Physicists create bizarre quantum state called fractional Fermi sea through interaction cycles

Cycling cesium atom interactions

The study describes how researchers can drive ultracold cesium atoms into an unusual quantum arrangement. The atoms are confined to move in one direction, allowing their interactions to be examined with control.

The team repeatedly adjusts the strength of the interactions, moving between strong repulsion and strong attraction. Calculations show that atoms beginning in the lowest-energy state can reach a highly excited state while retaining organization.

A modified Fermi sea

Fermions, including electrons, cannot share the same quantum state. At very low temperatures, they fill available energy levels in a pattern known as a Fermi sea.

Order far from equilibrium

The system is far from equilibrium, a condition in which a system’s properties have settled into a stable balance. Despite the high energy, the particles do not behave like a random collection.

The researchers found pronounced Friedel oscillations, or wave-like variations linked to interactions among quantum particles. They also identified unusual patterns in how particle correlations weaken with distance, including when repulsion is relatively weak.

Differences from established theory

The observed patterns differ from predictions associated with Tomonaga-Luttinger liquid theory, a framework used to describe interacting quantum particles in one dimension. The findings point to behavior that this familiar picture does not fully explain.

The work was published in Physical Review Letters by researchers in a group led by Hanns-Christoph Nägerl, in collaboration with theoretical physicist Alvise Bastianello of CNRS and Université Paris-Dauphine. The theoretical results provide a foundation for recent experimental research by Nägerl’s team at the Department of Experimental Physics.