Magnetic Field Control And Preliminary Evaluation Of The Cold Ytterbium Atomic Clocks

Recently,scientists make a breakthrough in the uncertainty and stability of the clock-transition frequency of optical clock.And a number of teams have built optical clocks that perform better than the best Cs fountain clock.The optical clock which could be a candidate of next generation of time and frequency standard would improve the accuracy of current time keeping and time service.Moreover,the ultrastable clock-transition frequency of the cold atoms could be a better reference to measure the fundamental physical constants,and applied into the study of quantum tangle,quantum communication and so on.Among the optical clocks under develepment,there are two kinds of clocks which are technical achievable and proven to have a brilliant perform.The better optical clocks among these two kinds have achieved an accuracy of 10^-18 magnitude.The best Ieutral atomic clock has even achieved a performance of 10^-18 magnitude both in uncertainty and stability.The single ion clock can only interrogate one ion in one cycle,while the neutral atomic clocks could interrogate about 10⁴ atoms in the lattice at one time.So the quantum project noise in neutral atomic clock is suppressed,the the stability of the clock-transition frequency of neutral atomic clock decreases more rapidly to 10^-18 magnitude than ion clock.This thesis introduces the research process and improvements of two ytterbium clocks?Yb ?,Yb 11?in detail.These optical clocks have successfully accomplished the first-stage and second-stage cooling processes.By optimizing the detuning and intensity of the cooling laser,the temperature of the atoms to be loaded into the lattice is low enough.Then the clock-transition spectrum is interrogated after loading the atoms into the optical lattice.By carefully adjusting the current ofthe three pairs of orthogonal perpendicular coils,the stray magnetic field in the center of chamber is compensated.Both clocks obtain the ultra narrow clock-transition spectrum with a linewidth of about 6 Hz which is close to the Fourier limit of the 150 ms interrogation time.During the process,we achieve the manipulation of clock-transition spectrum and obtain the spin-polarization spectrum.To cancel the first-order Zeeman shift in the frequency locking process,we lock the interrogation laser to the two ? transitions in the clock-transition spectrum which is separated out by applying a magnetic field parallel to the polarization of the interrogation laser.The fluctuation of magnetic field is supressed by a magnetic field controling system The frequency comparison is achieved while the time sequences of the two clocks are synchronized,and the result is indicated by Allan deviation.S ince the performances of the two clocks are the same,we employ one of them as the reference clock to evaluate the system effects such as light shift,Zeeman shift,and collision shift of another clock.This thesis focuses on the evaluation of the ytterbium clock and precise control of magnetic field.The careful compensation of the magnetic field in the first ytterbium clock?Yb I?reduces the linewidth of the clock-transition spectrum from 56 Hz to 6 Hz,which is close to the Fourier limit of the 150 ms long probe pulse.The noisy background magnetic field would cause the distortion of the lineshape and expand the linewidth obviously.By monitoring the fluctuation of the background magnetic field with a high resolution magnetometer,we find that the magnetic field fluctuation is one of limitations for improving the stability to 10^-18 magnitude.Though the background magnetic field would be quite stable when the large facility nearby are all shut down,the noisy background would disturb the long time running clock.A magnetic field detecting and controling system based on Labview is developed,and the minimal step of the current is 0.042 mA,corresponding to a variation of magnetic field of 0.011 ?T.In the time sequence,the magnetic field controling system could finish feedback before clock interrogation.The experimental simulation indicates a fluctuation of magnetic field within 0.1 ?T,and the standard deviation is?0.017 ?T.Before evaluating the Zeeman shift,the Zeeman splitting of the energy level of ytterbium atoms in weak magnetic field is theoretically analyzed.The dependence of the first-order and second-order Zeeman shift on the magnetic field is presented.We cancel the first-order Zeeman shift by averaging the frequency of two ? transitions.The stability degrade is analyzed by employing a binomial probability distribution model when the magnetic field fluctuation is taken into account in frequency locking.The affect of the stabilized magnetic field on the stability per second is less than 3×10-17.The second-order Zeeman shift coefficient ? can be calculated by State-mixing theory,and also can be measured experimentally.To prepare for the miniature optical clock,the theoretical analysis of permanent Zeeman slower is presented,and a miniature design is proposed.The permanent Zeeman slower needs neither water cooling nor extra power supplies.The slower for ytterbium is theoretically analyzed,and the optimal conditions such as intensity and detuning of the laser power are proposed.Since the miniature model is presented,a real permanent Zeeman slower is built to confirm the theory.