  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.

Optics;High Energy Physics - Phenomenology