  In this note, we show that the latest spin grating measurement of spin helix
by Koralek {\em et al.} [Nature {\bf 458}, 610 (2009)] provides strong evidence
of the infinite spin diffusion length of any spin polarization along the
direction perpendicular to the effective magnetic field from the
  Dresselhaus and Rashba spin-orbit couplings with identical strengths in
$(001)$ GaAs quantum wells, predicted by Cheng {\em et al.} [Phys. Rev. B {\bf
75}, 205328 (2007)].
