  By taking the spin and polarization of the electrons, positrons and photons
into account in the strong-field QED processes of nonlinear Compton emission
and pair production, we find that the growth rate of QED cascades in
ultra-intense laser fields can be substantially reduced. While this means that
fewer particles are produced, we also found them to be highly polarized. We
further find that the high-energy tail of the particle spectra is polarized
opposite to that expected from Sokolov-Ternov theory, which cannot be explained
by just taking into account spin-asymmetries in the pair production process,
but results significantly from "spin-straggling". We employ a kinetic equation
approach for the electron, positron and photon distributions, each of them
spin/polarization-resolved, with the QED effects of photon emission and pair
production modelled by a spin/polarization dependent Boltzmann-type collision
operator. For photon-seeded cascades, depending on the photon polarization, we
find an excess or a shortage of particle production in the early stages of
cascade development, which provides a path towards a controlled experiment.
Throughout this paper we focus on rotating electric field configuration, which
represent an idealized model and allows for a straightforward interpretation of
the observed effects.
