Formation of Monochromatic Atomic Beam for Atom interferometer

在激光囚禁、原子干涉仪、原子光刻等领域中，光诱导原子力被有效地用来操纵中性原子的运动。在这些实验中，要求强流原子束具有窄的纵向速度分布。在原子干涉仪中，原子束的速度扩展会降低干涉对比度。用通常的超声方法得到的速度扩散约为4%，这使干涉对比度降低到单色情况的28%。因此，制备单色原子束是非常重要的。本研究发展了一种中性原子束速度谱仪，用以制备单色原子束。为了根据原子束的纵向速度分散相对快的原子束，已经使用了具有大磁梯度的磁四重透镜。该装置由NdFeB永磁体制成。经测量，四极磁场梯度足够大（IT/cm），且沿径向沿着均匀。用该谱仪测量了800℃热Li束流的纵向速度分布，与荧光谱的结果符合得很好。利用所研制的速度谱仪，可以在一个紧凑的实验装置上容易地形成速度展宽小于0.1%的1000 m/s的单色Li原子束。用该光束进行干涉对比度为96.8%。与光学单色仪类似，通过使用更窄的狭缝可以实现更高的分辨率。然而，使用较窄的狭缝意味着可用原子束的严重损失。因此，我们采用了激光冷却的方法来压缩纵向速度分布在所需的速度。我们已经实现了1000 m/s的强Li原子束，比原始束亮41倍。

Light induced force on atoms has been effectively used to manipulate neutral atomic motion in laser trapping, atom interferometer, or atom lithography. In these experiments, an intense atomic beam with narrow longitudinal velocity distribution has been required. For the atom interferometer, the interference contrast is reduced by the velocity spread of the atomic beam. The velocity spread obtained by the usual supersonic method is about 4%, which reduces the interference contrast to 28% of the monochromatic case. Therefore, preparation of a monochromatic atomic beam is quite important. In this study, a velocity spectrometer for a neutral atomic beam has been developed to prepare a monochromatic atomic beam. In order to disperse a relatively fast atomic beam according to its longitudinal velocity, a magnetic quadruple lens with a large magnetic gradient has been used. The device was made by NdFeB permanent magnets. The magnetic gradient in the quadruple was measured to be sufficiently large (IT/cm) and uniform along radial direction. Using the present spectrometer, we have measured the longitudinal velocity distribution of the thermal Li beam at 800℃, which was in good agreement with the result from the fluorescence spectra. It is expected that a monochromatic Li atomic beam of 1000m/s with the narrower velocity spread than 0.1% can be easily formed with a compact experimental arrangement using the developed velocity spectrometer. The interference contrast using this beam is 96.8%. Similar to the optical monochrometer, a higher resolution can be achieved by using a narrower slit. However, usage of the narrower slit means the severe loss of the available atomic beam. So, we have adopted the laser cooling method to compress the longitudinal velocity distribution around the desired velocity. We have achieved to make an intense Li atomic beam of 1000m/s that is 41 times brighter than the original beam.