Study On Optical Lattices For Ytterbium Optical Clocks

Optical frequency standards based on ultra-cold neutral atoms and single trapped ion have outperformed the best133Cs fountain clock for lower fractional frequency uncertainty, and are becoming promising candidates for the next generation of primary frequency standards. They can not only be used for testing of fundamental physical constants but also essential technologies for Global Positioning System (GPS) and high-speed communication networks. Although the best fractional frequency uncertainty of10-18to date has been achieved in the aluminum ion clocks, the stability of the ion optical clock is limited by the quantum projection noise of a single ion. However, the optical lattice clocks of neutral atoms can simultaneously interrogate a great number of cold atoms in the optical lattice and therefore outperform the single ion clocks for higher stability. Alkaline-earth-metal alike atoms as well as ytterbium (Yb) atoms have attracted a lot of interest due to the narrow linewidth and the abundance of isotopes. To date, the frequency uncertainty of10-16has been demonstrated. However, the uncertainty is two orders of magnitude larger than the predicted value. For fermionic optical lattice clock, early experiments focused on87Sr atoms with the nuclear spin of9/2. In this paper we mainly focus on the171Yb atoms which have the simplest fermionic structure with the nuclear spin of1/2and are the best choice for fermionic optical clock. This dissertation mainly describes the experimental study on a second-stage magneto-optical trap (MOT) and the optical lattices of171Yb atoms. The experiments on the Mott-insulator and photoassociation of ytterbium atoms in optical lattices have been proposed. By realization of one atom per lattice site, we expect suppressing the collisional frequency shifts in the optical lattice and improving the optical frequency uncertainty.Firstly, we describe the experimental study on the second-stage MOT of the171Yb atoms using the1S0-3P1intercombination transition at556nm. We mainly introduce the556nm laser system and experiment on loading of556nm cold atoms.556nm laser light has been generated by frequency doubling of an1111.6nm fiber laser with a periodically poled MgO doped LiNbO3waveguide. By measuring the temperature tuning curve of second harmonic generation (SHG), optical inhomogeneities has been studied. We used the fitting parameters for identifying the uniformity of the optical waveguide. By carefully adjusting the crystal temperature, we could diminish thermal dephasing effect. By adjusting the distance between two coupling lenses and polarization of the fundamental light, we maximized the conversion efficiency. Finally,556nm power of111.8mW was obtained with213mW of the fundamental light power coupled into the waveguide, corresponding to52.5%conversion efficiency. We also investigated the beam properties of inhomogeneous waveguides. After realization of first-stage MOT using the strong399nm1S0-1P1transition, we successfully transferred the cold atoms into a second-stage MOT with the transferring efficiency of38%. About106atoms were captured in the556nmMOT and the lifetime was about381ms. The temperature of the cold atoms was about32(±10) μK, measured by the time of flight method.Secondly, theory of the optical lattice, the experimental apparatus and loading process of the optical lattice were described. We also calculated the lattice depth and trapping frequency, and analyzed the experimental results. After realization of the556nmMOT, we successfully transferred the ultracold atoms into the optical lattice formed by propagating beams at the759nm magic wavelength. The pictures of cold atoms in the optical lattice were taken by intensified charge coupled device. The temperature of the one-dimensional and two-dimensional optical lattice was determined to be78(±20)μK and94(±20)μK, respectively. We expect that by gradually ramping up the optical lattice potential, the temperature of cold atoms will be further decreased through adiabatic cooling.Thirdly, currently, one of the most important factors that limit the improvement of the frequency uncertainty of the174Yb optical lattice is the collisional frequency shifts. We proposed two methods for eliminating the collisional frequency shifts. By using mott-insulator and photoasociation in three dimensional optical lattice, we can realize one atom per lattice site and suppress the collisional frequency shifts. We first describe the Bose-Hubbard model and phase transition theory and propose the experiment. We also calculated the critical value for phase transition from superfluid to the Mott-insulator state for different atom number per lattice. Next we described three methods for photoassociation and proposed the photoassociation experiment in three dimensional optical lattices of ytterbium atoms.Fourthly, the experimental setup for detection of the spectrum of1SO-3PO clock transition has been built up. By observing the transition spectrum in399nmMOT and556nmMOT using556nm laser as a probe laser, we simulated the detection process, successfully observed the Zeeman sublevels of171Yb and made good preparation for the detection of clock transition spectrum.