Two-beam laser photon merging

Quasi-elastic scattering processes have long been thought of providing the most promising signal for a first experimental detection of quantum vacuum nonlinearity. A prominent example of such a process is vacuum birefringence. However, these signals are typically strongly background dominated. This problem can be circumvented by inelastic scattering processes. In this study, we investigate the inelastic process of laser photon merging in the collision of just two laser pulses under a finite angle, which provides signal photons of a distinct frequency outside the frequency spectrum of the background. As a key result, for the example of two laser beams of the same oscillation frequency we demonstrate that by using high-intensity optical lasers and choosing an optimal collision angle, photon merging should become accessible in experiment with state-of-the-art technology. In this case three frequency $\omega$ laser photons are merged to a single $3\omega$ photon.