The study demonstrates how a new critical phase of matter can emerge when quantum particles are pushed far from their normal equilibrium conditions. Using ultracold cesium atoms confined to one dimension, the researchers repeatedly altered how strongly the particles interacted with one another. The resulting state goes beyond the behavior predicted by the well-known Tomonaga-Luttinger liquid theory, a cornerstone for understanding one-dimensional quantum systems.

This publication provides the theoretical framework for recent experimental research conducted in the group of Hans-Christoph Nägerl 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 arrange themselves. As Alvise Bastianello explains:

"Fermions, for instance, stack neatly into the available energy states to form the so-called 'Fermi sea'. But what happens if one forces interacting atoms to continuously cycle through extreme conditions, smoothly shifting them from strongly repelling each other to strongly attracting each other?"

The researchers found that carefully repeating this interaction cycle drives atoms out of their normal ground state and into a highly excited yet remarkably organized configuration. They call this state a "fractional" Fermi sea because the particles appear to obey a reduced occupancy rule.

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

Hidden Order in an Excited Quantum State

The newly created state displays several unusual characteristics. Mathematical correlations between particles reveal pronounced ripples, known as Friedel oscillations, along with distinctive decay behavior across all levels of repulsive interactions.

Perhaps most importantly, the state exhibits properties that differ from those expected for Tomonaga-Luttinger liquids, which have long served as the standard description of one-dimensional quantum matter.

"This state is highly excited, but it is not random," says Hanns-Christoph Nägerl, the group leader. "It has a hidden order that becomes visible in its correlations."

He adds: "We are not yet sure how we should name these new quasiparticles. Perhaps 'super-Fermions'?"

A New Critical Phase of Matter

These distinctive signatures indicate the presence of an entirely new and exotic critical phase. The discovery offers a new route for investigating universal quantum behavior using cold-atom simulators.

As Hanns-Christoph Nägerl says: "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."

A companion paper describing the experimental realization of fractional Fermi seas through quantum simulation is currently under review.