We explore Schwinger pair production in rotating time-dependent electric fields using the real-time DHW formalism. We determine the time evolution of the Wigner function as well as asymptotic particle distributions neglecting back-reactions on the electric field. Whereas qualitative features can be understood in terms of effective Keldysh parameters, the field rotation leaves characteristic imprints in the momentum distribution that can be interpreted in terms of interference and multiphoton effects. These phenomena may seed characteristic features of QED cascades created in the antinodes of a high-intensity standing wave laser field.

We present a comparison of two methods to compute the momentum spectrum and the Schwinger pair creation rate for pulsed rotating electric fields: one based upon the real-time Dirac-Heisenberg-Wigner (DHW) formalism and a semiclassical approximation based on a scattering ansatz. For the semiclassical method we propose to either perform numerical calculations or an additional approximation based on an analytical solution for the constant rotating field. We find that both numerical methods are complementary with respect to computation time as well as accuracy. The approximate method shows the same qualitative features while being computationally much faster. We additionally find that the unequal production of pairs in different spin states reported for constant rotating fields with the scattering method is in agreement with the Wigner function method.

We show that coherent harmonic focusing provides an efficient mechanism to boost all-optical signatures of quantum vacuum nonlinearity in the collision of high-intensity laser fields. Assuming two laser pulses of given parameters at our disposal, we demonstrate a substantial increase of the number of signal photons measurable in experiments where one of the pulses undergoes coherent harmonic focusing before it collides with the fundamental-frequency pulse. Imposing a quantitative criterion to discern the signal photons from the background of the driving laser photons as well as accounting for the finite purity of polarization filtering, the discernible signal photons are found to arise from manifestly inelastic interaction processes of the driving laser fields. Our results suggest that coherent harmonic focusing offers a promising new route to the first detection of signatures of quantum vacuum nonlinearities at the high-intensity frontier in the laboratory.