Universal spin dynamics in two-dimensional Fermi gases

Harnessing spins as information carriers has emerged as an elegant extension to the transport of electrical charges'. The coherence of such spin transport in spintronic circuits is determined by the lifetime of spin excitations and by spin diffusion. Fermionic quantum gases allow the study of spin transport from first principles because interactions can be precisely tailored and the dynamics is on directly observable timescales(2-12). In particular, at unitarity, spin transport is dictated by diffusion and the spin diffusivity is expected to reach a universal, quantum-limited value on the order of the reduced Planck constant h divided by the mass m. Here, we study a two-dimensional Fermi gas after a quench into a metastable, transversely polarized state. Using the spin-echo technique(13) for strong interactions, we measure the lowest transverse spin diffusion constant(14,15) so far 0.25(3) (h) over bar /m. For weak interactions, we observe a collective transverse spinwave mode that exhibits mode softening when approaching the strongly interacting regime.