  We suggest an experiment to observe vacuum birefringence induced by intense
laser fields. A high-intensity laser pulse is focused to ultra-relativistic
intensity and polarizes the vacuum which then acts like a birefringent medium.
The latter is probed by a linearly polarized x-ray pulse. We calculate the
resulting ellipticity signal within strong-field QED assuming Gaussian beams.
The laser technology required for detecting the signal will be available within
the next three years.
