Manipulating cold atoms with atom chip

原子芯片集合了成熟的激光操纵原子的技术和微制造技术，能利用小型化的装置对原子进行复杂且精确地操控。在本论文中，首先详细地描述了用原子芯片操纵冷原子的实验，包括冷原子团的囚禁、压缩、导引、分裂和复合。其次研究了芯片表面引起的蒸发冷却效应。最后简单的介绍了利用射频蒸发冷却实现了玻色-爱因斯坦凝聚的实验过程。 利用镜面磁光阱从背景气体中捕获了大约1.5*10^7 个Rb原子，然后转移，压缩，偏振梯度冷却，光抽运。最后约4*10^6个处于F=2,m_f=2 态的原子被装载到芯片上的Z型微磁阱中。绝热地增加偏置场 ，原子团被绝热压缩，离芯片表面约85 μm。测量了压缩后磁阱的底部磁场和磁阱频率，研究了绝热压缩过程中原子团的位置、原子数、寿命、温度和弹性碰撞率的变化。在高度压缩的磁阱中，观察到了严重的加热现象，研究了加热率和磁阱频率的关系。 H型芯片上两根直导线通反向电流，同时加一个垂直于芯片表面的偏置场，在两根导线的中间会形成一个导引。成功地将U型微磁阱中的原子团装载到导引中，装载效率约60%。研究了原子团的在导引中运动特性，观察到了原子团在导引径向上的简谐振荡。研究了导引的位置和偏置场的关系。最后研究了重力势对反向电流导引和同向电流导引的影响。 利用双U型芯片上的两根U型线和一个外加偏置场，实现了原子团的对称分裂和复合。改变偏置场的大小和方向，观察到了原子团的非对称分裂。研究了分裂过程中原子团的轨迹变化和原子数比例的变化。 原子芯片表面可以认为是一种理想的射频刀，它可以选择性地蒸发掉所有能量高于表面势垒的原子。将原子团移向芯片表面的过程中，我们观察到了表面蒸发冷却效应。这种表面蒸发冷却的效率是位置相关的，也和磁阱的形状有关系。当原子团离芯片表面很近时，存在严重的加热现象。这种加热抑制了表面蒸发冷却的效果。 经过4.16 s的射频蒸发冷却，在芯片上实现了玻色-爱因斯坦凝聚。原子团中心的光学厚度突然的增加和飞行过程中纵横比的变化都证实了玻色-爱因斯坦凝聚的实现。Atom chip brings together the methods of manipulating neutral atoms with laser and the mature techniques of microfabrication. it offers a robust, scaleable platform which can realize sophisticated and precise manipulations of cold atoms. in this thesis, first, the manipulations of atom clouds with atom chip, including the trapping, adiabatically compressing, guiding, dynamically splitting and merging, are detailed. next, surface-induced evaporative cooling effect is investigated. last, the realization of bose-einstein condensation on atom chip by using rf-forced evaporative cooling is described simply. About 1.5*10^7 rb atoms were collected from the background vapor using uv mirror-mot. after an optimized loading process, about 4*10^6 atoms in f=2,m_f=2 state were loaded into the z-trap. atom cloud was compressed adiabatically and moved close to the surface of atom chip by increasing the bias field. the characteristic parameters of the compressed trap, such as the bottom field and trap frequency, were measured. the evolutions of the position, atom number, lifetime and collision rate of atom cloud during the compression were investigated. in highly compressed microtrap, severe heating was observed, which was dependent of the trap frequency. A guide is created by superposing the magnetic field of two parallel wires with counter-propagating equal currents and a vertical bias field. the atom cloud trapped in u-trap was transferred into the guide with efficiency of about 60%. atom cloud was guided along the axial direction of the guide and harmonically oscillated in the radical direction. increasing the bias field, the guide was moved to the surface of the chip. furthermore, we discussed the influence of the gravity on two different guide potentials. Symmetrical splitting and merging of atom cloud were demonstrated with two u-wires on the doulbe u-shaped chip. changing the magnitude and direction of the bias field, an asymmetrical splitting was observed. moreover, the trajectories of the symmetrical and asymmetrical splitting were investigated, which ware consistent with theoretical calculations. The surface of the chip, as a nearly ideal evaporative knife, could selectively evaporate all the hot atoms with energy in excess of the potential on the surface. when the atom cloud was moved to the surface, surface-induced evaporative cooling effect was observed. the efficiency of evaporative cooling was dependent of the atom-surface distance and the shape of the evaporative trap. when atom cloud was moved very close to the surface, severe heating is also observed, which dominated when the holding time is more than 8 ms. Using rf-forced evaporative cooling about 4.16 s, bose-einstein condensate was realized in the z-trap on the atom chip. the sudden increase of the peak od of the atom cloud and the nonisotropic shape evolution during free expansion. both confirmed the realization of bose-einstein condensate