Physicists have created an unusual quantum state known as aย fractional Fermi sea, revealing a new phase of matter that does not fit within conventional theoretical models.
The discovery was made using ultracold cesium atoms arranged in a one-dimensional system. By carefully controlling how the atoms interacted, researchers were able to push the system into a highly organized state that had never been observed before.
The work was carried out by the research group of Hanns-Christoph Nรคgerl at the University of Innsbruck, together with theoretical collaborator Alvise Bastianello of CNRS and Universitรฉ Paris-Dauphine. The findings were published in Physical Review Letters.
Engineering a New Quantum State
The researchers studied cesium atoms cooled to extremely low temperatures and confined in a single dimension. They then repeatedly changed the strength of interaction between the atoms, switching them between strong repulsion and strong attraction.
Instead of settling into a normal equilibrium state, the atoms entered a new type of critical quantum phase. This state behaved differently from what is predicted by the long-established Tomonaga-Luttinger liquid theory, which is commonly used to describe one-dimensional quantum systems.
The result shows that scientists can use quantum engineering to create forms of matter that do not naturally appear under ordinary conditions. By applying carefully designed interaction cycles, researchers can guide quantum systems into entirely new states.
What Makes It a Fractional Fermi Sea?
At very low temperatures, quantum particles usually organize themselves according to strict rules. Fermions, for example, fill available energy levels in a structure known as a Fermi sea.
In this experiment, however, the atoms did something unusual. The repeated cycling between attraction and repulsion reorganized them into a highly excited but ordered state. The researchers call this state
aย fractional Fermi sea because the particles appear to follow a reduced occupancy rule, unlike the standard arrangement expected in a normal Fermi sea.
Rather than simply heating up or becoming disordered, the system formed a new many-body state with its own structure and behavior.
Hidden Order in a Highly Excited State
One of the most striking features of the fractional Fermi sea is that it is not random. Although the system is highly excited, it contains a hidden order that becomes visible through particle correlations.
The researchers identified clear signatures of this order, including strong Friedel oscillations and distinctive decay patterns across different levels of repulsive interaction. These features distinguish the new state from ordinary Tomonaga-Luttinger liquids.
According to the researchers, this hidden structure suggests the presence of unusual quasiparticles that do not fit neatly into existing descriptions. Nรคgerl even suggested that these new quasiparticles may eventually need a new name, possibly โsuper-Fermions.โ
A New Direction for Quantum Simulation
The discovery opens a new path for studying exotic quantum matter. It shows that cold atom systems can do more than simulate known models; they can also be used to create and explore states that go beyond established physics.
This makes fractional Fermi seas an important new platform for investigating universal quantum behavior, non-equilibrium physics, and the limits of current theory.
A companion study describing the experimental realization of fractional Fermi seas through quantum simulation is currently under review.
Journal References:
Bastianello, A., Zeng, Y., Dhar, S., Wang, Z., Yu, X., Horvath, M., Astrakharchik, G. E., Guo, Y., Nรคgerl, H.-C., & Landini, M. (2026). Exotic Critical States as Fractional Fermi Seas in the One-Dimensional Bose Gas. Physical Review Letters. https://doi.org/10.1103/j3s5-gjpf
Zeng, Y., Bastianello, A., Dhar, S., Wang, Z., Yu, X., Horvath, M., Astrakharchik, G. E., Guo, Y., Nรคgerl, H.-C., & Landini, M. (2026). Realization of fractional Fermi seas. arXiv. https://doi.org/10.48550/arXiv.2602.17657