Observation of Shock Waves in a Strongly Interacting Fermi Gas

We study collisions between two strongly interacting atomic Fermi gas clouds. We observe exotic nonlinear hydrodynamic behavior, distinguished by the formation of a very sharp and stable density peak as the clouds collide and subsequent evolution into a boxlike shape. We model the nonlinear dynamics of these collisions by using quasi-1D hydrodynamic equations. Our simulations of the time-dependent density profiles agree very well with the data and provide clear evidence of shock wave formation in this universal quantum hydrodynamic system.

Figure 1

Collision between two strongly interacting Fermi gas clouds in a cigar-shaped optical trap. The clouds are initially separated by a repulsive 532 nm optical beam. After the 532 nm beam is extinguished (0 ms), the clouds approach each other. False color absorption images show the spatial profiles versus time. Initially, a sharp rise in density occurs in the center of the collision zone. At later times the region of high density evolves from a “peaklike” shape into a “boxlike” shape as the shock front propagates outward. The well defined edges of the central zone in the last four images provide evidence of shock wave formation in the strongly interacting Fermi gas.

Figure 2

1D density profiles divided by the total number of atoms versus time for two colliding strongly interacting Fermi gas clouds. The normalized density is in units of ${10}^{-2}/\mu \mathrm{m}$ per particle. Red dots show the measured 1D density profiles. Black curves show the simulation, which uses the measured trap parameters and the number of atoms, with the kinetic viscosity as the only fitting parameter.