Massless Dirac particle in a stochastic magnetic field: A solvable   quantum walk approximation
  A massless Dirac particle is considered, moving along the x-axis while Pauli-coupled by its anomalous magnetic moment to a piecewise constant magnetic field along the same axis, with stochastically varying sign. The motion is approximated as a quantum walk with unitary noise, for which the evolution can be found exactly. Initially ballistic, the motion approaches a classical diffusion on a time-scale determined by the speed of light, the size of the magnetic moment, the strength of the field and the time interval between changes in its direction. It is suggested that a process of this type could occur in the Sun's corona, significantly affecting the solar fluxes of one or more neutrino types. 
A. J. Bracken, D. Ellinas and I. Smyrnakis
quant-ph hep-th