Physicists create a bizarre new quantum state called a fractional fermi sea
Physicists have discovered a strange new state of quantum matter in which ultracold atoms defy familiar organizational rules, forming a highly excited yet surprisingly orderly configuration called a "fractional Fermi sea." The discovery challenges established quantum theories and hints at exotic quasiparticles that scientists have dubbed potential "super-Fermions."
Key points
- Physicists described a new quantum state, the fractional Fermi sea, in Physical Review Letters.
- The state forms when ultracold cesium atoms are cycled between strong repulsion and attraction.
- The highly excited state keeps a hidden order visible in its particle correlations.
Researchers led by Hanns-Christoph Nägerl, working with theoretical physicist Alvise Bastianello of the CNRS and Université Paris-Dauphine, have described how to engineer an exotic quantum state called a fractional Fermi sea. The study was published in Physical Review Letters.
The work involves ultracold cesium atoms confined to a one-dimensional system and repeatedly shifted between strong repulsion and strong attraction. The calculations showed the atoms could be driven from their ground state into a highly excited state while keeping an underlying organization.
Hidden order in the state
The researchers named the arrangement a fractional Fermi sea because its particles appear to follow a modified rule for occupying quantum states. The state is far from equilibrium but is not random, showing distinctive mathematical patterns.
These include ripples called Friedel oscillations and unusual patterns in how particle correlations weaken with distance, even when repulsive interactions are weak. The signatures differ from predictions of Tomonaga-Luttinger liquid theory, a long-established framework for interacting particles in one-dimensional systems.
This state is highly excited, but it is not random. It has a hidden order that becomes visible in its correlations.
— Hanns-Christoph Nägerl
What the researchers said
Yi Zeng, the study's leading author, said the interaction cycle reorganizes the atoms into a new many-body state rather than simply heating the system. Nägerl said the state has "a hidden order that becomes visible in its correlations."
Nägerl also raised the question of naming the system's unusual collective excitations, suggesting "super-Fermions" as a possibility. The paper sets out the theoretical framework; a companion study on the experimental realization is undergoing peer review.
