  A derivation of fully polarization-resolved probabilities is provided for
high-energy photon emission and electron-positron pair production in
ultrastrong laser fields. The probabilities resolved in both electron spin and
photon polarization of incoming and outgoing particles are indispensable for
developing QED Monte Carlo and QED-Particle-in-Cell codes, aimed at the
investigation of polarization effects in nonlinear QED processes in
ultraintense laser-plasma and laser-electron beam interactions, and other
nonlinear QED processes in external ultrastrong fields, which involve multiple
elementary processes of a photon emission and pair production. The quantum
operator method introduced by Baier and Katkov is employed for the calculation
of probabilities within the quasiclassical approach and the local constant
field approximation. The probabilities for the ultrarelativistic regime are
given in a compact form and are suitable to describe polarization effects in
strong laser fields of arbitrary configuration, rendering them very well suited
for applications.
