  Ultra strong electromagnetic fields can lead to spontaneous creation of
single or multiple electron-positron pairs. A quantum field theoretical
treatment of the pair creation process combined with numerical methods provides
a description of the fermionic quantum field state, from which all observables
of the multiple electron-positron pairs can be inferred. This allows to study
the complex multi-particle dynamics of electron-positron pair creation
in-depth, including multi-pair statistics as well as momentum distributions and
spin. To illustrate the potential benefit of this approach, it is applied to
the intermediate regime of pair creation between nonperturbative Schwinger pair
creation and perturbative multiphoton pair creation where the creation of
multi-pair states becomes nonnegligible but cascades do not yet set in.
Furthermore, it is demonstrated how spin and helicity of the created electrons
and positrons are affected by the polarization of the counterpropagating laser
fields, which induce the creation of electron-positron pairs.
