Transport of strongly interacting fermions governs modern materials -- from the high-$T_c$ cuprates to bilayer graphene --, but also nuclear fission, the merging of neutron stars and the expansion of the early universe. Here we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting Fermi gas of atoms by studying sound propagation and its attenuation via the coupled transport of momentum and heat. In the normal state, the sound diffusivity ${ D}$ monotonically decreases upon lowering the temperature $T$, in contrast to the diverging behavior of weakly interacting Fermi liquids. As the superfluid transition temperature is crossed, ${ D}$ attains a universal value set by the ratio of Planck's constant ${ h}$ and the particle mass ${ m}$. This finding of quantum limited sound diffusivity informs theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons and quarks.
Categories: cond-mat.quant-gas cond-mat.stat-mech cond-mat.str-el