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Conference on Precision Electromagnetic Measurements

A frequency standard based on a continuous beam of laser cooled atoms

P. Berthoud, Alain Joyet, P. Thomann, G. Dudle

Published Nov 27, 2002• 0 citations
Publisher

Abstract

A frequency standard based on a continuous beam of laser cooled atoms as an alternative to a pulsed scheme of an atomic fountain is currently investigated. A general overview of such a device and its main parts are presented. Two mechanisms of continuous extraction of atoms from an anisotropic magneto-optical trap are experimentally demonstrated. Stability and accuracy issues are discussed.

DOI
10.1109/cpem.1998.699817

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A frequency standard based on a continuous beam of laser cooled atoms as an alternative to a pulsed scheme of an atomic fountain is currently investigated. A general overview of such a device and its main parts are presented. Two mechanisms of continuous extraction of atoms from an anisotropic magneto-optical trap are experimentally demonstrated. Stability and accuracy issues are discussed.
Atomic clocks based on a continuous beam of cold atoms are a promising alternative to fountains mainly because the low atomic density of a beam would reduce collisional shifts to a negligible level; in addition dead times can be suppressed or greatly reduced. Several mechanisms allowing to continuously extract cold atoms from an anisotropic trap will be discussed, in particular atomic drift induced by a constant magnetic field and laser beam imbalance. Experimental results of trap loading times and time-of-flight measurements will be presented to assess the potential of these methods for the production of a continuous beam of cold atoms.
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.>
To achieve a primary frequency standard based on a continuous fountain of laser cooled cesium atoms, we report on the production of a bright, slow and cold continuous atomic beam. Atoms are cooled from a thermal vapor in a 45/spl deg/ laser geometry and launched directly from the optical molasses by the moving molasses technique. The effects of the source fluorescence on the continuous beam (average velocity, temperature and flux) have been observed, and ways to optimize simultaneously the atomic capture and the continuous beam are presented. Flux measurements rise to 2/spl times/10/sup 8/ at/s from a 2D magneto-optical trap at 7 m/s. The initial velocity is easily tunable from 1 to 10 m/s. If the longitudinal temperature is about 70 /spl mu/K, additional transverse cooling even reduces the transverse temperature in one dimension below 30 /spl mu/K. By extrapolating the obtained atomic flux to the continuous fountain under construction, we expect a short-term stability of 7/spl times/10/sup -14//spl tau//sup -1/2/. Other applications of continuous beams of cold atoms are suggested.
Cesium atomic fountain clock is the quantum frequency standard with very high accuracy. Research on laser cooling and trapping of cesium atom at Peking University began in 1993. A cesium atomic fountain frequency standard is being developed. A magneto-optical trap with some 108 trapped atoms and an optical molasses with the cold atoms of the temperature of 10/spl mu/K have been achieved previously. The device has been carefully designed and is under construction. It is composed of two main parts: vacuum system and laser system. The article describes the apparatus and presents a brief introduction to some considerations on the atomic clock.
Summary form only given.Laser cooling, where excess kinetic energy is extracted by a photon, is ideally suited to loading atoms into a trap; however, quantization and reabsorption of light, limit the maximum phase-space density attainable. A sequence of laser cooling followed by evaporative cooling, where the excess energy is extracted by escaping atoms, may be used to overcome the laser cooling limit; however, such a scheme dramatically reduces the number of trapped atoms. A remaining challenge is to achieve continuous loading without the phase-space density limit imposed by near-resonant light. We consider continuous evaporative loading of a magnetic trap using a magnetically insensitive buffer gas. The key is to deliver sufficient cold atoms to the trapping region such that the steady-state trap population is large. To achieve this we employ an atomic fountain guided by a far-off resonant laser beam. At the apex of the fountain, the density can be a factor of 30-40 higher than in a magneto-optical trapping region, and importantly, the atoms are spatially separated from the region of resonant laser light. We propose to use this high-density region as a reservoir for continuous evaporative loading of a trap.