Physicists have engineered a new quantum state, called a fractional Fermi sea, in ultracold cesium atoms confined to one dimension. By repeatedly cycling the atoms' interactions between strong repulsion and strong attraction, the team pushed the system far from equilibrium. Instead of descending into disorder, it settled into a highly excited but organized configuration.
According to the study in Physical Review Letters, the state shows Friedel oscillations and correlation patterns that do not match Tomonaga-Luttinger liquid theory, the usual framework for one-dimensional quantum gases. The authors say this points to an exotic critical phase with hidden order.
The work was led by Hanns-Christoph Nägerl at the University of Innsbruck, with theoretical input from Alvise Bastianello of CNRS and Université Paris-Dauphine. Lead author Yi Zeng says the interaction cycle offers a controlled way to explore quantum matter beyond equilibrium, and the team suggests the collective excitations could become new quasiparticles, perhaps called 'super-Fermions.'