Hydrodynamic Properties of the Unitary Fermi Gas*

The unitary Fermi gas is a paradigmatic model for other strongly interacting Fermi systems, from atomic nuclei to neutron stars, and can be efficiently realized with ultracold atoms near a Feshbach resonance. Strong interactions and fermion antisymmetry render theoretical predictions highly challenging, in particular for transport properties such as density, spin, heat and momentum transport. Here, we prepare a spin-balanced, homogeneous gas of ⁶Li atoms at unitarity, trapped within a homogeneous box potential that removes complications from non-uniform density. We observe the response of the gas to local density and temperature perturbations in both the normal and superfluid phases and extract the associated diffusivities. In the degenerate regime, and near the superfluid critical temperature, these diffusivities attain a Heisenberg limit. This behavior contrasts with that expected for Fermi liquids, where instead diffusivities would strongly rise at low temperatures due to Pauli blocking. Our precision measurements of transport coefficients can serve as a benchmark for many-body theories of strongly interacting fermionic matter.