  Quantum field theory predicts a nonlinear response of the vacuum to strong
electromagnetic fields of macroscopic extent. This fundamental tenet has
remained experimentally challenging and is yet to be tested in the laboratory.
A particularly distinct signature of the resulting optical activity of the
quantum vacuum is vacuum birefringence. This offers an excellent opportunity
for a precision test of nonlinear quantum electrodynamics in an uncharted
parameter regime. Recently, the operation of the high-intensity laser ReLaX
provided by the Helmholtz International Beamline for Extreme Fields (HIBEF) has
been inaugurated at the High Energy Density (HED) scientific instrument of the
European XFEL. We make the case that this worldwide unique combination of an
x-ray free-electron laser and an ultra-intense near-infrared laser together
with recent advances in high-precision x-ray polarimetry, refinements of
prospective discovery scenarios, and progress in their accurate theoretical
modelling have set the stage for performing an actual discovery experiment of
quantum vacuum nonlinearity.
