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冷镱原子钟跃迁谱的精密测量

Precision Measurement of the Clock-transition Spectrum in a Cold Ytterbium Atomic Clock

【作者】 周敏

【导师】 徐信业;

【作者基本信息】 华东师范大学 , 光学, 2014, 博士

【摘要】 六十多年以来,原子频率标准在很多领域起着非常重要的作用,诸如精密测量、基础物理研究和技术应用等方而。激光冷却与囚禁技术的飞速发展使得原子或离子的操控变得灵活和方便。从2000年开始,人们已经可以利用光学频率梳将光学频率与微波频率联系起来,对光频进行直接和可靠的测量。另外,作为本地振荡的超稳激光器已经可以实现超窄的激光线宽。所有这些都使得光学原子钟的研制成为可能,光学原子钟与微波原子钟相比优势在于,它具有更小的频率不确定度和更优的稳定度。目前,国际单位制中的时间单位“秒”由铯微波频率标准实现,也许在不久的将来,将会用原子的光跃迁频率对它重新定义。用于研制光钟的原子有很多,镱原子是其中很有潜力的候选者之一。在华东师范大学精密光谱科学与技术国家重点实验室,目前正在研制的有两台冷镱原子光钟。多年来我们一直致力于此,已经实现了镱原子的一级冷却、二级冷却以及初步的光晶格装载,本小组的先前论文已经详细介绍了这些工作。本文将介绍各个实验部分的改进和优化,重点研究镱原子钟跃迁谱线的精密测量。一级冷却399nm激光可通过Digilock110模块进行远程操控。我们采用的是调制转移光谱进行激光稳频,对比两种连线方式后选择有利的一种,充分利用了Digilock110的优势,最后获得了高信噪比的误差信号。对于二级冷却,借助PDH技术将556nm激光器频率锁定在高精细度的光学谐振腔(FP腔)上,激光线宽被压窄至约3kHz,获得了低噪声的556nm二级冷却光。同时,我们优化了556nm磁光阱的相关参数,比如激光频率失谐量、激光功率以及磁场,获得了典型温度为20μK的冷171Yb原子团。通过小心地调节光路,在三维方向上搭建了“(1,1,1)”激光组态的光晶格。实验上实现了各种光晶格结构的原子装载,在一维、二维以及三维魔术波长光晶格中都可以观察到171Yb原子的荧光信号。在一维光晶格中,我们测量了17Yb原子的温度、数目和寿命。优化参数后,一维光晶格中的171Yb原子将被用于钟跃迁探询。为消除原子数日起伏带来的影响,我们采用电子搁置法探测镱原子钟跃迁谱。对一个双次通过的声光调制器进行总共几百kHz的频率扫描,可以获得171Yb的钟跃迁谱,包含载波,红边带和蓝边带三种成分。一维光晶格的非谐振特性使得纵向与横向的自由度发生混杂,导致边带结构频谱的展宽和不对称:背对着载波的一面十分陡峭,而面朝载波的一面下降平缓。对典型的边带谱进行拟合,可以得知光晶格阱深和原子温度等参数。我们在实验上证明了,对不同的晶格阱深,纵向的晶格振动能级间隔频率与晶格光功率的开平方成正比,这与光晶格谐振近似理论完全符合。当晶格光处于魔术波长时,载波跃迁是纯电子能级跃迁,非零的振动能级激发或者晶格横向囚禁势都不会对载波跃迁产生影响。最大的问题可能就是边带对载波会造成谱线牵引频移。实际上,几kHz的钟跃迁载波谱已经可以用于钟激光的频率锁定。谱线的展宽主要是因为还存在剩余磁场,使得钟跃迁磁子能级的四条特征跃迁谱线简并解除,发生塞曼增宽。如果我们放大载波跃迁区域,可以清楚地看到这四条跃迁谱线的互相分离。移除578nm钟探询光功率展宽后,通过补偿磁场将剩余磁场补偿掉,在垂直钟探询光传播方向外加一个磁场,一束线偏振的钟激光脉冲就只能激发“π”跃迁。此时,谱线的宽度在16Hz左右,这已经快接近我们目前实验参数下的傅里叶极限了。另外,本论文还进行了相关的理论分析。晶格光若在魔术波长处,其引起某共振跃迁谱线上下能级的光位移将一致,从而消除了差分光位移。我们计算了镱原子两条跃迁谱线的魔术波长,其中6s21S0-6s6p3P1跃迁谱线的魔术波长目前还没有文献报道,可能对556nm光缔合谱图的精确绘制很有帮助。对于镱原子光钟,本文将系统分析黑体辐射频移的影响,给出具体的评估方案。理论上详细分析了利用激光偏转原子束进行同位素分离的可能,这为实验上获取纯度高的同位素提供了理论指导。同时,激光偏转的思想还可用于黑体辐射频移的抑制,本文只概述基本的物理图像。

