【作者】 蒋海灵；

【导师】 徐信业；

【作者基本信息】 华东师范大学 ， 光学， 2012， 博士

【摘要】 基于超冷中性原子和单个囚禁离子的光频标频率不确定度远远低于当前最好的铯喷泉原子钟,是下一代时间频率基准的有力竞争者。光钟不仅可以用来测量基本物理常数的变化,也是全球定位系统和高速通信网络中的重要技术。目前最好的光钟是铝离子光钟,其不确定度已经达到了10-18量级,但是离子光钟的稳定度受到量子投影噪声的限制。而中性原子光晶格钟能同时探测大量囚禁在光晶格中的原子,其稳定度远远高于离子光钟。镱(Yb)原子具有毫赫兹的超窄线宽和丰富的同位素种类,是中性原子光晶格钟中的研究热点。目前国际上镱原子光钟的频率不确定度达到了10-16,但是与理论期望值还差两个数量级,镱原子光钟的不确定度还有很大的提高空间。对于费米子光晶格钟早期的研究集中在费米原子87Sr上,核自旋为9/2。本文我们选用核自旋为1/2的171Yb原子,具有最简单的费米子结构,是研究费米原子光晶格钟的最佳原子。本论文主要介绍了171Yb原子的二级冷却和光晶格的实验研究,提出了通过制备镱原子Mott绝缘态和光缔合效应,制备一个晶格格点一个原子,以期达到操控光晶格中冷镱原子的量子特性,从而抑制碰撞频移,提高镱原子光钟的频率不确定度。第一、我们介绍了利用556nm的1S0-3P1的窄带跃迁冷却的实验。其中主要介绍了556nm冷却激光系统和556nm冷原子团的装载。我们将周期极化掺氧化镁铌酸锂波导和1111.6nm光纤激光器相结合,通过倍频产生了556nm二级冷却光源。通过研究倍频光功率随温度的变化关系,解释了波导中的光学非均匀现象,所获得的参量可以用来鉴别波导的非均匀性。通过精细地调节晶体温度,减小了热相移,并通过调节两耦合透镜的距离和基频光偏振优化了倍频转换效率。在最佳条件下,当耦合到波导内的基频光功率为213mW时,产生了111.8mW的倍频光功率,获得了最大的倍频转换效率为52%。此外,我们还进一步研究了光波导的光束特性随着波导的非均匀性的变化关系。在镱原子的399nm的1S0-1P1宽带跃迁的一级冷却的基础上,我们成功地实现了将171Yb冷原子团从399nm磁光阱转移到556nm磁光阱中,转移效率约为38%。556nm磁光阱捕获的原子数约为106个,寿命为381ms,用飞行时间方法测量了冷原子团温度,约为32(±10),uK。第二、我们介绍了光晶格理论、实验装置和装载过程,估算了实验中的光晶格阱深和囚禁频率,并对实验结果做了分析。在556nm磁光阱的基础上,成功地将171Yb超冷原子装载到759nm魔术光波长形成的一维和二维光晶格中,并用ICCD拍摄到了清晰的光晶格图像。还用飞行时间方法测量了一维和二维光晶格的温度,分别为78(±20)μK和94(±20)μK,希望通过缓慢升高光晶格阱深,经绝热冷却进一步降低冷原子温度。第三、目前限制174Yb玻色子光钟的频率不确定度提高的主要因素之一是碰撞频移。为了消除碰撞频移,我们提出了通过制备mott绝缘态和冷镱原子三维光晶格中的光缔合效应两种方法,制备一个晶格格点一个原子,从而消除目前限制光钟不确定度提高的碰撞频移。文章首先介绍了玻色-哈伯特模型以及量子相变理论,提出了超流态到Mott绝缘态的实验方案,并估算了我们的实验中超流态到Mott绝缘态的量子相变临界值。接着介绍了光缔合实验的三种方案,提出了冷镱原子三维光晶格中的光缔合实验方案。第四、我们搭建了578nm探测实验装置。在399nmMOT和556nmMOT中模拟了谱线探测过程,用556nm激光作为探测光,成功地观察到了1S0-3P1的塞曼子能级跃迁谱线。为成功探测578nm钟态跃迁谱打下了坚实的基础。更多还原

【Abstract】 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.更多还原