All-optical quantum vacuum signals in two-beam collisions

We study the collision of two optical laser pulses in a pump-probe setup using beams with circular and elliptic cross section estimate number discernible signal photons induced by quantum vacuum nonlinearities. In this analysis we strategies to optimize from background driving lasers. One main results is that maximally focused lasers does not lead best signal. Instead, widening focus typically improves separation far field. For petawatt class lasers, an optimal choice waist yields several per shot contrast no for tight focusing. Further enhancement possible elliptical waist.