Transverse Demagnetization Dynamics of a Unitary Fermi Gas*

Understanding the quantum dynamics of strongly interacting fermions is a challenge raised by diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasmas. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions, where the system is both scale-invariant and known to obey universal thermodynamics in equilibrium. Here we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We find that demagnetization is caused by diffusive spin transport with a diffusion constant that saturates at low temperatures to the conjectured quantum-mechanical lower bound $\hbar/m$, where $m$ is the particle mass. The development of pair correlations is observed by measuring Tan's contact parameter.