| Summary: | 博士 === 國立清華大學 === 物理學系 === 95 === The Q & A experiment [1,2],aiming at the detection of vacuum birefringence predicted by quantum electrodynamics,
consists mainly of a suspended 3.5 m Fabry-Perot interferometer, a rotating permanent dipole magnet and an ellipsometer. The permanent magnet,
with maximum central magnetic field of 2.3 T and field region length of 0.6 m, is used to polarize the vacuum (or gas) traversed by the light resonating inside the Fabry-Perot interferometer. To increase the detection sensitivity, the magnet is rotated at 5~10 rev/s to produce a time-dependent signal at twice the rotation frequency.
The time-dependent signal is amplified 2F/\pi times by the Fabry-Perot interferometer, where F is the finesse of Fabry-Perot interferometer. For the last 3 months, the finesse F of the Fabry-Perot cavity is increased to 28,000~32,000 corresponding to a gain factor of 17,800~20,370. The feedback control system is improved to be more stable and less noisy, and an alignment control is successfully implemented on the Fabry-Perot interferometer.
The ellipsometer is formed by a pair of Glan-Taylor type polarizing prism, a quarter-wave plate and a polarization rotation modulator. Without the quarter-wave plate, the ellipsometer becomes a polarization-rotation measuring instrument. The extinction ratio of the polarizing prism is lower than 5*10^{-9}. The apparatus is calibrated by performing the measurement of gaseous Cotton-Mouton effect (CME). The measurement of nitrogen CME is taken at several different pressures and room temperature of ~19.5 deg. C.
The birefringence in nitrogen is measured to be \Delta n(N_2)=-(2.66+-0.12)*10^{-13}(B/1 T)^2(P/1 atm).
The CME of air at the atmospheric pressure and room temperature 23.5 deg. C is also measured and the birefringence in air is \Delta n(air)=-(6.73+-0.26)*10^{-13}(B}/1 T)^2(P/1 atm). These results agree with other measurements found in the literature [70,79]. At present,
the sensitivity for ellipticity detection is 5*10^{-7}rad.Hz}^{-1/2} at 10~20 Hz and that for polarization-rotation detection is 1.4*10^{-6}rad.Hz}^{-1/2} at 10~20 Hz.
With this sensitivity, it is possible to check the polarization rotation effect recently observed by the PVLAS collaboration [29]. Our first results give (2.8+-2.8)*10^{-13} rad/pass, at B=2.3 T with 18,700 passes through a L=0.6 m long magnet (B^2L^2=1.6 T^2m^2). From the absence of an optical rotation, we were able to set a limit on axion coupling to two photons of g_{a\gamma\gamma}<1.7*10^{-6} GeV^{-1}, for axion mass m_a<2.2 meV.
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