Emergent interaction-driven elliptic flow of few fermionic atoms

Hydrodynamics is a successful framework for effectively describing the dynamics of complex many-body systems, ranging from subnuclear to cosmological scales. It applies coarse-grained assumptions about the microscopic constituents of a system to define macroscopic fluid cells, which are large compared to the interparticle spacing and mean free path. In high-energy heavy-ion collisions, hydrodynamic behaviour is inferred from the observation of elliptic flow, which is the elliptical deformation of the particle momentum distribution. Here we demonstrate the emergence of elliptic flow in a mesoscopic system with a few strongly interacting ultracold atoms. In our system, a hydrodynamic description is a priori not applicable, as all relevant length scales—the system size, the interparticle spacing and the mean free path—are comparable. The single-particle resolution and the deterministic control over the number of particles and interaction strength in our experiment allow us to explore the boundaries between a microscopic description and a hydrodynamic framework, and we show that elliptic flow appears as an interaction-driven effect. Our results demonstrate the emergence of collective behaviour in a regime where hydrodynamics is not usually applicable. Hydrodynamics can describe the collective behaviour of many-body systems when all the relevant length scales are separable. Now, an experiment with ultracold atoms shows signatures of hydrodynamic behaviour in a regime where length scales are comparable.