In a study published in Physical Review Letters, researchers from the group led by Hanns-Christoph Nägerl, working with theoretical physicist Alvise Bastianello of the CNRS and Université Paris-Dauphine, describe how to engineer an exotic quantum state known as a “fractional Fermi sea.”

The breakthrough involves pushing ultracold cesium atoms far beyond their normal conditions by repeatedly changing how strongly they attract or repel one another. Rather than simply becoming disordered, the particles rearrange themselves into an unusual configuration that behaves differently from the quantum systems scientists have traditionally studied.

The theoretical findings provide the foundation for recent experimental research conducted by Nägerl’s team at the Department of Experimental Physics.

Pushing Quantum Particles Beyond Their Normal Limits

Under ordinary conditions, quantum particles follow well-established rules that determine how they arrange themselves, particularly at extremely low temperatures.

One important example involves fermions, a category of particles that includes electrons. These particles cannot all occupy the same quantum state, so they fill available energy levels according to specific restrictions. At very low temperatures, this arrangement produces what physicists call a Fermi sea.

But what happens when researchers deliberately disrupt the conditions that normally govern how particles behave?

As Alvise Bastianello explains: “Fermions, for instance, stack neatly into the available energy states to form the so-called ‘Fermi sea’. But what happens if one forces interacting atoms to continuously cycle through extreme conditions, smoothly shifting them from strongly repelling each other to strongly attracting each other?”

To investigate this possibility, the researchers studied ultracold cesium atoms confined to a one-dimensional system. This means the atoms are restricted to moving along a single direction, creating an environment in which their quantum interactions can be studied with exceptional control.

The team examined what happens when the interactions between these atoms are repeatedly adjusted, cycling between powerful repulsion and attraction.

Their calculations revealed something unexpected. Starting from the ground state, the lowest energy configuration available to the system, the atoms could be driven into a highly excited state without losing their underlying organization.

The researchers call this unusual arrangement a fractional Fermi sea because its particles appear to obey a modified rule governing how quantum states are occupied.

“Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state,” says Yi Zeng, the leading author of this study. “This gives us a controlled way to explore quantum matter beyond the usual equilibrium paradigms.”

A Strange New Quantum State With Hidden Order

One of the most intriguing features of the fractional Fermi sea is that its particles remain organized even though the system has been pushed far from equilibrium.

In physics, equilibrium describes a condition in which a system’s overall properties have settled into a stable balance. Systems driven far from equilibrium can behave in complicated and unexpected ways, making them particularly challenging to understand.

Yet the fractional Fermi sea is not simply a collection of excited particles behaving randomly. Instead, it contains distinctive mathematical patterns that reveal a deeper structure.

Among these patterns are pronounced ripples called Friedel oscillations. These are wave-like variations associated with the way quantum particles influence one another. The researchers also identified unusual patterns in how correlations between particles weaken with increasing distance, even when the repulsive interactions are relatively weak.

These signatures are important because they differ from predictions associated with Tomonaga-Luttinger liquid theory, a long-established framework for describing how interacting quantum particles behave in one-dimensional systems.

The findings therefore point toward a previously unexplored type of quantum behavior that cannot be fully described using this familiar theoretical picture.

“This state is highly excited, but it is not random,” says Hanns-Christoph Nägerl, the group leader. “It has a hidden order that becomes visible in its correlations.”

The strange behavior also raises questions about the effective particles that emerge within the system. Physicists call these collective excitations quasiparticles because they can behave like individual particles even though they arise from the coordinated motion of many interacting particles.

Nägerl suggests that the unusual excitations might eventually deserve a name of their own.

“We are not yet sure how we should name these new quasiparticles. Perhaps ‘super-Fermions’?”

Opening a New Frontier in Quantum Physics

The discovery suggests that fractional Fermi seas represent an exotic critical phase of quantum matter, with characteristic patterns of particle correlations that extend beyond existing descriptions of one-dimensional quantum systems.

It also demonstrates the growing power of quantum simulation, a technique that allows scientists to use carefully controlled quantum systems to investigate physical behavior that would otherwise be difficult to reproduce or study.

Ultracold atoms are particularly useful for this purpose because researchers can precisely adjust their interactions and observe how the resulting quantum states change.

Rather than limiting these experiments to recreating phenomena already understood through established theories, scientists can now explore deliberately engineered states that reveal entirely new possibilities.

As Hanns-Christoph Nägerl says: “The discovery of fractional Fermi seas shows how far we can push quantum simulation: not only reproducing known models, but creating and probing states that go beyond established paradigms.”

The current publication establishes the theoretical framework for fractional Fermi seas. A companion study describing their experimental realization through quantum simulation is undergoing peer review.

