  We demonstrate that a homogeneous spin polarization in one-dimensional
structures of finite length in the presence of Bychkov-Rashba spin-orbit
coupling decays spontaneously toward a persistent spin helix. The analysis of
formation of spin helical state is presented within a novel approach based on a
mapping of spin drift-diffusion equations into a heat equation for a complex
field. Such a strikingly different and simple method allows generating robust
spin structures whose properties can be tuned by the strength of the spin orbit
interaction and/or structure's length. We generalize our results for
two-dimensional case predicting formation of persistent spin helix in
two-dimensional channels from homogeneous spin polarization.
