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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

An electrostatic light shutter for atomic fountain clocks

L. Marmet, R.M. Thomson

Published Jun 26, 2003• 3 citations
Publisher

Abstract

Most cesium fountain clocks built today operate in sequential steps where an atomic cloud is trapped, launched, interrogated and detected. Except for the interrogation, every other step requires laser beams to illuminate the atoms and produce fluorescence. The microwave interrogation is, on the other hand, severely disturbed by any fluorescence which produces a light shift on the atoms. Since it is desirable to minimize the dead time between the interrogation of the atoms, it is tempting to try to operate the fountain with many steps occurring simultaneously. In that case, the fluorescence light produced by the trap and the detection must be blocked before it enters the interrogation region. This is accomplished in some fountains by a using various shutters or by deflecting the trajectory of the atomic cloud with a non-uniform magnetic field. We propose here a new type of shutter that operates using an electrostatic force to move a rotor. A hole in the rotor lines up with the axis of the fountain at regular intervals to let the atomic cloud through, but then the shutter is closed when the laser beams are turned on. We give here the details of the operations of such a shutter and present the results of attenuation measurements.

DOI
10.1109/freq.2002.1075923

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Most cesium fountain clocks built today operate in sequential steps where an atomic cloud is trapped, launched, interrogated and detected. Except for the interrogation, every other step requires laser beams to illuminate the atoms and produce fluorescence. The microwave interrogation is, on the other hand, severely disturbed by any fluorescence which produces a light shift on the atoms. Since it is desirable to minimize the dead time between the interrogation of the atoms, it is tempting to try to operate the fountain with many steps occurring simultaneously. In that case, the fluorescence light produced by the trap and the detection must be blocked before it enters the interrogation region. This is accomplished in some fountains by a using various shutters or by deflecting the trajectory of the atomic cloud with a non-uniform magnetic field. We propose here a new type of shutter that operates using an electrostatic force to move a rotor. A hole in the rotor lines up with the axis of the fountain at regular intervals to let the atomic cloud through, but then the shutter is closed when the laser beams are turned on. We give here the details of the operations of such a shutter and present the results of attenuation measurements.
Our recent work showed the atomic microwave clock configuration beyond fountain is possible by trapped Rb or Cs atoms in optical lattice formed by trapping laser operating at magic wavelength for microwave clock transition where light shifts on both clock levels are matching exactly, and an accuracy of 2 times 10-17at this clock configuration is expected. In this report, we analyze the possibility of 20 s coherent interrogation time of stable Rb or Cs clock transition, which corresponds to a 2 km height atomic fountain. We propose a simple experimental scheme to measure the needed magic wavelengths for atomic microwave clocks in conventional fountain configuration. Moreover, we discuss a new clock scheme combining fountain and lattice configurations, thus provide an easy way to improve fountain clock performance by adding a trapping laser beam in conventional fountain system without any further complicated modifications
We propose an atomic fountain clock with a truncated cold atomic beam to achieve both a low collisional frequency shift and high frequency stability. In this clock, the launching velocity of a cold atomic beam can be swept to reduce the atomic density in the interrogation region for the Ramsey resonance and to increase the atomic density in the detection region. Before the top of the beam arrives at the interrogation region, the cold atomic beam is truncated by turning off the cooling laser beams to remove the unnecessary light shift. The atomic density in the interrogation region is theoretically evaluated to be 0.04 times that in an ordinary atomic fountain with optical molasses for the same number of detected atoms. The frequency stability limit due to quantum projection noise is calculated to reach $6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ in 1 s from the number of detected atoms while the fractional collisional shift is estimated to be $~\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$.
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.
The well-known MOT technique is used to trap and cool 133 Cs atoms to reproduce the SI second in an atomic fountain. About 10 8 cesium atoms are caught in a magnetooptical trap from a vapor cell at room temperature. The fountain has a (1,1,1)-geometry for the laser cooling and launching operation. The three upper beams have the same polarization, opposite to the three lower ones. The interrogation and preparation cavities have also been constructed, as well as characterized, concerning the temperature tunability in vacuum. The microwave synthesizer has been constructed in collaboration with BNM-SYRTE, time-and-frequency group. The synthesizer has been compared to a very stable oscillator at BNM-SYRTE resulting in a short-term stability of 9.7 × 10 -14 τ -1/2 .
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.>
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.