An unusual new type of quantum state has been revealed by theoretical physicists, which they say demonstrates a critical new phase of matter that arises from pushing particles to their limits.
Known as “fractional Fermi seas,” this class of quantum states can be created on demand by exciting quantum particles beyond their typical equilibrium states, as detailed in new research published in Physical Review Letters, based on findings by theoretical physicist Alvise Bastianello of CNRS and Université Paris-Dauphine, working in collaboration with the Nägerl group.
To achieve this, the team relied on ultracold cesium atoms, which they were able to successfully confine to a single dimension. Then, by manipulating the strength of the interactions between these particles, the unusual quantum state they induced produced characteristics the team says are far beyond predicted models.
Going Beyond What Models Predict
Specifically, the findings reveal characteristics that extend well beyond what physicists call the Tomonaga-Luttinger liquid theory, a well-characterized model that describes the way electrons or other fermions operate within a one-dimensional conductor, such as carbon nanotubes that restrict their movement to a single dimension.
“Fermions, for instance, stack neatly into the available energy states to form the so-called ‘Fermi sea’,” according to researcher Alvise Bastianello, who notes that quantum particles generally behave predictably at low temperatures and follow very specific rules in terms of their self-arrangement.
“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?” he asks.
To address this, the team implemented experiments where they created conditions that repeated this interaction cycle, intentionally forcing atoms from their typical ground state into more highly excited ones. Despite their excited state, the atoms still maintained a surprising degree of organization as the so-called “fractional Fermi sea” was induced.
“Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state,” according to lead author Yi Zeng, who added that the team’s observations provided “a controlled way to explore quantum matter beyond the usual equilibrium paradigms.”
Rise of the “Super-Fermions”
One of the team’s key findings had been the odd conditions they observed within the newly created state, which include what are characterized as Friedel oscillations, a term for the ripples the team observed as anticipated based on mathematical correlations between particles. Additionally, the team says they observed decay behavior that was present across all levels of the repulsive interactions between particles.
“This state is highly excited, but it is not random,” according to team leader Hanns-Christoph Nägerl. This is significant, he says, since the state the research team achieved is markedly different from what Tomonaga-Luttinger liquid theory predicts.
“It has a hidden order that becomes visible in its correlations,” Nägerl said of the team’s unique discovery, adding that they have not yet determined the name they plan to give these newly characterized quasiparticles, although ‘super-Fermions’ is a possible contender.
Evidence of Exotic States
Also of key importance, the team believes these specific signatures could point to the presence of a new and exotic type of critical phase which has never been previously recognized, and which could point to promising new approaches to studying quantum mysteries with the help of cold atom simulation technologies.
Presently, the team says a companion paper is in the works, which elaborates on the team’s experiments and the creation of fractional Fermi seas through quantum simulations.
“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,” Nägerl concluded.
Presently, the team’s work, detailed in the papers “Exotic critical states as fractional Fermi seas in the one-dimensional Bose gas,” and “Realization of fractional Fermi seas” is available on the preprint arXiv.org server.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.