Is a gas of strongly interacting atomic fermions a nearly perfect fluid?

Abstract

We use all-optical methods to produce a highly-degenerate Fermi gas of spin-1/2 $^6$Li atoms. A magnetic field tunes the gas near a collisional (Feshbach) resonance, producing strong interactions between spin-up and spin-down atoms. This atomic gas is a paradigm for strong interactions in nature, and provides tests of current quantum many-body calculational methods for diverse systems, including very high temperature superconductors, nuclear matter in neutron stars, and the quark-gluon plasma of the Big Bang. We have measured properties of a breathing mode over a wide range of temperatures. At temperatures both below and well above the superfluid transition, the frequency of the mode is nearly constant and very close to the hydrodynamic value. However, explaining both the frequency and the damping rate in the normal collisional regime has not been achieved. Our measurements of the damping rate as a function of the energy of the gas are used to estimate an upper bound on the viscosity. Using our new measurements of the entropy of the gas, we estimate the ratio of the shear viscosity to the entropy density, and compare the result with the lower bound for quantum viscosity recently predicted using string theory methods.