  We show that a laser beam can be diffracted by a more concentrated light
pulse due to quantum vacuum effects. We compute analytically the intensity
pattern in a realistic experimental configuration, and discuss how it can be
used to measure for the first time the parameters describing photon-photon
scattering in vacuum. In particular, we show that the Quantum Electrodynamics
prediction can be detected in a single-shot experiment at future 100 petawatt
lasers such as ELI or HIPER. On the other hand, if carried out at one of the
present high power facilities, such as OMEGA EP, this proposal can lead either
to the discovery of non-standard physics, or to substantially improve the
current PVLAS limits on the photon-photon cross section at optical wavelengths.
This new example of manipulation of light by light is simpler to realize and
more sensitive than existing, alternative proposals, and can also be used to
test Born-Infeld theory or to search for axion-like or minicharged particles.
