Q & A (QED[Quantum ElectroDynamics] Test and Axion search)實驗是一個以探測真空雙折射效應為目標之實驗 [1,2], 其主要由一3.5米懸吊式Fabry-Perot干涉儀, 一轉動永久磁鐵及一組橢圓率偵測儀所構成。 實驗中所使用之旋轉永久磁鐵, 中心磁場最大為2.3 T, 磁場區域長度為0.6米, 可使真空(或氣體)產生偏極化而導致雙折射性。 此外, 為了提高偵測靈敏度, 我們將磁鐵旋轉在5~10 rev/s的速度之間, 將雙折射訊號提高至10~20 Hz的兩倍轉動頻率區間。
當線偏振雷射光通過此磁場作用區域後,其偏振態會由線偏振轉換為橢圓偏振。 利用光在Fabry-Perot干涉儀內來回往返之特性, 我們可將橢圓偏振之橢圓率放大為2F/\pi倍,其中F為Fabry-Perot干涉儀之精細度。F值在最近幾個月的實驗中, 提升至28,000~32,000之間;
所對應之光在干涉儀中的來回次數或橢圓率之放大倍數為17,800~20,370倍。本階段實驗研究的一項主要工作是改進Q & A實驗中的回饋控制系統, 使其更加穩定, 更加安靜,並完成Fabry-Perot干涉儀曲面鏡之準直控制。本階段實驗研究的另一項主要工作是改善光偏振測量的靈敏度。橢圓率偵測儀由一對Glan-Taylor型式之偏極化鏡, 一四分波片及一偏振旋轉調制器所組成。偏極化鏡之消光比低於5*10^{-9}。 移除四分波片後,此橢圓率偵測儀可以用來量測偏振旋轉效應。我們對氣體進行了Cotton-Mouton效應(CME)之量測實驗, 以校正實驗裝置。氮氣在數個不同氣壓下進行CME量測(溫度~19.5 deg. C), 所得之氮氣雙折射效應為\Delta n(N_2)=-(2.66+-0.12)*10^{-13}(B/1T)^2(P/1atm)。空氣在一大氣壓, 溫度23.5 deg. C下, 雙折射效應\Delta n(air)=-(6.73+-0.26)*10^{-13}(B/1T)^2(P/1atm)。 此實驗結果, 與文獻中其它實驗所得到之結果一致 [70,79]。目前, Q & A實驗的橢圓率偵測靈敏度在頻率10~20 Hz間為5*10^{-7} rad*Hz^{-1/2}。移除四分波片, 偏振旋轉之偵測靈敏度在頻率10~20 Hz間為1.4*10^{-6} rad*Hz^{-1/2}。 此靈敏度, 使得Q & A實驗有機會來驗證PVLAS所得到之偏振旋轉效應 [29]。我們在磁場大小B=2.3 T、 磁場作用區域長度L=0.6 m (B^2L^2=1.6 T^2 m^2), 以及雷射光在磁場作用區域內來回往返18,700次的實驗狀態下,進行了首次偏振旋轉的量測實驗, 所測得之偏振旋轉效應為(2.8+-2.8)*10^{-13} rad/pass。 由於沒有明顯之偏振旋轉訊號, 我們可以由此結果給出軸子耦合至兩個光子之耦合常數上限,
為g_{a\gamma\gamma}<1.7*10^{-6} GeV^{-1}, 在軸子質量m_a<2.2 meV 之範圍內。

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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