We extract the shear viscosity to entropy density ratio $\eta /s$ of cold fermionic atoms in the unitarity limit from experimental data on the damping of collective excitations. We find that near the critical temperature $\eta /s$ is roughly equal to 1/2 in units of $\hslash /k_B$. With the possible exception of the quark gluon plasma, this value is closer to the conjectured lower bound $1/\left(4\pi \right)$ than any other known liquid.

• Received 26 January 2007

DOI:https://doi.org/10.1103/PhysRevA.76.063618