  The spin-orbit interaction (SOI) in zincblende semiconductor quantum wells
can be set to a symmetry point, in which spin decay is strongly suppressed for
a helical spin mode. Signatures of such a persistent spin helix (PSH) have been
probed using the transient spin grating technique, but it has not yet been
possible to observe the formation and the helical nature of a PSH. Here we
directly map the diffusive evolution of a local spin excitation into a helical
spin mode by a time- and spatially resolved magneto-optical Kerr rotation
technique. Depending on its in-plane direction, an external magnetic field
interacts differently with the spin mode and either highlights its helical
nature or destroys the SU(2) symmetry of the SOI and thus decreases the spin
lifetime. All relevant SOI parameters are experimentally determined and
confirmed with a numerical simulation of spin diffusion in the presence of SOI.
