Physicists have discovered a strange new form of quantum behavior in which particles organize themselves in ways that defy established theories. Even more surprising is that the particles maintain a remarkable hidden order despite being driven to an extremely energetic state. In a study published in Physical Review Letters, researchers from the group led by Hanns-Christoph Nägerl, in collaboration with theoretical physicist Alvise Bastianello of the CNRS and Paris-Dauphine University, describe how to engineer an exotic quantum state known as a “fractional Fermi sea.” The breakthrough involves pushing the ultracold cesium atoms far beyond their normal conditions by repeatedly changing the strength with which they attract or repel each other. Instead of simply becoming disordered, the particles rearrange themselves into an unusual configuration that behaves differently from the quantum systems that scientists have traditionally studied. The theoretical findings provide the basis for recent experimental research by Nägerl’s team at the Department of Experimental Physics. Pushing quantum particles beyond their normal limits Under normal conditions, quantum particles follow well-established rules that determine how they organize, particularly at extremely low temperatures. An important example is fermions, a category of particles that includes electrons. Not all of these particles can occupy the same quantum state, so they fill the available energy levels according to specific constraints. At very low temperatures, this arrangement produces what physicists call a Fermi sea. But what happens when researchers deliberately alter the conditions that normally govern particle behavior? As Alvise Bastianello explains: “Fermions, for example, clearly stack up in the available energy states to form the so-called ‘Fermi sea’. But what happens if one forces the interacting atoms to circulate continuously through extreme conditions, smoothly changing them from strongly repelling each other to strongly attracting each other?” To investigate this possibility, the researchers studied ultracold cesium atoms confined in a one-dimensional system. This means that atoms are restricted to moving in 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, alternating between powerful repulsion and attraction. His calculations revealed something unexpected. Starting from the ground state, the lowest energy configuration available to the system, the atoms could be brought to 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 that governs how quantum states are occupied. “Rather than simply heating the system, the interaction cycle rearranges the atoms into a new many-body state,” says Yi Zeng, the lead 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 moved away from equilibrium. In physics, equilibrium describes a condition in which the overall properties of a system have stabilized in a stable equilibrium. Systems far from equilibrium can behave in complicated and unexpected ways, making them particularly difficult to understand. However, the fractional Fermi sea is not simply a collection of excited particles that behave randomly. Instead, it contains distinctive mathematical patterns that reveal a deeper structure. Among these patterns are pronounced waves called Friedel oscillations. These are wave variations associated with the way quantum particles influence each other. The researchers also identified unusual patterns in how correlations between particles weaken with increasing distance, even when repulsive interactions are relatively weak. These signatures are important because they differ from predictions associated with the Tomonaga-Luttinger theory of liquids, 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 very exciting, but it is not a coincidence,” says Hanns-Christoph Nägerl, the group’s leader. “It has a hidden order that becomes visible in its correlations.” The strange behavior also raises questions about the actual particles emerging 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 these unusual excitations could eventually deserve a name of their own. “We’re still not sure what we should name these new quasiparticles. Maybe ‘superfermions’?” Opening a new frontier in quantum physics The discovery suggests that Fermi fractional 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 behaviors that would otherwise be difficult to reproduce or study. Ultracold atoms are particularly useful for this purpose because researchers can fine-tune their interactions and observe how the resulting quantum states change. Instead of limiting these experiments to recreating phenomena already understood through established theories, scientists can now explore deliberately designed 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 just reproducing known models, but creating and testing states that go beyond established paradigms.” The current publication establishes the theoretical framework for fractional Fermi seas. A complementary study describing its experimental implementation using quantum simulation is undergoing peer review. Taken together, the research points to 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 beyond their normal conditions.
