Double-slit vacuum polarization effects in ultraintense laser fields

The influence of the strong laser-driven vacuum on a propagating electromagnetic probe wave has been studied in detail. We investigate two scenarios, which comprise a focused probe laser beam that passes through a region of vacuum polarized by an ultraintense laser field. By splitting this strong field into two separated monochromatic Gaussian pulses that counterpropagate in a plane perpendicular to the probe-field axis, we demonstrate a leading-order light-by-light diffraction effect that generates an interference pattern reminiscent of the classic double-slit experiment. We calculate the total number of probe photons diffracted as well as the number diffracted into regions where the vacuum polarization signal is higher than the probe background. In addition, we calculate the induced ellipticity and polarization rotation in the probe beam and show how, in the realistic situation in which the centers of the two strong fields are not exactly aligned, certain ranges of beam separation and observation distance may actually lead to an increase over the idealized case of a single strong laser beam.