behave in one-dimensional environments. The fractional Fermi sea points toward an unexplored type of quantum behavior.

“This state is highly excited, but it is not random,” Nägerl states. “It has a hidden order that becomes visible in its correlations.”

The collective excitations emerging from this coordinated motion behave like individual particles. Nägerl suggests a potential identifier for these phenomena. “We are not yet sure how we should name these new quasiparticles. Perhaps ‘super-Fermions’?”

Experimental Realization via Quantum Simulation

The breakthrough establishes fractional Fermi seas as an exotic critical phase of matter. The research highlights the capabilities of quantum simulation, where researchers manipulate controlled systems to investigate physical behavior otherwise difficult to replicate.

Ultracold atoms provide the ideal medium for this manipulation because researchers can adjust interactions directly. While the published paper outlines the theoretical framework, a companion study detailing the experimental realization of the fractional Fermi sea is currently undergoing peer review.

“The discovery of fractional Fermi seas shows how far we can push quantum simulation: not only reproducing known models, but creating and probing states that go beyond established paradigms,” Nägerl concludes.