Is an Ultra-Cold Strongly Interacting Fermi Gas a Perfect Fluid?

Fermi gases with magnetically tunable interactions provide a clean and controllable laboratory system for modeling interparticle interactions between fermions in nature. The s-wave scattering length, which is dominant a low temperature, is made to diverge by tuning near a collisional (Feshbach) resonance. In this regime, two-component Fermi gases are stable and strongly interacting, enabling tests of nonperturbative many-body theories in a variety of disciplines, from high temperature superconductors to neutron matter and quark-gluon plasmas. We have developed model-independent methods for measuring the entropy and energy of this model system, providing a benchmark for calculations of the thermodynamics. Our experiments on the expansion of rotating strongly interacting Fermi gases in the normal fluid regime reveal extremely low viscosity hydrodynamics. Combining the thermodynamic and hydrodynamic measurements enables an estimate of the ratio of the shear viscosity to the entropy density. A strongly interacting Fermi gas in the normal fluid regime is found to be a nearly perfect fluid, where the ratio of the viscosity to the entropy density is close to a universal minimum that has been conjectured by string theory methods.