【作者】 陈海霞；

【导师】 张靖；

【作者基本信息】 山西大学 ， 光学， 2009， 博士

【摘要】 自从单组份玻色气体的玻色爱因斯坦凝聚和双组份自旋极化费米气体的费米简并实现以后,超冷量子气体领域迅速扩展到具有不同统计规律,不同俘获性质、质量和相互作用不同的玻色费米混合气体的研究。这一领域为多体物理、长程相互作用、强关联位相以及量子模拟等的研究提供了一个理想的平台,尤其当操控原子的强有力的技术手段周期性的强束缚光晶格,以及通过Feshbach共振产生的强共振相互作用应用于这一领域时,玻色费米混合气体的实验研究展现了美好的前景。我们的工作是建立一套冷却⁴⁰K和⁸⁷Rb原子的实验装置,并在实现玻色爱因斯坦凝聚（BEC）和费米量子简并（DFG）的基础上进行相关研究工作。本论文的工作主要是在原先建立的⁴⁰K和⁸⁷Rb两级磁光阱实验装置的基础上构建了一套实现量子气体简并的实验装置,并在此装置上实现了⁸⁷Rb原子的玻色爱因斯坦凝聚和⁴⁰K原子的简并费米气体。这是国内首次完成的费米量子简并的实验。实验方案是:采用了水平放置的双磁光阱装置;采用玻色子⁸⁷Rb和费米子⁴⁰K作为工作原子。首先在第一级真空气室（Collection Cell）中对⁸⁷Rb和⁴⁰K进行激光冷却与俘获,得到两种原子的磁光阱（MOT）。然后使用推送光把冷原子推到第二级真空气室（ScienceCell）中再一次进行MOT的冷却与俘获,最后把冷原子样品装入QUIC磁阱中进行蒸发冷却,实现⁸⁷Rb原子的BEC,并通过⁸⁷Rb原子和⁴⁰K原子之间的弹性碰撞,将⁴⁰K原子同步冷却实现DFG。本论文的主要工作有以下几个部分:1,在原先建立的冷却⁸⁷Rb和⁴⁰K原子的激光器系统上进行改进,首先,将原先TA1和TA2分别由三束光耦合注入放大(⁸⁷Rb原子的冷却光,⁴⁰K原子的冷却光以及再抽运光)输出用作MOT1和MOT2的冷却,改为TA1由⁸⁷Rb原子的冷却光注入放大和TA2由⁴⁰K原子的冷却光和再抽运光的耦合光注入放大,然后TA1和TA2放大输出耦合再分成两束分别用作MOT1和MOT2的冷却。这样可以避免在放大器中由于两个模式之间的放大竞争造成的⁸⁷Rb和⁴⁰K原子冷却光的功率输出不稳定;⁸⁷Rb原子的冷却光全部用于注入,由于耦合而造成的功率损失被避免,⁴⁰K原子的冷却光和再泵浦光耦合时漏掉的光被用于光泵浦阶段的泵浦光,提高了光束的利用率;其次,在实验中我们将两级MOT由光纤滤波改为使用pinhole进行滤波,由于放大器输出光对光纤的耦合效率比较低,最高只能60%左右,这样用于冷却的光功率会比较低,且存在保偏问题,严重影响了原子的俘获和冷却,改用pinhole进行滤波,光斑模式得到改善的情况下,衍射效率也比较高,大约为90%左右,而且,不存在保偏的问题;采用脉冲光推送的方式,提高了从MOT1到MOT2原子的传输效率;建立了实现简并费米气体所需要的各种光束,包括探测光,抽运光,再抽运光等;搭建了实验中所需的吸收成像探测系统。2,设计并且制作了实验系统中所需的各类电路。在实现量子简并气体的过程中,需要很精确的时序控制,而时序控制的实现需要很多外围的电路,因此在实验中自制了一些外围的电路,主要包括信号隔离电路,机械开关驱动电路,电流源电源开关电路,电流开关电路等,通过这些电路实现了对光场和磁场的计算机控制。3,实现了⁴⁰K原子的DFG和⁸⁷Rb原子的BEC。在MOT2中重新俘获⁴⁰K原子和⁸⁷Rb原子,通过压缩阱,偏振梯度冷却,光抽运阶段之后,将两种原子装载到四级阱中,然后将原子转移到QUIC阱中进行蒸发冷却得到了⁴⁰K原子的DFG和⁸⁷Rb原子的BEC。在实验中理论分析了BEC和DFG的空间密度分布并对实验结果进行了拟合,得到了BEC凝聚体的相变温度为500nK,凝聚体的原子数目为10⁵,⁴⁰K原子达到量子简并时的粒子数为7.59×10⁵,系统的费米温度为T_F=961 nK,且T/T_F=0.28。4,将⁸⁷Rb原子的BEC非绝热地装载到一维的光学晶格中,通过Kapitza-Dirac散射测量了一维光学晶格势阱的深度,而且将一维光晶格形成的脉冲相位光栅应用于⁸⁷Rb原子的BEC,观测到了物质波的Talbot效应。更多还原

