In a study published in Physical Review Letters, researchers led by Hans-Christoph Nagel, in collaboration with theoretical physicist Alvise Bastianello of CNRS and Paris Dauphine–PSL University, describe how to engineer an exotic quantum state known as a fractional Fermi sea.

The theoretical breakthrough involves pushing cryogenic cesium atoms far beyond their usual state by repeatedly changing the strength of the forces that attract and repel them. Instead of simply becoming disordered, the particles rearrange themselves into unusual configurations that behave differently from the quantum systems traditionally studied by physicists.

This theoretical discovery provides the foundation for recent experimental research conducted by Nagel’s team in the Department of Experimental Physics.

Driving quantum particles into extreme states

Under normal conditions, quantum particles follow established rules that determine how they are arranged, particularly at extremely low temperatures.

One important example involves fermions, a category of particles that includes electrons. Because identical fermions cannot occupy the same quantum state, they fill available energy levels according to specific restrictions. At very low temperatures, this arrangement produces what physicists call a Fermi sea.

But what happens when researchers deliberately disrupt the conditions that normally control particle behavior?

Alvise Bastianello explains: “For example, fermions stack neatly into the available energy states, forming the so-called ‘Fermi sea.’ But what if we forced interacting atoms to continually cycle through their extreme states, smoothly transitioning from strongly mutually repelling to strongly mutually attracting states?”

To investigate this possibility, the researchers studied cryogenic cesium atoms confined in a one-dimensional system. In this setup, the atoms are restricted to move along a single direction, creating an environment in which their quantum interactions can be controlled and examined with high precision.

The researchers repeatedly adjusted the interactions between the atoms, cycling between strong repulsive and attractive forces.

Their calculations revealed an unexpected result. Starting from the ground state—the lowest-energy configuration available to the system—the atoms could be driven into highly excited states without losing their underlying organization.

The researchers call this unusual configuration a fractional Fermi sea because the particles appear to follow modified rules governing how quantum states are occupied.

“Rather than simply heating the system, the interaction cycles reorganize the atoms into new many-body states,” said Yi Zeng, lead author of the study. “This provides a controlled way to explore quantum matter beyond the usual equilibrium paradigm.”

A far-from-equilibrium quantum state with hidden order

One of the most intriguing features of the fractional Fermi sea is that its particles remain organized even though the system is far from equilibrium.

In physics, equilibrium describes a state in which a system’s overall properties have settled into a stable balance. Systems far from equilibrium can behave in complex and unexpected ways, making them especially difficult to understand.

However, the fractional Fermi sea is not simply a collection of randomly behaving excited particles. It contains mathematical patterns that reveal a deeper structure.

Among these patterns are ripples known as Friedel oscillations. These wave-like variations are related to the way quantum particles interact with one another. The researchers also identified an unusual pattern in which correlations between particles weaken as the distance between them increases, even when repulsive interactions are relatively weak.

These signatures are significant because they differ from predictions based on Tomonaga–Luttinger liquid theory, a long-established framework for describing interacting quantum particles in one-dimensional systems.

The findings therefore point to a previously unexplored type of quantum behavior that cannot be fully explained by this established theoretical picture.

“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.”

The unusual behavior also raises questions about the effective particles that emerge within the system. Physicists call these collective excitations quasiparticles because they can behave like individual particles even though they arise from the coordinated motion of many interacting particles.

Nagel suggests that these unusual excitations may eventually deserve their own name.

“We still don’t know what to name these new quasiparticles. Maybe ‘superfermions’?”

Why the discovery matters for quantum simulation

The discovery suggests that the fractional Fermi sea represents an exotic critical phase of quantum matter with a distinctive pattern of particle correlations that extends beyond existing descriptions of one-dimensional quantum systems.

It also demonstrates the growing potential of quantum simulation. This technique enables scientists to use carefully controlled quantum systems to investigate physical behaviors that are difficult to reproduce or study directly.

Ultracold atoms are especially useful for this purpose because researchers can precisely tune their interactions and observe how the resulting quantum state changes.

Rather than limiting experiments to phenomena already explained by established theories, scientists can use controlled manipulations to explore conditions that reveal entirely new possibilities.

Hans-Christoph Nagel said: “The discovery of the fractional Fermi sea shows how far we can push quantum simulations. We can not only reproduce known models, but also create and explore conditions beyond established paradigms.”

The current publication establishes a theoretical framework for fractional Fermi seas. A related study describing their experimental realization through quantum simulation is under peer review.

At the same time, the work offers new ways to investigate fundamental properties of quantum matter, including anomalous collective excitations and hidden forms of order that emerge when particles are driven far from their usual state.