Researchers led by Hanns-Christoph Nägerl have identified an exotic quantum state termed a fractional Fermi sea, in which particles exhibit unexpected organizational behavior when subjected to extreme conditions. The discovery was detailed in Physical Review Letters through collaboration between Nägerl’s experimental group and theoretical physicist Alvise Bastianello of CNRS and Université Paris-Dauphine. The research demonstrates that particles can be driven far beyond equilibrium while retaining distinctive structural patterns, contradicting assumptions that such states would simply become disordered.
The team conducted experiments using ultracold cesium atoms confined to a one-dimensional system. By repeatedly cycling the interactions between atoms between strong repulsion and strong attraction, researchers observed that particles could transition into highly excited states without losing underlying organization. Yi Zeng, the study’s lead author, explains that this controlled manipulation provides a new approach to studying quantum matter outside traditional equilibrium frameworks. The fractional Fermi sea gets its name from the observation that particles appear to follow modified rules governing quantum state occupation, distinguishing it from standard Fermi seas found in conventional quantum systems.
The fractional Fermi sea exhibits characteristics that diverge from established theoretical models, including distinctive wave-like variations called Friedel oscillations and unusual correlation patterns between particles. These signatures suggest behavior that cannot be fully explained by Tomonaga-Luttinger liquid theory, the prevailing framework for describing interacting quantum particles in one-dimensional systems. Nägerl notes that the state is highly excited yet maintains hidden order visible in particle correlations. The research also raises questions about previously unknown collective excitations, potentially warranting distinct classification as novel quasiparticles.
The findings highlight the expanding capabilities of quantum simulation, a technique using carefully controlled quantum systems to investigate physical phenomena difficult to study otherwise. Ultracold atoms prove particularly valuable for such research due to researchers’ ability to precisely adjust interactions and observe resulting quantum state changes. The current publication establishes the theoretical framework for fractional Fermi seas, with a companion study describing experimental realization under peer review. Together, these studies suggest new avenues for investigating fundamental quantum matter behavior and exploring exotic forms of order emerging when particles are pushed beyond normal operating conditions.
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