At the coldest edge of physics, atoms stop behaving like atoms.

Physicists have created what they call a fractional Fermi sea, a new phase of quantum matter produced by cooling about 70,000 cesium atoms to a few nanokelvin, or billionths of a degree above absolute zero, then driving them through repeated cycles of strong repulsion and attraction.

The work was published in Physical Review Letters and is also available on arXiv.

At these temperatures, quantum statistics govern how particles behave. Bosons, including photons and the Higgs boson, can occupy the same quantum state. Fermions, including electrons and the quarks that make up protons and neutrons, cannot, because only one fermion may occupy a quantum state under the Pauli exclusion principle.

“Fermions, for instance, stack neatly into the available energy states to form the so-called ‘Fermi sea’,” said Alvise Bastianello, a theoretical physicist at the French National Centre for Scientific Research and the Université Paris-Dauphine.

“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 test that, researchers first created a Bose gas using the cesium atoms. In those conditions, the atoms begin acting as a unified entity. The team then confined the matter inside one-dimensional tubes created by a two-dimensional optical lattice, using lasers to trap the atoms for observation.

Researchers then repeatedly pushed the atoms through interaction cycles, making them strongly repel and then strongly attract each other.

The result was the fractional Fermi sea.

In a standard Fermi sea, fermions fill quantum states one by one. In this new state, quantum states can be only partially occupied, a feature the researchers say may appear only in lower-dimensional experiments.

The repeated pulses also produced an outcome that ran against expectations.

“Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state,” said lead author Yi Zeng, a condensed matter physicist at the University of Innsbruck in Austria.

Research leader Hanns-Christoph Nägerl, a professor at the University of Innsbruck who works in experimental quantum physics, said: “This state is highly excited, but it is not random. It has a hidden order that becomes visible in its correlations.”

The team also reported apparent Friedel oscillations, which the article describes as the “smoking gun” evidence of a fractional Fermi sea.

The researchers are still working out what to call the new quasiparticles involved.

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

“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 said.

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