Researchers have shown that it is possible to intentionally create a rare type of quantum state known as a “fractional Fermi sea,” according to a new study published in . physical review letter. The research was carried out by the Nägerl group, together with theoretical physicist Alvise Bastianello from CNRS and Paris-Dauphine University.
The study shows how new critical phases of matter emerge when quantum particles are pushed far from their normal equilibrium state. Using ultracold cesium atoms confined in one dimension, the researchers iteratively changed how strongly the particles interacted with each other. The resulting state exceeds the behavior predicted by the well-known Tomonaga-Luttinger liquid theory, which is fundamental to understanding one-dimensional quantum systems.
This publication provides a theoretical framework for recent experimental research carried out in Hans-Christoph Nagel’s group at the Department of Experimental Physics.
Creating a fractional Fermi sea
At very low temperatures, quantum particles typically follow strict rules that determine how they are arranged. Alvise Bastianello explains:
“For example, fermions stack neatly into available energy states, forming what’s called a ‘Fermi ocean.’ But what if we forced interacting atoms to continually cycle through their extreme states, smoothly transitioning from strongly repelling to strongly attracting each other?”
The researchers discovered that by carefully repeating this cycle of interactions, the atoms move from their normal ground state to highly excited yet remarkably organized configurations. They call this state a “fractional” Fermi Sea because the particles appear to follow reduced occupancy rules.
“Rather than simply heating the system, the interaction cycles rearrange the atoms into new many-body states,” says Yi Zeng, lead author of the study. “This provides a controlled way to explore quantum matter beyond the usual equilibrium paradigm.”
Hidden order in excited quantum states
The newly created state has some unusual characteristics. Mathematical correlations between particles reveal pronounced ripples known as Friedel oscillations and unique damping behavior across all levels of repulsive interactions.
Perhaps most importantly, this state exhibits properties different from those expected for Tomonaga-Luttinger liquids, which have long served as the standard description of one-dimensional quantum matter.
“This condition is very exciting, but it is not random,” said group leader Hans-Christoph Nagel. “There is a hidden order to it, which is revealed through correlation.”
He added: “We still don’t know what to name these new quasiparticles. Maybe ‘superfermions’?”
A new important stage of the problem
These characteristic signs indicate the existence of an entirely new and exotic critical stage. This discovery provides a new route to investigating universal quantum behavior using cold atom simulators.
Hans Christoph Nagel said: “The discovery of the fractional Fermi sea shows how far quantum simulation can be pushed. We can not only reproduce known models, but also create and explore conditions beyond established paradigms.”
A related paper describing the experimental realization of a fractional Fermi sea by quantum simulation is currently under consideration.
