News Overview

Macroscopic fluid dynamics is usually thought to emerge from vast numbers of microscopic particles. Now, scientists have studied fluid-like behaviour in systems of as few as ten ultracold lithium atoms.

How many particles does it take to form a fluid? A new study led by STRUC­TURES researchers, published in Nature Physics, reveals that fluid-like collective behaviour can emerge with as few as ten ultracold lithium atoms. Inspired by observations in high-energy nuclear collisions, where similar phenomena are seen in systems with only a few dozen constituents, the researchers explored the onset of collectivity in quan­tum systems.

By precisely controlling the number of atoms and the strength of their interactions, they observed elliptic flow – a striking inversion of the initial aspect ratio that is a hallmark of hydrodynamic behaviour. This phenomenon, typically associated with much larger systems, challenges the conventional understanding that elliptic flow requires vast numbers of particles.

The study not only challenges long-held assumptions but also provides access to observables that remain elusive in high-energy nuclear collisions. This interdisciplinary effort, combining advanced experiments and theo­re­ti­cal modelling, paves the way for a deeper understanding of collective phenomena in quan­tum systems and opens exciting new avenues for re­search at the interface of quan­tum physics and particle physics.

Further information:

• Original Publication: Brandstetter, S., Lunt, P., Heintze, C. et al. Emergent interaction-driven elliptic flow of few fermionic atoms. Nat. Phys. (2025).
• News & Views: Fluids constructed atom by atom.
• STRUC­TURES Blog post: The Curious Case of the World’s Smallest Fluid.
• Jochim Labs: Ultracold Quan­tum Gases.