Fractional Fermi sea discovered in ultracold cesium atoms

Physicists have identified a fractional Fermi sea in ultracold cesium atoms driven far from equilibrium. The research involved a team from the University of Innsbruck led by Hanns-Christoph Nägerl and theoretical physicist Alvise Bastianello of the CNRS and Université Paris-Dauphine.

Starting from the ground state, the atoms were driven into a highly excited state without losing underlying organization. The breakthrough involved pushing ultracold cesium atoms beyond normal conditions by repeatedly changing how strongly they attract or repel one another. Researchers alternated between significant repulsion and attraction in the atomic interactions. Yi Zeng, the leading author, explained that instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state. This provides a controlled way to explore quantum matter beyond the usual equilibrium paradigms.

Hidden order persists despite high excitation

Even in a highly excited state, the system retains internal order through correlations, diverging from random thermal behavior. Hanns-Christoph Nägerl stated that even in its excited state, the system is not random and exhibits hidden order through correlations. The fractional Fermi sea retains a characteristic structure and does not become a random collection of excited particles. The calculations showed that the system can be transferred from its ground state into a highly excited state without losing its internal order.

Signature patterns reveal quantum correlations beyond standard models

The state exhibits Friedel oscillations and unusual correlation weakening with distance, not predicted by Tomonaga-Luttinger liquid theory. Pronounced ripples called Friedel oscillations were observed. These are wave-like changes in particle density due to quantum interactions. Correlations between particles weaken with distance in a pattern differing from standard theoretical predictions, even under weak repulsion. These correlation patterns occur even with comparatively weak repulsive interaction and diverge from established Tomonaga-Luttinger liquid theory predictions.

Theoretical framework established for fractional Fermi sea

The study provides a theoretical foundation for the fractional Fermi sea as a possible critical phase of quantum matter with exotic quasiparticles. The arrangement is called a fractional Fermi sea because particles appear to obey a modified rule governing quantum state occupation. The discovery suggests fractional Fermi seas represent an exotic critical phase of quantum matter. The state may be associated with atypical quasiparticles arising from collective excitations. Hans-Christoph Nägerl suggested that such quasiparticles may in the future be called superfermions, noting it is currently only a hypothesis. The results do not fit entirely within the framework of Tomonaga-Luttinger liquid theory.

Experimental realization under peer review

A companion study detailing experimental creation of the fractional Fermi sea via quantum simulation is currently undergoing peer review. The research demonstrates the power of quantum simulation using ultracold atoms to engineer and study states beyond established paradigms. A companion study describing experimental realization through quantum simulation is undergoing peer review.