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European Frequency and Time Forum

Continuous extraction mechanisms from a Cs magneto-optical trap

N Sagna

Published Jan 1, 1996• 0 citations
Publisher

Abstract

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.

DOI
10.1049/cp:19960050

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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.
Summary form only given. We discuss our compact clock physics package and report the realization of a continuous, cold Cs atomic beam from a vapor-cell magneto-optical trap (MOT) of Cs atoms. Briefly, we generate the cold Cs beam by perturbing the trap potentials with a small opaque disk inserted in one of the MOT beams. Along the narrow dark column thus formed, Cs atoms are continuously accelerated out of the trap volume to form a Cs beam while the MOT remains in functioning of capturing and cooling Cs atoms from the Cs vapor. The fluorescence image of the Cs trap and the continuous Cs atomic beam are shown. Our MOT captures and confines 8 /spl times/ 10/sup 7/ Cs atoms at a loading rate of 2 /spl times/ 10/sup 8/ atoms/s and cools the atoms to a thermal temperature of 300 /spl mu/K. The longitudinal velocity of the cold Cs beam is measured by a time of flight (TOF) method. A single-shot laser-induced fluorescence signal recorded at a distance of 18 cm downstream from the MOT is shown. Under the nominal experimental conditions, the TOF measurement determines that our cold Cs atomic beam travels at 7 m/s and has a velocity spread of 0.9 m/s. With the cold Cs beam thus formed and a compact Ramsey cavity of 20 cm in length, we have estimated a short-term Allan deviation of 1.8 /spl times/ 10/sup -13/ /spl tau//sup -1/2/ (T is the averaging time) for the atomic clock under development.
An anisotropic magneto-optical trap is studied as the potential source of a continuous beam of laser-cooled atoms. Capture efficiency and cloud temperature are measured experimentally and found to be lower than in an isotropic situation with the same cooling parameters. Intensity imbalance and static magnetic field are investigated as possible extraction mechanisms. While only small drift velocities are induced by means of intensity imbalance, velocities of tens of are observed with a static magnetic field; the shape of the observed two-peaked velocity distributions is related to the average depth of local light-shift-induced potential wells.
Starting from a 2D magneto-optical trap where cesium atoms are permanently subjected to 3D sub-Doppler cooling and 2D magneto-optical trapping, we have produced a beam of cold atoms continuously extracted along the trap axis. The simplest extraction mechanism, presently used, is the drift velocity induced by a constant magnetic field. We have used this continuous beam of atoms to produce Ramsey fringes in a microwave cavity as a first demonstration of an atomic resonator operating continuously with laser cooled atoms. The shape of the resonance pattern allows an estimate of the axial temperature, typically 200 μK. The average velocity can be adjusted from 0.7 to 3 m/s; the trap-to-atomic-beam conversion efficiency is close to one.
In this paper we report our design and experimental results towards demonstration of a magneto optical trap (MOT)-based cold Cs-beam atomic clock. We generated a continuous cold Cs beam from a Cs MOT by perturbing the MOT potential and measured the longitudinal velocity of the Cs beam to be 7 m/s. In order to separate the atomic beam from the MOT laser beam, we used a one-dimensional optical molasses to deflect the Cs beam through an angle of 30° before the atomic beam enters the microwave cavity. Our Cs beam has an instantaneous atomic flux of 3.6 × 1010atoms/s when operated in pulsed mode and a continuous flux of 2 × 108atoms/s. Under current experimental conditions, the observed cold Cs atomic beam parameters infer a short-term, shot-noise limited Allan standard deviation of 2.7 × 10-13τ-1/2(τ is the averaging time) for the cold Cs beam clock under development.
In this paper, we present our design and prospects of a magneto-optical trap (MOT)-based cold Cs-beam atomic clock physics package and report the realization and characterization of a continuous, laser-cooled Cs atomic beam from a Cs MOT. We have determined the longitudinal velocity of the cold Cs beam by time of flight (TOF) method to be 7 m/s with a velocity spread of 1 m/s. By adjusting the MOT parameters, we are able to tune the Cs-beam velocity from 5 m/s to 8.5 m/s while the velocity spread remains to be 1 m/s. The Cs beam has an instantaneous atomic flux of 3.6 times 1010atoms/s when operated in pulsed mode and a continuous beam flux of 2 times 108atoms/s. Our theoretical simulation reveals that the MOT inhomogeneous magnetic field and the varying Doppler shift along the atomic beam propagation play an important role in determining the longitudinal velocity of the Cs atomic beam. With the cold Cs beam thus formed and a compact Ramsey cavity of 13 cm in length, we have estimated a short-term, shot-noise limited Allan standard deviation of 2.7 times 10-13pi-frac12(pi is the averaging time) for the atomic master oscillator under development