First 5-sigma vacuum birefringence detection with an x-ray laser and optical laser
In plain words
An x-ray beam passing through the focus of a very intense laser should have a tiny fraction of its photons flip polarization because the vacuum acts like a crystal. The question is whether a planned experiment can see these few flipped photons above all background.
Precise statement
Euler-Heisenberg theory predicts that a linearly polarized x-ray probe crossing a focused intense optical pulse acquires a small perpendicular polarization component; the signal is the number of polarization-flipped photons per shot behind a high-extinction polarimeter. Determine whether the HIBEF configuration (European XFEL photons at about 10 keV colliding with the ReLaX laser, letter of intent published 2025) or another setup detects flipped photons at $>5\,\sigma$ over polarimeter leakage, diffuse scattering and timing jitter, and whether the measured flip probability agrees with the Euler-Heisenberg prediction.
What would settle it
An XFEL-plus-laser polarimetry measurement giving a flipped-photon signal at $>5\sigma$ that scales with laser intensity as predicted.
Status in the literature
Unverified note
A letter of intent for the HIBEF experiment was published in High Power Laser Science and Engineering in 2025; no detection is known to this compilation as of 2026.