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Conference on Precision Electromagnetic Measurements

Precision measurements using laser cooled atoms

Steven Chu, Mark A. Kasevich, Kathryn A. Moler, Erling Riis, David S. Weiss

Published Dec 4, 2002• 1 citation
Publisher

Abstract

A number of experimental applications of laser cooling of neutral atoms are described. In an experiment on atomic fountains, laser-cooled atoms pushed up on a vertical trajectory by radiation pressure were observed to turn around due to gravity. A compact cesium fountain that would give three to four orders of magnitude increase in the counting rate over the first experiment can be designed, and the trapping and cooling can be done with diode lasers, suggesting a practical clock. In order to increase the time-averaged flux of such a fountain and also create a continuous fountain, an atomic funnel that can produce an atomic beam of very low velocity ( approximately 270 cm/s) at densities of 10/sup 8/ atoms/cm/sup 2/ has been devised.>

DOI
10.1109/cpem.1990.109897

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A cesium frequency standard based on the atomic fountain configuration has the potential of improving the accuracy of primary frequency standard. For the first step toward a realization of slow vertical atomic beam (that is an atomic fountain), we have succeeded to decelerate the horizontal cesium atomic beam by laser cooling with the Zeeman tuning method. Then those slowed atoms are confined with a magneto-optical trap by four laser beams in the tetrahedral configuration. By using these trapped atoms, one can launch them upward with the upstream laser pulses.>
Summary form only given. The long observation times afforded by laser cooling of neutral atoms ushered in a new era of accuracy and stability for cesium atomic clocks. Traditional thermal beam atomic clocks are limited to interaction times of only a few milliseconds, while with laser cooling times of order 1 second have been realized with high stability. Most laser cooled clocks use a "fountain" geometry, in which the atoms are thrown up against the pull of gravity and realize longer times before falling back to the bottom of the apparatus. We have been developing a laser-cooling apparatus for flight on the International Space Station (ISS), with the intention of demonstrating linewidths of the cesium clock transition narrower than can be realized on the ground.
The measurement on energy levels suitable for time standards can be made more precise with the use of neutral atom manipulation techniques. We have created a fountain of atoms where the measurement time of atoms in ballistic free-fall can be on the order of a fraction of a second. With these long measurement times, the linewidth of the transition between the ground state hyperfine transition of sodium was measured to be 2 Hz. Transitions in the optical frequency range could then have Q's of 1014 to 1015. In addition to creating high Q resonances, it is desirable to have a continuous, high density fountain of atoms. Towards this end, we have constructed an "atomic funnel" capable of creating a cold, high flux stream of atoms suitable for injection into a continuous atomic fountain. In the first embodiment of the funnel, the brightness of the atomic beam (defined as the number of atoms per second/( ΔvxΔvyΔvz) was increased by a factor of 3 × 103. With more recent improvements for both funneling more atoms from the hot atomic beam and better use of polarization gradient cooling to launch the atoms, we will describe how to improve the brightness enhancement by another four orders of magnitude.
Present status of laser cooling experiments in NRLM is presented. Laser cooling and trapping experiments for cesium atoms are continued in NRLM to develop an atomic fountain frequency standard. As a first step, we performed Cs atomic beam deceleration and trapping experiments. Atomic beam cesium atoms were decelerated by counterpropagating radiation from semiconductor laser diodes. The laser frequency was chirped to keep it in resonance with the decelerated atoms. The decelerated atoms were trapped by six mutually orthogonal laser beams in a magneto-optical trap configuration. The atoms could also be trapped directly from ambient cesium gas in a glass cell without using the deceleration laser beam. The characteristics of the trapped atoms are reported.
In our Cs atomic fountain, we laser cooled and trapped 10(exp 10) Cs atoms in 0.4 s. By shifting the frequencies of the laser beams, the atoms were launched upwards at 2.5 m/s and a fraction of the atoms were optically pumped into the F=3 ground state. The unwanted atoms in the F=4 ground state were removed from the fountain with radiation pressure from a laser beam tuned to excite only those atoms. The Cs atoms in the F=3 state traveled ballistically upwards, were excited by the microwave cavity, and then returned back through the same cavity in the atomic fountain configuration.
Summary form only given. We have constructed a prototype of an /sup 87/Rb fountain clock. We collect 10/sup 10/ /sup 87/Rb atoms in a vapor cell magneto-optic trap (MOT). The laser system for trapping and cooling is entirely solid state--a grating stabilized laser injects a separate slave laser for each of the six beams of the MOT. We launch the atoms by frequency shifting the three downward propagating laser beams and then increasing the detuning and gradually reducing the intensity of the slave lasers to adiabatically cool the atoms to low temperatures in the fountain.