  The Q & A experiment, aiming at the detection of vacuum birefringence
predicted by quantum electrodynamics, consists mainly of a suspended 3.5 m
Fabry-Perot cavity, a rotating permanent dipole magnet and an ellipsometer. The
2.3 T magnet can rotate up to 10 rev/s, introducing an ellipticity signal at
twice the rotation frequency. The X-pendulum gives a good isolation ratio for
seismic noise above its main resonant frequency 0.3 Hz. At present, the
ellipsometry noise decreases with frequency, from 1*10^{-5} rad Hz^{-1/2} at 5
Hz, 2*10^{-6} rad Hz^{-1/2} at 20 Hz to 5*10^{-7} rad Hz^{-1/2} at 40 Hz. The
shape of the noise spectrum indicates possible improvement can be made by
further reducing the movement between the cavity mirrors. From the preliminary
result of yaw motion alignment control, it can be seen that some peaks due to
yaw motion of the cavity mirror was suppressed. In this paper, we first give a
schematic view of the Q & A experiment, and then present the measurement of
transfer function of the compound X-pendulum-double pendulum suspension. A
closed-loop control was carried out to verify the validity of the measured
transfer functions. The ellipsometry noise spectra with and without yaw
alignment control and the newest improvement is presented.
