Study Of The Spin Self-sustaining Method And Its Applications In The Magnetic Field Measurements

High precision magnetic field measurements based on the atomic systems have find various applications in the area such as geophysics,resource exploration,navigation,bio-science&medicine and fundamental physics.The high sensitivity atomic magnetometers are the key to realize such precision measurements.Generally,atomic magnetometers perform the absolute measurement of the external magnetic field by monitoring the fre-quency of the Larmor precession of the spins.In these quantum systems,the sensitivity,however,is usually limited by the coherence time ?0 of the atomic spin,beyond which,the uncertainty decreases only as 1/(?) with the measurement time ?.In this thesis,we investigate the methods to improve the sensitivity performance through two aspects:one is increasing the coherence time ?0,while the other is developing new methods which can surpass the 1/(?) rule at a time scale larger than ?0.First,the thesis explores the way to increase the coherence time of the spin:the study of the fabrication of alkali metal vapor cells with inner coatings.The improvement of the fabrication of the cells is based on laborious and empirical effort.The best results achieved by the thesis are a-olefin coated spherical cells with longitudinal relaxation time of the electron spin over 3 s and cylindrical cells with hyperfine relaxation time about 30 ms.The thesis further investigates new methods surpassing the 1/(?) rule at the time scale larger than ?0.We show that by non-destructively measuring the phase of the Lar-mor precession and coherent optical pumping,one can prepare the initial phase of the newly polarized spin in coherence with that of the previous precession,regenerating the spin coherently.Consequently,the self-sustaining Larmor precession signal can persist indefinitely and the precision of the magnetometer increases following a much faster 1/?rule at a time scale lager than the coherent time of the atom.Based on the 85Rb atoms,the thesis builds up the spin self-sustaining magnetometer prototype to investigate the coherence of the phase signal of the spin precession vs mea-surement time in the self-sustaining method.By synchronously pumping with the same frequency and phase of the Larmor procession,the spin signal can self-sustain and persist indefinitely.The sensitivity of the magnetometer improves with time as a faster 1/? even beyond the coherent time of the spin.The turning point of the 1/(?) emerges after 300 ms,which is nearly 10 times longer than the ?0.The sensitivity of the self-sustaining magnetometer is 240 fT/(?) @ 1 Hz,which is close to the spin projection noise of the atomic spin,limited by the temperature of the current coated cell.The magnetometer delivers uninterrupted signal which is convenient for signal pro-cessing such as filter and mixing.The thesis continue to study the magnetic gradient measurements based on the self-sustaining magnetometer.The thesis builds up the gra-diometer prototype and achieves a gradient sensitivity of 300 fT/(?) @ 1 Hz.Given the continuous phase signal of gradiometer,we successfully realize a new magnetic locking method by phase locking the Larmor precession signal to a local oscillator.Compared to the amplitude locking methods,the noise in the phase locking method decreases faster as 1/?.The thesis shows the 1/? property of the self-sustaining magnetometer at a time scale beyond the coherent time of the spin,while the ideas lying in the self-sustaining method itself is general.This method of coherence regeneration may also find important applications in improving the performance of atomic clocks.The noise of the quantum system with a 1/? property can decrease much faster with time averaging to a level close to the quantum shot noise,greatly increasing the sensitivity.Even more important is the fact that the accumulated error for a 1/? measurement is constant while increases follow-ing a (?) rule with time for the 1/(?) one.Methods able to realize the 1/? property like self-sustaining method have a great potential in applications such as frequency standard and inertial navigation where the accumulated error dominates the entire performance.