Physicists at the University of Innsbruck have driven a cloud of ultracold atoms into a previously unobserved quantum state, which they are calling a "fractional Fermi sea." The findings, published this month in Physical Review Letters, describe a form of matter that behaves unlike anything predicted by the standard theory used to understand one-dimensional quantum systems.
The team, led by Hanns-Christoph Nägerl's experimental group with theoretical input from Alvise Bastianello of CNRS and Université Paris-Dauphine, worked with cesium atoms cooled to within a hair's breadth of absolute zero and confined to move along a single dimension, like beads on a wire. Rather than simply heating the atoms, the researchers repeatedly cycled the strength of the atoms' interactions between strongly repulsive and strongly attractive.
Instead of settling into disorder, as a system driven this hard might be expected to do, the atoms reorganized into a new, highly excited but distinctly ordered state. "This state is highly excited, but it is not random," Nägerl said. "It has a hidden order that becomes visible in its correlations."
A Modified Occupation Rule
In an ordinary Fermi sea — the state electrons settle into inside a metal, for instance — particles stack into available energy levels according to strict quantum rules. Bastianello said the team's atoms only partially obey that law. "Fermions, for instance, stack neatly into the available energy states to form the so-called 'Fermi sea,'" he said, describing the new state as a kind of in-between case.
The resulting state shows wavelike ripples in particle density known as Friedel oscillations, and the way correlations between particles fade with distance departs from Tomonaga-Luttinger liquid theory, the framework physicists have relied on for decades to describe one-dimensional quantum matter. The team suggests the collective excitations of the new state may amount to a distinct type of quasiparticle, though they haven't settled on a name. "We are not yet sure how we should name these new quasiparticles," Nägerl said. "Perhaps 'super-Fermions'?"
Lead author Yi Zeng said the interaction-cycling technique gives physicists a controlled way to push quantum matter far from equilibrium without simply scrambling it. "Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state," Zeng said.
The Physical Review Letters paper lays out the theoretical case; a companion paper describing the experimental realization of the same state, posted as a preprint on arXiv, is still undergoing peer review. Because that experimental confirmation has not yet cleared review, independent physicists have not had a public chance to weigh in on whether the cycling technique generalizes to other atomic systems or temperatures.
If confirmed, researchers say the technique could give physicists a new way to engineer exotic quantum phases on demand, rather than waiting to find them in nature — a tool that could eventually prove useful for quantum simulation experiments probing materials that are difficult to study directly.
