  We show that coherent harmonic focusing provides an efficient mechanism to
boost all-optical signatures of quantum vacuum nonlinearity in the collision of
high-intensity laser fields, thereby offering a promising route to their first
experimental detection. Assuming two laser pulses of given parameters at our
disposal, we demonstrate a substantial increase of the number of signal photons
measurable in experiments where one of the pulses undergoes coherent harmonic
focusing before it collides with the fundamental-frequency pulse. Imposing a
quantitative criterion to discern the signal photons from the background of the
driving laser photons and accounting for the finite purity of polarization
filtering, we find that signal photons arising from inelastic scattering
processes constitute a promising signature. By contrast, quasi-elastic
contributions which are conventionally assumed to form the most prospective
signal remain background dominated. Our findings may result in a paradigm shift
concerning which photonic signatures of quantum vacuum nonlinearity are
accessible in experiment.