【Abstract】 For about60years, the atomic frequency standards have played a crucial role in the precision measurements, fundamental physical researches and technical applications. Tremendous progress on the laser cooling and trapping techniques has enabled the flexible manipulation of atoms or ions. Since year2000, it is also possible to make a direct and reliable frequency measurement from the optical domain to the microwave domain via the optical frequency comb. Moreover, the ultrastable laser with an extremely narrow linewidth, which acts as a local oscillator, has been successfully achieved. All these possibilities allow us to build optical frequency standards with a lower fractional frequency uncertainty and higher stability. Maybe in the future the second, one of the International System of Units that is currently realized by the microwave caesium frequency standards, will be redefined by an optical atomic transition.Ytterbium is regarded as one of the promising candidates for the optical clocks. At present, two optical clocks based on cold ytterbium atoms are being developed in the State Key Laboratory of Precision Spectroscopy, East China Normal University. We have been working on the realization of ytterbium optical clocks for years and we have accomplished the first-stage cooling, second-stage cooling and preliminary optical lattice confinement of ytterbium atoms, which have been detailedly described in the previous works of our group.This thesis presents some improvements in each experimental part and emphasizes the precision measurement of the clock-transition spectrum in a cold ytterbium optical clock. In the first cooling stage, the399nm laser is remotely controlled via a Digilock110module. For laser frequency stabilization utilizing the modulation transfer spectroscopy method, we compare two connection schemes to fully exploit the Digilockl10, and finally obtain an error signal with a high signal to noise ratio. In the second cooling stage, a low noise556nm laser is achieved by locking the frequency to a high finesse Fabry Perot (FP) cavity with the Pound-Drever-Hall (PDH) technique. The laser linewidth is narrowed to about3kHz. We optimize the parameters for the556nm magneto-optical trap (MOT) phase, such as the556nm laser frequency, laser intensity and the B-field. The typical temperature of the171Yb atomic cloud is measured to be about20μK. By careful alignment, we construct a so-called "(1,1,1)" light configuration for the optical lattice on three dimensions. Experiments on trapping ytterbium atoms in various optical lattices are presented. The ultracold171Yb atoms are visibly confined in the one-, two-, and three-dimensional optical lattices operating at the Stark-free wavelength, respectively. For the one-dimensional lattice, the temperature, number and lifetime of cold171Yb atoms are measured. After optimization, the one-dimensional optical lattice with171Yb is readily used for the clock laser interrogation.Precision measurement of the clock-transition spectrum is performed using the electron shelving technique, which cancels out the shot-to-shot atom number fluctuations. The clock-transition spectrum, taken by scanning a double-passed acousto-optic modulator (AOM) for a total range of a few hundred kHz, consist of a carrier, red sideband and blue sideband spectral structure. The anharmonicity of the lattice results in the coupling between the longitudinal and transverse degrees of freedom. So the sideband structures appear to be broad and asymmetric:a sharp edge with its back to the carrier and a slope towards the carrier. A typical sideband spectral shape is modeled to know about the efficient lattice depth and the temperature of in-lattice atoms. For different lattice depths, the longitudinal oscillation frequency is demonstrated experimentally to be proportional to the square root of the lattice beam power, which is consistent with the harmonic approximation of the lattice. As the carrier favors the purely electronic transition when the lattice is at the magic wavelength, generally it has nothing to do with the nonzero motional excitation and transverse confinement. Maybe the main problem is the line pulling induced by the tail of the sideband spectrum. In fact, the carrier spectrum with a linewidth of several kHz can already be used to lock the clock laser. The linewidth broadening mainly results from the residual magnetic field around the lattice region so that the degeneration of four characteristic Zeeman sub-level transitions are removed. If we zoom in the carrier, all four transitions are separated from each other. After the power broadening effect of the578nm clock laser is eliminated, the residual B-field can be well compensated with our compensation coils. With an external applied B-field perpendicular to the clock laser propagation direction, a linear-polarized clock laser pulse allow only "π" transitions. The spectral lines feature a linewidth of about16Hz, which approaches the Fourier limit for our experimental parameters.This thesis presents some analysis as well. The differential light shifts induced by lattice beams can be cancelled out by operating at the magic wavelength. Here magic wavelengths of two ytterbium transition lines are calculated. The Stark free wavelength of6s21S0-6s6p3P1, which has not been reported yet, may be helpful in accurately mapping the ytterbium photoassciation spectrum using this556-nm intermediate line. For the ytterbium optical clock, I will give the detailed evaluation of the blackbody radiation shift. Isotope separation by laser deflecting an atomic beam is analyzed theoretically, which will give a guideline for simply obtaining pure isotopes of various elements. Meanwhile, the concept of laser deflection can be applied to the blackbody radiation suppression, of which I will just outline the physics here.

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