Together, the research points toward new ways of investigating the fundamental behavior of quantum matter, including unusual collective excitations and hidden forms of order that emerge only when particles are pushed far outside their normal conditions.

To investigate this question, researchers in Canada examined the effects of psilocybin on the amphibious mangrove rivulus fish (Kryptolebias marmoratus). Their findings, published in Frontiers in Behavioral Neuroscience, suggest that even a brief exposure to a low dose can significantly reduce aggressive behavior without disrupting normal social interactions.

“We show that an acute, low dose of psilocybin significantly reduces activity and aggressive attack behavior during social interactions in adult mangrove rivulus fish, a species that is naturally highly aggressive,” said first author Dayna Forsyth, a research associate and former MSc student at Acadia University in Nova Scotia.

“These findings provide the first evidence that psilocybin can selectively reduce escalated aggression in a vertebrate model without suppressing social interaction,” added senior author Dr. Suzie Currie, a biologist at The University of British Columbia.

Why Scientists Studied Aggressive Mangrove Rivulus Fish

Mangrove rivulus fish are naturally combative, particularly when they encounter another member of their species. Their aggressive responses are easy to recognize and measure, allowing researchers to detect even small changes in behavior.

These fish also have an unusual reproductive ability. They can fertilize themselves, producing offspring that are genetically identical. This makes them especially useful for experiments because scientists can minimize genetic variation and more confidently attribute behavioral differences to psilocybin rather than inherited traits.

For the experiment, the researchers worked with three genetically distinct lines of fish bred in the laboratory. One group received psilocybin, another provided the fish used to provoke social interactions, and the third helped researchers measure how much psilocybin entered and accumulated in the animals’ bodies.

The researchers first established a baseline for each fish’s behavior. They placed a fish into a tank already occupied by another fish, with the two separated by a fiberglass mesh barrier and an opaque cover. The mesh prevented physical contact while allowing the fish to see and smell each other once the cover was removed.

After allowing five minutes for the fish to adjust to their shared environment, the team removed the opaque cover and recorded their interactions.

The following day, 24 hours after the initial observation, the researchers placed the same fish in water containing dissolved psilocybin. Following 20 minutes of exposure, they returned it to the tank with the same fish it had encountered previously. The opaque cover was then removed again so the researchers could observe any changes in behavior.

Magic Mushroom Compound Reduces Aggressive Attacks

The results revealed a striking change. Fish exposed to psilocybin moved around less and displayed fewer bursts of aggressive swimming than fish that had not received the substance.

To assess these changes, the scientists measured overall activity (time spent moving) and several signs of aggression (including swimming bursts). These rapid bursts are particularly important because they indicate that an encounter is escalating toward an attack, even when the fish cannot make physical contact.

“Swimming bursts are high-energy attack behaviors that represent an escalation of aggression towards the stimulus fish without making physical contact,” explained Currie. “Other types of aggressive behaviors, like head-on displays, are more about communication and social assessment and require very little energy.”

Interestingly, psilocybin did not suppress every form of aggressive or social behavior. While the fish became less likely to engage in intense, energy-demanding attacks, their less physically demanding social displays remained largely unchanged.

“Psilocybin’s calming effect appears to selectively reduce energetically costly, escalated behaviors while lower-energy social display behaviors remained largely unchanged,” said Forsyth. “This suggests that this compound can selectively dampen escalated social conflict rather than shutting down behavior altogether.”

The compound also reduced general activity. When placed alongside another member of their species, fish treated with psilocybin spent less time swimming than untreated controls.

Together, these findings suggest that psilocybin may influence specific aspects of aggressive behavior rather than simply making animals less responsive to their surroundings.

What the Findings Could Mean for Psychedelic Research

The researchers believe animal studies like this one could eventually help scientists better understand how psychedelic compounds influence behavior and identify possible directions for therapeutic research.

By examining which social responses are most sensitive to psilocybin, researchers may be able to uncover biological mechanisms that are difficult to investigate directly in people. Such insights could ultimately inform studies of potential clinical applications.

However, the team emphasized that the findings should not be interpreted as evidence of a treatment for aggression in humans. The experiment involved fish, did not evaluate any clinical intervention, and cannot establish whether similar effects would occur in people.

There are also important questions about how long the calming effect lasts. The researchers examined only single doses and short exposure periods. They did not investigate repeated treatments, long-term behavioral changes, or whether the fish might adapt to the substance over time.

Additional experiments will be necessary to determine whether psilocybin’s ability to reduce aggressive attacks can persist beyond the brief period examined in this study.

“Future studies can build on this work to explore how psilocybin alters neural signaling, which serotonin pathways are involved, and why some aspects of social behavior are affected while others are not,” concluded Currie. “These are questions that are difficult or impossible to answer directly in humans.”