無屏蔽不受自旋交換緩和影響的鉀原子測磁計

Abstract

精準測磁計的應用範圍相當的廣泛-從測試基本物理對稱性到生物磁場的測量。低溫 超導體量子干涉裝保持著最低的磁場雜訊的紀錄 － 1 fT / Hz 。然而超導體量子干 涉裝置具巨大的儀器體積和昂貴的降溫系統限制了這項儀器的運用。近年來，利用鉀 或銣的鹼金屬的光學測磁計已經接近了超導體量子干涉裝置的磁場雜訊。這類型的光 學測磁計是利用量測金屬原子自旋極化的拉馬進動進而得到磁場的大小。而這類型光 學測磁計的基本雜訊限制就是量子雜訊，其大小和垂直磁場方向的自旋緩和速率有 關。 在零磁場附近操作的金屬原子測磁計可以減低此種自旋緩和的影響，進而得 到約0.3 fT / Hz 的磁場雜訊。為了屏蔽環境的磁場，有些系統使用高磁導率的金屬 屏蔽或是使用荷姆霍茲線圈去抵銷外在磁場[8]。此論文利用參考文獻[8]的基 礎，提出一個改進過後的無屏蔽系統，此系統改進要項為:資料利用電腦跟資料擷取 卡處理、使用電光晶體雷射功率穩定器來降低系統雜訊，還有減少地磁抵銷所需的時 間等項目。

Sensitive magnetometers find wide range of applications, from tests of fundamental symmetries to measurements of biological magnetic fields. Low temperature SQUID detector presently hold the record as the most sensitive magnetometers with a noise level of about 1fT / Hz . However, its huge bulk due to the complex structure and high cost of cryogenic cooling confine its applications. Recently years, Alkali-metal magnetometers using Potassium and Rubidium approach this level of sensitivity. The mechanism of atomic magnetometers relies on a measurement of Larmor precession of spin-polarized atoms in a magnetic field. The fundamental noise is limited by the quantum shot noise which is related to the transverse relaxation time of the spin polarized atoms.The atomic magnetometers operating near zero field which eliminates spin exchange collision as the source of relaxation shows a fundamental noise limit which is about 0.3 fT / Hz . To block out environmental tray field, these systems apply μ -metal shield or Helmholtz coil compensate system. This thesis proposes an improved version of the un-shielded system in reference : the optical detection of the signals is processed by a DAQ card and a computer to simplify the system, an EO stabilizer is introduced to lower the optical noise level in the detection, and a improved method is developed to reduce the time spent in field compensating.