MIRROR BIREFRINGENCE IN A FABRY-PEROT CAVITY AND THE DETECTION OF VACUUM BIREFRINGENCE IN A MAGNETIC-FIELD

Quantum electrodynamics (QED) theory predicts that vacuum under the in fluence of a strong magnetic field is birefringence. Recently, several groups have proposed to used a high finesse Fabry-Perot cavity to inc rease the average path length of the light in the magnetic field. This together with the state-of-the-art dipole magnets, should bring the e ffect within reach of observation. However, the mirrors used in the FP are known to have intrinsic birefringence which is of orders of magni tude larger than the birefringence of the vacuum. In this letter, we a nalyze the effect of uncontrollable variations of mirror birefringence on two recently proposed optical schemes. The first scheme,(1) which we called the frequency scheme, is based on measurement of the beat fr equency of two orthogonal polarized laser beams in the cavity. We show that mirror birefringence contributes to the detection uncertainties in first order, resulting in a high susceptibility to variations of it s value. In the second scheme, which we called the polarization scheme , laser polarized at 45 degrees relative to the B-field is injected in to the cavity. The ellipticity and polarization rotation of the light exiting the cavity is measured.(2) Under this scheme, mirror birefring ence contributes as a correction of the QED effect, greatly reducing i ts sensitivity to the undesirable changes.