Neutral atom trapping and laser cooling has opened the way to large prospects for the present generation of atomic frequency standards [1].A cold cesium fountain device has been built for the first time at the Observatoire de Paris and is now operating since 1994.Its frequency stability, 4 10 -14 -1/2 where is the integration time, has been the best one ever measured for a while.This letter shows how accurate can be the measurement of low static magnetic fields within the fountain device and the variations along the longitudinal and transverse axis.Thus the device is no more operating as a clock but constitutes a very precise magnetometer.
Neutral atom trapping and laser cooling has opened the way to large prospects for the present generation of atomic frequency standards [1].A cold cesium fountain device has been built for the first time at the Observatoire de Paris and is now operating since 1994.Its frequency stability, 4 10 -14 -1/2 where is the integration time, has been the best one ever measured for a while.This letter shows how accurate can be the measurement of low static magnetic fields within the fountain device and the variations along the longitudinal and transverse axis.Thus the device is no more operating as a clock but constitutes a very precise magnetometer.
The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P
The realisation of cold and slow atomic beams has opened the way to a series of precision measurements of high scientific interest, as atom interferometry, Bose-Einstein Condensation and atomic fountain clocks. The latter are used since several years as reference clocks, given the high performance that they can reach both in accuracy and stability. The common philosophy in the construction of atomic fountains has been the pulsed technique, where an atoms sample is launched vertically and then falls down under the effect of the gravity. The Observatoire de Neuchâtel had a different approach and has built a fountain clock (FOCS1) operating in a continuous mode. This technique offers two main advantages: the diminution of the undesirable effects due to the atomic density (e.g. collisions between the atoms and cavity pulling) and to the noise of the local oscillator (intermodulation effect). To take full advantage of the continuous fountain approach however, we need to increase the atomic flux. The techniques chosen to reach this goal are an efficient transverse collimation together with more atoms to begin with. Here we report on a study of different transverse collimation techniques performed in a two-dimensional phase stable optical lattice (namely gray molasses and magnetically induced laser cooling) as well as the development and characterisation of a 2D-magneto-optical trap used to load the fountain. Best performances are reached in a 2D+-MOT configuration of such a pre-source, for which we detect an atomic flux 20 times greater than the one measured when the fountain is loaded by a Cs vapour. This gain could improve the atomic clock stability by a factor 6. An even better stability is expected after introducing a pre-cooling stage before performing the transverse collimation. This new configuration is currently under investigation and will be implemented in a second continuous fountain (FOCS2).
The realisation of cold and slow atomic beams has opened the way to a series of precision measurements of high scientific interest, as atom interferometry, Bose-Einstein Condensation and atomic fountain clocks. The latter are used since several years as reference clocks, given the high performance that they can reach both in accuracy and stability. The common philosophy in the construction of atomic fountains has been the pulsed technique, where an atoms sample is launched vertically and then falls down under the effect of the gravity. The Observatoire de Neuchâtel had a different approach and has built a fountain clock (FOCS1) operating in a continuous mode. This technique offers two main advantages: the diminution of the undesirable effects due to the atomic density (e.g. collisions between the atoms and cavity pulling) and to the noise of the local oscillator (intermodulation effect). To take full advantage of the continuous fountain approach however, we need to increase the atomic flux. The techniques chosen to reach this goal are an efficient transverse collimation together with more atoms to begin with. Here we report on a study of different transverse collimation techniques performed in a two-dimensional phase stable optical lattice (namely gray molasses and magnetically induced laser cooling) as well as the development and characterisation of a 2D-magneto-optical trap used to load the fountain. Best performances are reached in a 2D+-MOT configuration of such a pre-source, for which we detect an atomic flux 20 times greater than the one measured when the fountain is loaded by a Cs vapour. This gain could improve the atomic clock stability by a factor 6. An even better stability is expected after introducing a pre-cooling stage before performing the transverse collimation. This new configuration is currently under investigation and will be implemented in a second continuous fountain (FOCS2).
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.>
At the moment the National Center of Metrology (CENAM) develops to the necessary technology for the creation of a frequency standard of high accuracy denominated CsF1 atomic fountain. The first stage of development of this clock consists of laser cooling and trapping neutral Cs-133 atoms. The mechanism of laser cooling and trapping of atoms requires of the accomplishment of a magneto-optical trap (MOT) in where is indispensable to maintain a strict control in its parameters of operation. In this experiment, the MOT operates with three set of red detuning |62s1/2, F=4|-> |62s3/2, F'=5| cyclical transition of Cs-133, mutually orthogonal, counterpropagating laser beams, with opposite circular polarization, intersecting at the center of a magnetic field which is generated by a pair of coil in anti-Helmholtz configuration. Additionally, a beam of light that comes from a repumping laser, tuned to the frequency of the |62s1/2, F=3|->|62s3/2, F'=4| transition of Cs-133, is injected to the MOT to maintain an atoms population in the |62s1/2, F=4| state. To cool and trap atoms to so low temperatures, of the order of the microkelvins on the zero absolute, by several seconds, minutes or even by several hours, provided the fundamentals tools to us to study the temporal and spatial behavior of the fluorescence of the atomic cloud and to make exact measurements in the count of such, in order to deduce the parameters of operation of the MOT that allows us to trap 1010 atoms or more. In this work the theoretical concepts as well as the experimental details of the parameters of operation of the MOT for the formation of the cold atom cloud are approached.