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.
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