In a new study published in Physical Review Letters, a team of the Nägerl group jointly with theory collaborator Alvise Bastianello from the CNRS and the Université Paris-Dauphine demonstrates that highly unusual quantum states known as "fractional Fermi seas" can be quantum engineered.
By driving quantum particles, here ultracold Cesium atoms under one-dimensional confinement, far out of equilibrium through cyclic changes of the particle interaction, a novel critical phase of matter is found to emerge, going beyond what is known from the celebrated Tomonaga-Luttinger liquid theory. The new publication serves as the theoretical companion and the foundation for recent experimental work in the group of Hans-Christoph Nägerl at the Department of Experimental Physics .
Usually, particles in the quantum world follow strict rules about how they organize themselves at low temperatures. 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?" The researchers show that a specific interaction cycle forces the initial ground-state atoms into a highly excited, yet highly ordered, non-equilibrium configuration. This has been named a "fractional" Fermi sea, where the particles seemingly follow a reduced occupancy rule.
"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."
The consequences of this fractional state are striking. The mathematical correlations between the particles show prominent ripples-known as Friedel oscillations-and distinct decay patterns at any level of repulsive interaction. Crucially, this new state shows features that distinct from the Tomonaga-Luttinger liquids, which has long been the established model for understanding one-dimensional quantum systems. "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." He adds: "We are not yet sure how we should name these new quasiparticles. Perhaps 'super-Fermions'?"
The appearance of these specific signatures points to an entirely new, exotic critical phase, opening up fresh pathways for exploring universal behavior in cold-atom quantum simulators. 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 sister publication on the experimental realization of fractional Fermi seas in the spirit of quantum simulation is yet in the reviewing process.
Publications:
Exotic critical states as fractional Fermi seas in the one-dimensional Bose gas. A. Bastianello, Y. Zeng, S. Dhar, Z. Wang, X. Yu, M. Horvath, G. E. Astrakharchik, Y. Guo, H.-C. Nägerl, M. Landini Physical Review Letters 136, 230402 (2026) DOI: 10.1103/j3s5-gjpf , preprint at arxiv.org/abs/2602.17656
Realization of fractional Fermi seas. Yi Zeng, Alvise Bastianello, Sudipta Dhar, Zekui Wang, Xudong Yu, Milena Horvath, Grigori E. Astrakharchik, Yanliang Guo, Hanns-Christoph Nägerl, Manuele Landini. preprint at arxiv.org/abs/2602.17657
