Cesium Atomic Magnetometer And Its Application In Magnetic Field Stabilization

Highly sensitive measurements of weak magnetic signals are very important for ap-plications such as geophysical exploration,bio-magnetic field detection,ultralow-field nuclear magnetic resonance and tests of fundamental physics.The alkali-metal atomic magnetometer is one of the most sensitive magnetometers,whose sensitivity can reach the order of f T/Hz^1/2(1 f T=10^-15T).Because the alkali-metal atomic magnetometer has the advantages of non-cryogenic operation,movable,customizable design,and low price,it is thus attracting attention in more and more application fields.In order to achieve high-sensitivity～f T/Hz^1/2,most alkali-metal atomic magne-tometers need to be operated in passive magnetic shields,which help to suppress ambient magnetic field noise.However,the passive magnetic shield is expensive and its internal space is crowded,which raises the threshold for the use of alkali-metal atomic magne-tometers and limits its wide application.Elimination of magnetic shields can significantly reduce the cost of the magnetometer systems.However,in a magnetically unshielded environment,even if some methods like gradiometer detection or active magnetic-field stabilization are used to suppress the ambient magnetic noise,the sensitivity of the alkali-metal atomic magnetometer is still one to three orders of magnitude lower than that achieved inside the magnetic shield,which limits its application in areas requiring high sensitivity.Therefore,how to improve the performance of the alkali-metal atomic magnetometer in a magnetically unshielded environment,thereby expanding its application fields,lower-ing the application threshold,and giving full play to its advantages of high sensitivity,portability and low cost,is still an important open question in this field.Cesium atomic magnetometer is a widely used alkali-metal atomic magnetometer.The author’s main research interest is cesium atomic magnetometers and the magnetic field stabilization system based on the cesium atomic magnetometer,especially the fac-tors that limit the performance of the cesium atomic magnetometer in magnetically un-shielded environment,as well as the potential solutions.The results of this research can be extended to atomic magnetometers based on other alkali-metal species or even other types of magneometers like superconducting quantum interference devices.The author’s research works mainly include the following parts:1.Building the cesium atomic magnetometer system.Firstly,a Bell-Bloom cesium atomic magnetometer operated in a magnetic shield is built,and the closed-loop operation of this atomic magnetometer is realized.Meanwhile,according to the automatic control theory,the author proposes a general model to quantitatively depict and optimize the per-formance of the closed-loop magnetometer,which is verified by experiments.This model provides a quantitative guidance on the bandwidth expansion of the magnetometer and the better suppression of the ambient magnetic noise in subsequent experiments.Then,the author designs and manufactures miniaturized sensors for cesium atomic magnetometers through 3D printing technology,which have become key components of the magnetically unshielded magnetometer system.2.Validating and studying the influence of the nonlinear Zeeman effect on the fre-quency response of the cesium atomic magnetometer.The author develops a density-matrix-equation-based model to quantitatively depict the frequency response of the ce-sium magnetometer when the nonlinear Zeeman effect is non-negligible and verify the results experimentally.The proposed model provides a general guidance on analyzing the frequency response of the alkali atomic magnetometer operating in the Earth’s magnetic field.A full and precise knowledge of the frequency response of the atomic magnetometer is important for optimization of the feedback control systems such as the closed-loop mag-netometers and the active magnetic field stabilization with magnetometers.This work is thus important for application of alkali atomic magnetometers in unshielded geomagnetic environment.3.Proposing a general model to quantitatively depict and optimize the performance of active magnetic-field stabilization,which aims at actively compensate the environmental magnetic field noise,and experimentally verify the model using cesium atomic magne-tometers.We experimentally demonstrate a magnetic-field noise rejection ratio of larger than～800 at low frequencies and an environment with a magnetic-field noise floor of～40f T/Hz^1/2in unshielded Earth’s field.The proposed model provides a general guidance on analyzing and improving the performance of active magnetic-field stabilization with magnetometers.This work offers the possibility of sensitive detections of magnetic-field signals in a variety of unshielded natural environments.4.Recording brain activities in unshielded Earth’s field with cesium atomic magne-tometers.On the basis of the above works,we build an atomic magnetic gradiometer,together with magnetic field stabilization to reduce the environment magnetic field noise.We demonstrate a magnetic-field noise floor of～20 f T/Hz^1/2and successfully observe the magnetic signals related to human brain activities.This is the first time in the world to measure brain magnetic signals with atomic magnetometer in a magnetically unshielded environment.Our method is promising to realize a practical wearable and movable un-shielded MEG system and bring new insights into medical diagnosis of brain symptoms.