  The tremendous progress in high-intensity laser technology and the
establishment of dedicated high-field laboratories in recent years have paved
the way towards a first observation of quantum vacuum nonlinearities at the
high-intensity frontier. We advocate a particularly prospective scenario, where
three synchronized high-intensity laser pulses are brought into collision,
giving rise to signal photons, whose frequency and propagation direction differ
from the driving laser pulses, thus providing various means to achieve an
excellent signal to background separation. Based on the theoretical concept of
vacuum emission, we employ an efficient numerical algorithm which allows us to
model the collision of focused high-intensity laser pulses in unprecedented
detail. We provide accurate predictions for the numbers of signal photons
accessible in experiment. Our study paves the way for a first verification of
quantum vacuum nonlinearity in a well-controlled laboratory experiment at one
of the many high-intensity laser facilities currently coming online.
