Level 1 — Absolute Beginner

Scientists have found a new kind of matter. It is not solid, liquid, or gas. It is called a fractional Fermi sea. It only exists when atoms are very, very cold.

Scientists in Austria made this new matter in a lab. They used special atoms called caesium. They cooled the atoms to almost the coldest temperature possible.

This discovery could help us build better computers in the future. These computers would use rules from quantum physics to solve very hard problems.

matter

anything that has mass and takes up space, such as solids, liquids, and gases

atom

the smallest unit of a chemical element

temperature

a measure of how hot or cold something is

quantum

relating to the very small world of atoms and particles

experiment

a scientific test done to find out something or prove an idea

discovery

finding something new that was not known before

laboratory

a room or building used for scientific experiments

physics

the science that studies matter, energy, force, and motion

Level 2 — Elementary

Scientists at the University of Innsbruck in Austria have created a new state of quantum matter. They call it the fractional Fermi sea. To make it, they cooled caesium atoms to temperatures just above absolute zero, the coldest point that is physically possible.

The researchers trapped the atoms in a very narrow, tube-like space so they could only move in one direction. They then applied cycles of changing forces to push the atoms out of their normal state. This drove the atoms into a completely new quantum phase that scientists had never seen before.

The discovery is important because it cannot be explained by existing theories. It was published in the journal Physical Review Letters in June 2026. Scientists hope it will improve our understanding of exotic materials and lead to new types of computers that use quantum effects.

absolute zero

the lowest possible temperature, equal to minus 273.15 degrees Celsius

quantum phase

a distinct state of matter defined by quantum mechanical properties

theory

a carefully reasoned explanation for how or why something happens

exotic material

a substance with unusual properties not found in ordinary matter

trap

a device used to hold atoms in a fixed region using electromagnetic fields or lasers

cycle

a repeated sequence of events or changes

equilibrium

a state of balance where conditions are not changing

journal

a scientific publication where researchers share their findings

Level 3 — Intermediate

A research team led by Hanns-Christoph Nagerl at the University of Innsbruck, in collaboration with theorist Alvise Bastianello at CNRS and Universite Paris-Dauphine, has experimentally observed a new quantum state of matter they call the fractional Fermi sea. The experiment used ultracold caesium atoms confined in a one-dimensional optical trap, cooled to temperatures within a billionth of a degree above absolute zero.

To create the fractional Fermi sea, the team applied cyclic modulation of the inter-particle interactions. This process drove the atomic gas far out of equilibrium, causing it to enter a critical quantum phase that had not been previously observed. The state is characterised by fractional occupation of quantum energy levels, a behaviour that existing theoretical frameworks, including the well-established Tomonaga-Luttinger liquid theory, cannot fully describe.

The results, published in Physical Review Letters in June 2026, represent a challenge to the field's standard models of one-dimensional quantum systems. The researchers suggest that the fractional Fermi sea may be related to phenomena such as fractional quantum Hall states and topological phases, both of which are being explored for their potential applications in fault-tolerant quantum computing.

optical trap

a device using focused laser beams to hold and position atoms or particles

Tomonaga-Luttinger liquid

a theoretical model describing the behaviour of interacting particles in one-dimensional systems

cyclic modulation

a repeating variation in a physical quantity, such as the strength of interaction between particles

fractional occupation

the quantum state in which energy levels are filled in fractions rather than whole numbers

topological phase

a state of matter defined by global, non-local properties that are stable against small perturbations

fault-tolerant

designed to continue operating correctly even when individual components fail

inter-particle interaction

the forces that act between individual particles in a physical system

one-dimensional

confined to movement or variation along a single spatial direction

Level 4 — Advanced

A collaboration between the experimental group of Hanns-Christoph Nagerl at the University of Innsbruck and theorist Alvise Bastianello at CNRS and Universite Paris-Dauphine has produced the first experimental realisation of a state of quantum matter they term the fractional Fermi sea. The platform is an ultracold gas of caesium-133 atoms confined within a one-dimensional optical lattice, cooled to temperatures on the order of a few nanokelvin. The key perturbation that drives the system into this novel phase is a cyclic modulation of the inter-particle contact interactions, which leverages a Feshbach resonance to tune the scattering length periodically in time.

The resulting non-equilibrium steady state exhibits fractional occupation numbers at quantum energy levels, a signature that lies outside the descriptive scope of Tomonaga-Luttinger liquid theory, the canonical framework for one-dimensional interacting Fermi systems. The term fractional Fermi sea is chosen by the authors to signal an analogy with the fractional quantum Hall effect in two dimensions, where quasiparticles carry fractional charge, while marking the distinct physical mechanism at play: periodic driving rather than Landau level mixing. The theoretical work by Bastianello identified the relevant non-equilibrium fixed point via a generalised hydrodynamics framework that incorporates Floquet driving.

The implications for the broader field are substantial. One-dimensional quantum systems have long served as exactly solvable laboratories for testing many-body physics, but the behaviour of such systems under strong periodic driving has remained poorly understood. The fractional Fermi sea provides the first direct experimental anchor for theoretical work in this regime, opening the possibility of engineering exotic quasiparticle statistics relevant to topological quantum error correction. The authors note that similar phases may be accessible in other platforms, including cold atoms in cavity quantum electrodynamics setups and designer quantum materials at low temperature.

Feshbach resonance

a phenomenon in ultracold physics where inter-atomic interactions can be tuned by an external magnetic field

scattering length

a parameter that characterises the strength and nature of interactions between quantum particles

non-equilibrium fixed point

a point in the space of physical parameters where a driven system settles into a stable but non-static state

generalised hydrodynamics

a theoretical framework that extends fluid dynamics to describe the behaviour of integrable quantum systems

Floquet driving

the periodic time-varying perturbation of a quantum system, named after the mathematician Gaston Floquet

quasiparticle statistics

the effective statistical properties of collective excitations in a many-body system