  In very intense electromagnetic fields, the vacuum refractive index is
expected to be modified due to nonlinear quantum electrodynamics (QED)
properties. Several experimental tests using high intensity lasers have been
proposed to observe electromagnetic nonlinearities in vacuum, such as the
diffraction or the reflection of intense laser pulses. We propose a new
approach which consists in observing the refraction, i.e. the rotation of the
waveplanes of a probe laser pulse crossing a transverse vacuum index gradient.
The latter is produced by the interaction of two very intense and ultra short
laser pulses, used as pump pulses. At the maximum of the index gradient, the
refraction angle of the probe pulse is estimated to be $0.2 \times
(\frac{w_0}{10 \mathrm{\mu m}})^{-3} \times \frac{I}{1
\mathrm{J}}$~picoradians, where $I$ is the total energy of the two pump pulses
and $w_0$ is the minimum waist (fwhm) at the interaction area. Assuming the
most intense laser pulses attainable by the LASERIX facility ($I = 25$~J, 30~fs
fwhm duration, 800~nm central wavelength) and assuming a minimum waist of $w=10
\mathrm{\mu m}$ (fwhm) (corresponding to an intensity of the order of
$10^{21}$~W/cm$^2$), the expected maximum refraction angle is about
5~picoradians. An experimental setup, using a Sagnac interferometer, is
proposed to perform this measurement.
