A recent study published in Physical Review Letters reveals that a team from the Nägerl group, in collaboration with theoretical partner Alvise Bastianello from CNRS and Université Paris-Dauphine, has successfully demonstrated the quantum engineering of highly unusual quantum states referred to as "fractional Fermi seas."
Ultracold cesium atoms locked into a hidden, ordered state after being cycled between repulsive and attractive interactions. Image Credit: University of Innsbruck
By manipulating quantum particles, specifically ultracold Cesium atoms confined in one dimension, and driving them far from equilibrium through cyclic variations in particle interactions, a new critical phase of matter has been discovered, which extends beyond the established Tomonaga-Luttinger liquid theory. The study acts as both a theoretical framework and a basis for the latest experimental endeavors conducted by Hans-Christoph Nägerl's group at the Department of Experimental Physics.
Particles adhere to stringent principles regarding their arrangement at low temperatures in the quantum realm.
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?
Alvise Bastianello, CNRS
The researchers demonstrate that a particular interaction cycle compels the initial ground-state atoms into a non-equilibrium configuration that is both highly excited and highly ordered. This phenomenon has been referred to as a "fractional" Fermi sea, in which the particles appear to adhere to a diminished occupancy rule.
Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state. This gives us a controlled way to explore quantum matter beyond the usual equilibrium paradigms.
Yi Zeng, Study Leading Author, University of Innsbruck
The implications of this fractional state are remarkable. The mathematical relationships among the particles exhibit significant ripples, referred to as Friedel oscillations, and clear decay patterns regardless of the degree of repulsive interaction. Importantly, this novel state displays characteristics that are different from those of the Tomonaga-Luttinger liquids, which have traditionally served as the foundational model for comprehending one-dimensional quantum systems.
This state is highly excited, but it is not random. It has a hidden order that becomes visible in its correlations. We are not yet sure how we should name these new quasiparticles. Perhaps ‘super-Fermions’?
Hanns-Christoph, Group Leader, University of Innsbruck
The emergence of these particular signatures indicates a completely novel and exotic critical phase, thereby creating new avenues for investigating universal behavior in cold-atom quantum simulators.
“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,” said Hanns-Christoph Nägerl.
The related publication on the experimental realization of fractional Fermi seas in a quantum simulation setting is currently under review.
Download the PDF of this page here
Journal Reference:
Bastianello, A., et al. (2026) Exotic Critical States as Fractional Fermi Seas in the One-Dimensional Bose Gas. Physical Review Letters. DOI: 10.1103/j3s5-gjpf. https://journals.aps.org/prl/abstract/10.1103/j3s5-gjpf.