【Abstract】 Starting with the observation of Bose-Einstein condensation in single-component bosonic gases and Fermi degeneracy in spin-polarized Fermi gases,the field of ultra-cold quantum gases is rapidly expanding to studies of mixed systems of different atomic species with different statistics, different trapping properties,masses and interaction.The studies of the mixed systems open up the new avenue for the many-body physics,long-range interacting systems,strongly correlated phases and quantum simulation. Especially,when the means of strong,periodic confinement as demonstrated in experiments with optical lattices and the strong resonant interactions which can be produced by Feshbach resonances are applied into this regime,the researches of the Bose-Fermi mixed gases show the beautiful perspective.The goal of our work is to establish the experimental apparatus for ⁸⁷Rb BEC and ⁴⁰K DFG and perform the associated scientific researches.The contribution of this thesis is doing a series of work to realize the quantum degeneracy of ⁴⁰K and ⁸⁷Rb,based on the established double-MOT cooling experimental setup,and achieving the BEC of ⁸⁷Rb and the DFG of ⁴⁰K.This is the first time to achieve the DFG at home.The experimental protocol:the double-MOT structure is adopted by our lab and the mixtures of ⁴⁰K and ⁸⁷Rb are selected as the workhorse.Firstly,fermionic ⁴⁰K and bosonic ⁸⁷Rb atoms is simultaneously magneto-optical trapped in collection cell,and then the precooled atoms is pushed into the science cell in which the MOT is performed again.Finally,the cold atoms are loaded into the QUIC trap and the evaporative cooling is performed.Through the above process,the BEC of ⁸⁷Rb can be achieved,and the DFG of ⁴⁰K can also be realized by sympathetic cooling with the evaporated Rb.The thesis mainly includes the following parts:1,some improvement has been made about the semiconductor laser system used in the experiment.Firstly,the injection configuration is changed from the three beams of ⁸⁷Rb cooling,⁴⁰K cooling and repumping injected into TA1（TA:Tapered amplifier） and TA2 respectively to ⁸⁷Rb cooling light injected into TA1 and ⁴⁰K cooling coupled with ⁴⁰K repumping light injected into TA2,to avoid the instability of the output power;Secondly,the pinhole possessing the coupling efficiency 90%is used for the filter of the beam mode instead of the optical fiber possessing the coupling efficiency 60%,this method increases the power for cooling atoms and polarization-maintaining problem do not exist.Thirdly,the transport efficiency from MOT1 to MOT2 is improved through the way of pulse-loading,and the required beams in the experiment including the probe beam,the pump beam,and repumping beam are set up.Finally,the detection system of absorption image is established.2,Designing and making all kinds of circuits required for the experiment.The exact experimental sequence is needed in the experiment and the realization of the sequence needs many circuits containing the opto-coupler circuits,driver circuits for the mechanical shutter,and the switch circuits for the current etc.These circuits are self-made and used to the control of the optical field and magnetic field.3,The BEC of ⁸⁷Rb and the DFG of ⁴⁰K are achieved in the experiment. After the magneto-optical trapping of the two species of atoms,the process of compressed MOT,molasses,and optical pump is performed,and then the cold atoms is compressed in the quadrupole magnetic trap and transferred into the QUIC trap.Through the evaporation cooling in the QUIC trap,the quantum degeneracy of Bose-Fermi mixed gases is achieved.The density distributions gained from the absorption images are theoretically analyzed and the experimental date is fitted.The critical temperature for BEC is about 500nK with the atom number 10⁵,and the quantum behavior in the case of ⁴⁰K is also analyzed.4,We study ⁸⁷Rb BEC loading into the pulse of the one-dimensional optical lattice experimentally,in which the lattice is turn on abruptly,held constant for a variable time and then turn off abruptly.The measurement of the depth of the optical lattice is obtained by Kapitza-Dirac scattering.The temporal matter-wave-dispersion Talbot effect with Rubidium BEC is observed by applying a pair of pulsed standing wave（as phase gratings） with the separation of a variable delay.更多还原