Physicists Engineer a Bizarre New Quantum State From Cold Atoms
Researchers in Austria and France have built a strange, highly ordered quantum state called a fractional Fermi sea using ultracold cesium atoms. The work appeared in Physical Review Letters on June 29, 2026, from the Nägerl group at the University of Innsbruck and theorist Alvise Bastianello of the CNRS and Université Paris-Dauphine. It points to a phase of matter that sits outside the standard playbook for one-dimensional quantum systems.
What the team did
According to the University of Innsbruck report, the physicists trapped cesium atoms in one dimension and pushed them far from their resting state. They did this by cycling the interaction between the atoms back and forth, moving from strong repulsion to strong attraction and around again. Most people would expect that kind of treatment to just heat everything up. Instead, the atoms settled into an excited but tidy arrangement.
Normally, fermions stack into available energy slots and form what physicists call a Fermi sea. “Fermions, for instance, stack neatly into the available energy states to form the so-called ‘Fermi sea,'” Bastianello explained. The cycling reorganized the atoms so they appeared to follow a reduced occupancy rule, which is where the “fractional” label comes from.
Yi Zeng, the study’s lead author, said the interaction cycle reorganizes the atoms into a new many-body state rather than simply heating the system. He described it as a controlled way to study quantum matter outside the usual equilibrium rules.
Why this state looks different
The new state carries clear fingerprints. Correlations between particles show pronounced ripples known as Friedel oscillations, along with distinct decay patterns at every level of repulsion. Those features set it apart from Tomonaga-Luttinger liquids, the model that has long described one-dimensional quantum systems.
“This state is highly excited, but it is not random,” said group leader Hanns-Christoph Nägerl. “It has a hidden order that becomes visible in its correlations.” He added that the team hasn’t settled on a name for the quasiparticles involved, half-joking that “super-Fermions” might work.
The signatures suggest an exotic critical phase that cold-atom simulators could now explore. Nägerl said the result shows how far quantum simulation can be pushed, creating and probing states that go beyond established models.
The published paper acts as the theoretical companion to an experiment from the same group. That sister paper, on the experimental realization of fractional Fermi seas, is still under review. Both preprints are posted on arXiv, and the published work carries the DOI 10.1103/j3s5-gjpf.