Brief
Physicists engineer a fractional Fermi sea in ultracold cesium atoms
The team demonstrated that cycling interactions in a one‑dimensional cesium gas can produce a highly excited yet ordered many‑body state.
By Felo News Desk · Published
Physicists have demonstrated a novel quantum state dubbed a “fractional Fermi sea” by repeatedly driving ultracold cesium atoms through extreme interaction regimes, according to a study published in Physical Review Letters and reported by ScienceDaily on 18 October 2026.
The research was conducted by a group led by Hanns‑Christoph Nägerl, with theoretical input from Alvise Bastianello of the CNRS and Université Paris‑Dauphine. Yi Zeng, the study’s lead author, explained that the experiment involved confining cesium atoms to a one‑dimensional trap and cyclically varying the inter‑atomic forces from strong repulsion to strong attraction. Rather than heating the system into disorder, the interaction cycle reorganised the atoms into a new many‑body configuration that retains a hidden order despite being highly excited.
In conventional quantum physics, fermions such as electrons obey the Pauli exclusion principle, filling available energy levels to form a “Fermi sea” at very low temperatures. The researchers asked what would happen if these fermionic atoms were forced to repeatedly cross between opposite interaction extremes. Their calculations showed that the atoms could be driven from the ground state into a highly excited state while still obeying a modified occupancy rule, which they term a fractional Fermi sea.
“Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many‑body state,” Yi Zeng said, as quoted by ScienceDaily. The theoretical framework provides a controlled pathway for exploring exotic quantum phases that have so far been inaccessible in laboratory settings.
The findings build on earlier work in ultracold atom physics, including recent advances in measuring quantum energy ladders and observing slow electron motion in novel materials, topics previously covered by Felo News. The fractional Fermi sea could open avenues for studying non‑equilibrium quantum dynamics and may eventually inform the design of quantum simulators that mimic complex many‑body phenomena.
Key facts
- The study was published in Physical Review Letters. (sciencedaily.com)
- Researchers cycled ultracold cesium atoms between strong repulsion and attraction in a one‑dimensional trap. (sciencedaily.com)
- The resulting state is called a fractional Fermi sea, a highly excited yet ordered many‑body configuration. (sciencedaily.com)
- Lead experimental team was headed by Hanns‑Christoph Nägerl; theoretical work involved Alvise Bastianello. (sciencedaily.com)
- Yi Zeng is the leading author of the paper. (sciencedaily.com)
Sources
- [1] sciencedaily.com — originally reported as “Physicists create a bizarre new quantum state called a fractional fermi sea”
