Emergent interaction-driven elliptic flow of few fermionic atoms

Hydrodynamics provides a successful framework to effectively describe the dynamics of complex many-body systems ranging from subnuclear to cosmological scales by introducing macroscopic quantities such as particle densities and fluid velocities. According to textbook knowledge, it requires coarse graining over microscopic constituents to define a macroscopic fluid cell, which is large compared to the interparticle spacing and the mean free path. In addition, the entire system must consist of many such fluid cells. In high energy heavy ion collisions, hydrodynamic behaviour is inferred from the observation of elliptic flow. Here, we demonstrate the emergence of elliptic flow in a system of few strongly interacting atoms. In our system a hydrodynamic description is a priori not applicable, as all relevant length scales, i.e. the system size, the inter-particle spacing, and the mean free path are comparable. The single particle resolution, deterministic control over particle number and interaction strength in our experiment allow us to explore the boundaries between a microscopic description and a hydrodynamic framework in unprecedented detail.