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The Q & A experiment, aiming at the detection of vacuum birefringencepredicted by quantum electrodynamics, consists mainly of a suspended 3.5 mFabry-Perot cavity, a rotating permanent dipole magnet and an ellipsometer. The2.3 T magnet can rotate up to 10 rev/s, introducing an ellipticity signal attwice the rotation frequency. The X-pendulum gives a good isolation ratio forseismic noise above its main resonant frequency 0.3 Hz. At present, theellipsometry noise decreases with frequency, from 1*10^{-5} rad Hz^{-1/2} at 5Hz, 2*10^{-6} rad Hz^{-1/2} at 20 Hz to 5*10^{-7} rad Hz^{-1/2} at 40 Hz. Theshape of the noise spectrum indicates possible improvement can be made byfurther reducing the movement between the cavity mirrors. From the preliminaryresult of yaw motion alignment control, it can be seen that some peaks due toyaw motion of the cavity mirror was suppressed. In this paper, we first give aschematic view of the Q & A experiment, and then present the measurement oftransfer function of the compound X-pendulum-double pendulum suspension. Aclosed-loop control was carried out to verify the validity of the measuredtransfer functions. The ellipsometry noise spectra with and without yawalignment control and the newest improvement is presented.Comment: 7 pages, 5 figures, presented in 6th Edoardo Amaldi Conference on  Gravitational Waves, June 2005, Okinawa Japan and submitted to Journal of  Physics: Conference Series. Some modifications are made according to the  referee's comments: mainly to explain the relation between the displacement  of cavity mirror and the ellipticity noise spectru

Ellipsometry noise spectrum, suspension transfer function measurement and closed-loop control of the suspension system in the Q & A experiment