Physicists have proposed a theoretical description of an unusual state of quantum matter — a fractional Fermi sea (fractional Fermi sea). In it, particles retain an ordered structure even when the system is driven far beyond ordinary conditions. To achieve this, the scientists propose repeatedly switching the interaction between ultracold atoms from strong repulsion to strong attraction.

The work was published in the journal Physical Review Letters. The study may help explore forms of quantum behavior that cannot be fully explained by existing theories of one-dimensional systems, according to ScienceDaily.

How atoms go beyond familiar behavior

Under ordinary conditions, the behavior of quantum particles follows certain rules. For example, fermions — a category that includes electrons — cannot occupy the same quantum state at the same time. At very low temperatures, they fill the available energy levels, forming what is known as a Fermi sea.

The researchers set out to find what would happen if the interaction conditions between particles were changed continuously. To do this, they considered ultracold cesium atoms restricted to motion along a single direction. Such a one-dimensional system makes it possible to control quantum interactions more precisely and study their consequences.

The scientists modeled repeated switching of the interaction between strong repulsion and strong attraction. The calculations showed that a system initially in its ground state — the configuration with minimum energy — can transition into a highly excited state without losing its internal organization.

The resulting structure was named a fractional Fermi sea by the researchers. The name reflects an unusual way of filling quantum states that differs from the familiar picture of a Fermi sea.

“Instead of simply heating the system, the interaction-switching cycle rearranges the atoms into a new many-body state,” explained the study’s first author, Yi Zeng.

An excited state in which order is preserved

Usually, systems far from thermodynamic equilibrium exhibit complex behavior that is difficult to describe. However, according to the calculations, the fractional Fermi sea does not turn into a random collection of excited particles: it retains a characteristic structure.

One sign of this order is pronounced Friedel oscillations. These are wave-like changes in particle density that arise because of quantum interactions. The researchers also found an unusual dependence of correlations between particles on distance: the connections between them weaken according to a pattern that differs from standard theoretical predictions even under comparatively weak repulsion.

In particular, the results obtained do not fit entirely within the framework of Tomonaga–Luttinger liquid theory. This theory is widely used to describe interacting quantum particles in one-dimensional systems.

This indicates that the fractional Fermi sea may represent a special type of quantum state whose properties require additional theoretical explanation.

“This state is highly excited, but it is not random. Its hidden order reveals itself in the correlations between particles,” noted the head of the research group, Hans-Christoph Nägerl.

What new particles may emerge in the system

The unusual state may also be associated with the appearance of atypical quasiparticles. This is how physicists refer to collective excitations that behave like individual particles, even though they arise from the coordinated motion of many interacting objects.

According to Nägerl, the observed excitations may turn out to be so unusual that they will require a separate name. As one possible option, he suggested the term “superfermions,” although for now this is only a hypothesis rather than an established scientific concept.

The researchers view the fractional Fermi sea as a possible critical phase of quantum matter — a state with characteristic correlation patterns that go beyond familiar models.

Why create unusual quantum states

The work shows how quantum simulation helps study phenomena that are difficult to reproduce or investigate by other means. In such experiments, scientists use well-controlled quantum systems — for example, ultracold atoms — as a model for more complex physical processes.

Researchers can precisely tune the interactions between atoms and observe how the system responds to these changes. This makes it possible not only to test existing theories but also to search for new forms of organization in quantum matter.

For now, the published work establishes the theoretical foundation for fractional Fermi seas. A companion study devoted to the experimental creation of such a state using quantum simulation is currently under peer review.

Thus, the existence of this unusual structure still has to be confirmed in a separate experimental publication. If the results are confirmed, they will help broaden our understanding of how quantum particles can maintain order even under conditions far from equilibrium.