Physicists Create a New Quantum State: The Fractional Fermi Sea
Physicists have uncovered a strange new form of quantum behavior in which particles organize themselves in ways that challenge established theories. Surprisingly, these particles maintain a hidden order despite being in an extremely energetic state.
In a study published in Physical Review Letters, researchers led by Hanns-Christoph Nägerl, in collaboration with Alvise Bastianello of CNRS and Université Paris-Dauphine, describe how to engineer an exotic quantum state known as a ‘fractional Fermi sea.’
This breakthrough involves pushing ultracold cesium atoms beyond their normal conditions by changing their interactions. Rather than becoming disordered, the particles rearrange into a configuration that behaves differently from traditional quantum systems.
Pushing Quantum Particles Beyond Their Normal Limits
Under ordinary conditions, quantum particles follow established rules for arrangement, especially at low temperatures. Fermions, such as electrons, cannot occupy the same quantum state, filling energy levels according to specific restrictions, leading to what physicists call a Fermi sea.
Researchers investigated what happens when these conditions are disrupted. Alvise Bastianello noted, “Fermions stack neatly into available energy states to form the so-called ‘Fermi sea.’ But what if one forces interacting atoms to cycle through extreme conditions, shifting from repulsion to attraction?”
The team studied ultracold cesium atoms confined to a one-dimensional system, allowing exceptional control over their quantum interactions. They adjusted the interactions between these atoms, cycling between strong repulsion and attraction.
Their calculations revealed that starting from the ground state, the atoms could be driven into a highly excited state while retaining their underlying organization. This unusual arrangement is termed a fractional Fermi sea, where particles obey a modified rule for occupying quantum states.
“Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state,” said Yi Zeng, the study’s lead author.
A Strange New Quantum State With Hidden Order
One intriguing feature of the fractional Fermi sea is that its particles remain organized despite being far from equilibrium. Systems driven far from equilibrium can behave unpredictably, yet the fractional Fermi sea exhibits distinctive mathematical patterns indicative of deeper structure.
Among these patterns are Friedel oscillations, wave-like variations linked to quantum particle interactions. The researchers also noted unusual correlations between particles that weaken with distance, even under weak repulsive interactions. These findings differ from predictions of Tomonaga-Luttinger liquid theory, suggesting a new type of quantum behavior.
“This state is highly excited, but it is not random,” said Nägerl. “It has a hidden order that becomes visible in its correlations.” This behavior raises questions about the effective particles that emerge, termed quasiparticles, which behave like individual particles despite arising from many interacting particles. Nägerl suggested these excitations might be called “super-Fermions.”
Opening a New Frontier in Quantum Physics
The discovery indicates that fractional Fermi seas represent an exotic phase of quantum matter, with particle correlation patterns extending beyond existing descriptions. It highlights the power of quantum simulation, allowing scientists to use controlled quantum systems to explore physical behaviors that are otherwise difficult to study.
Ultracold atoms are particularly useful as researchers can precisely adjust interactions and observe changes in quantum states. This research enables exploration of engineered states that reveal new possibilities beyond established theories.
As Nägerl stated, “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, with a companion study on their experimental realization undergoing peer review. Together, this research points toward new ways to investigate the fundamental behavior of quantum matter and the unusual excitations that emerge when particles are pushed beyond normal conditions.
Source: ScienceDaily