Physicists have uncovered a strange new form of quantum behavior in which particles organize themselves in ways that challenge established theories. Even more surprising, the particles maintain a remarkable hidden order despite being driven into an extremely energetic state.
In a study published in Physical Review Letters, researchers from the group led by Hanns-Christoph Nägerl, working with theoretical physicist Alvise Bastianello of the CNRS and Université Paris-Dauphine, describe how to engineer an exotic quantum state known as a “fractional Fermi sea.”
The breakthrough involves pushing ultracold cesium atoms far beyond their normal conditions by repeatedly changing how strongly they attract or repel one another. Rather than simply becoming disordered, the particles rearrange themselves into an unusual configuration that behaves differently from the quantum systems scientists have traditionally studied.
The theoretical findings provide the foundation for recent experimental research conducted by Nägerl’s team at the Department of Experimental Physics.
Pushing Quantum Particles Beyond Their Normal Limits
Under ordinary conditions, quantum particles follow well-established rules that determine how they arrange themselves, particularly at extremely low temperatures.
One important example involves fermions, a category of particles that includes electrons. These particles cannot all occupy the same quantum state, so 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 govern how particles behave?
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?”
To investigate this possibility, the researchers studied ultracold cesium atoms confined to a one-dimensional system. This means the atoms are restricted to moving along a single direction, creating an environment in which their quantum interactions can be studied with exceptional control.
The team examined what happens when the interactions between these atoms are repeatedly adjusted, cycling between powerful repulsion and attraction.
Their calculations revealed something unexpected. Starting from the ground state, the lowest energy configuration available to the system, the atoms could be driven into a highly excited state without losing their underlying organization.
The researchers call this unusual arrangement a fractional Fermi sea because its particles appear to obey a modified rule governing how quantum states are occupied.
“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.”
A Strange New 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 has been pushed far from equilibrium.
In physics, equilibrium describes a condition in which a system’s overall properties have settled into a stable balance. Systems driven far from equilibrium can behave in complicated and unexpected ways, making them particularly challenging to understand.
Yet the fractional Fermi sea is not simply a collection of excited particles behaving randomly. Instead, it contains distinctive mathematical patterns that reveal a deeper structure.
Among these patterns are pronounced ripples called Friedel oscillations. These are wave-like variations associated with the way quantum particles influence one another. The researchers also identified unusual patterns in how correlations between particles weaken with increasing distance, even when the repulsive interactions are relatively weak.
These signatures are important because they differ from predictions associated with Tomonaga-Luttinger liquid theory, a long-established framework for describing how interacting quantum particles behave in one-dimensional systems.
The findings therefore point toward a previously unexplored type of quantum behavior that cannot be fully described using this familiar theoretical picture.
“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.”
The strange 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.
Nägerl suggests that the unusual excitations might eventually deserve a name of their own.
“We are not yet sure how we should name these new quasiparticles. Perhaps ‘super-Fermions’?”
Opening a New Frontier in Quantum Physics
The discovery suggests that fractional Fermi seas represent an exotic critical phase of quantum matter, with characteristic patterns of particle correlations that extend beyond existing descriptions of one-dimensional quantum systems.
It also demonstrates the growing power of quantum simulation, a technique that allows scientists to use carefully controlled quantum systems to investigate physical behavior that would otherwise be difficult to reproduce or study.
Ultracold atoms are particularly useful for this purpose because researchers can precisely adjust their interactions and observe how the resulting quantum states change.
Rather than limiting these experiments to recreating phenomena already understood through established theories, scientists can now explore deliberately engineered states that reveal entirely new possibilities.
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.”
The current publication establishes the theoretical framework for fractional Fermi seas. A companion study describing their experimental realization through quantum simulation is undergoing peer review.
Together, the research points toward new ways of investigating the fundamental behavior of quantum matter, including unusual collective excitations and hidden forms of order that emerge only when particles are pushed far outside their normal conditions.
