Cambridge physicists create fractional Fermi sea using cooled lithium-6 atoms

Physicists at the Quantum Dynamics Laboratory in Cambridge have reported the creation of a new quantum state called a fractional Fermi sea. The team cooled a cloud of lithium‑6 atoms to a few nanokelvin and trapped them in a three‑dimensional optical lattice. By tuning the interaction strength with a Feshbach resonance, the atoms arranged themselves in a highly excited but surprisingly ordered pattern that does not match the conventional Fermi‑liquid picture. The researchers published their findings in the journal *Nature Physics* on Tuesday.

The discovery matters because it challenges the long‑standing assumption that fermionic particles fill momentum states up to a sharp Fermi surface. In a normal Fermi sea, each quantum state is either empty or fully occupied. In the new fractional Fermi sea, the occupancy of each state is fractional, suggesting the emergence of new quasiparticles that behave like “super‑Fermions.” These exotic excitations could provide a bridge between fermionic and bosonic behavior, offering fresh insights into high‑temperature superconductors and other strongly correlated systems that have resisted explanation for decades.

Experimentally, the researchers used time‑of‑flight imaging to map the momentum distribution of the atoms after releasing them from the lattice. The images showed a smeared‑out Fermi surface with a continuous occupation probability between zero and one, rather than a step function. The team also measured correlation functions that revealed long‑range coherence despite the system’s high energy density. Colleagues at MIT and the University of Chicago praised the work as a “landmark” for quantum simulation, noting that it opens a new playground for testing theories of quantum matter that were previously only accessible in solid‑state materials.

The next step is to explore how the fractional Fermi sea behaves under different conditions. The researchers plan to vary the lattice geometry and introduce disorder to see whether the super‑Fermions remain robust. Theoretical groups are already working on extensions of the Hubbard model that could capture the observed fractional occupancy. If the state can be stabilized in solid‑state systems, it might lead to new ways to engineer materials with tailored electronic properties, potentially impacting quantum computing and energy‑efficient electronics.