Formation of Monochromatic Atomic Beam for Atom interferometer

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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.