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

Progress on the construction of an atomic cesium fountain at NIM

Tianchu Li, Mingshou Li, Huang Bingying, Qian Jin, Lin Pingwei

Published Nov 13, 2002• 1 citation
Publisher

Abstract

An atomic cesium fountain has been designed for a new primary standard of frequency in China. Cold atomic clouds collected in the magneto-optical trap (MOT) were previously reported (1995). To further improve the performance of the MOT we have optimized the laser-optical system and operation of the MOT. Some optical elements were replaced with new ones with better quality; frequency stabilization of lasers and arrangement of the optical system was modified; and polarization maintaining (PM) optical fibers were adopted to transfer lights from the optical system to MOT. We also installed the fluorescence detecting - amplifying assembles to take the time of flight (TOF) measurement of the cold atoms.

DOI
10.1109/freq.2001.956169

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An atomic cesium fountain has been designed for a new primary standard of frequency in China. Cold atomic clouds collected in the magneto-optical trap (MOT) were previously reported (1995). To further improve the performance of the MOT we have optimized the laser-optical system and operation of the MOT. Some optical elements were replaced with new ones with better quality; frequency stabilization of lasers and arrangement of the optical system was modified; and polarization maintaining (PM) optical fibers were adopted to transfer lights from the optical system to MOT. We also installed the fluorescence detecting - amplifying assembles to take the time of flight (TOF) measurement of the cold atoms.
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
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.
We report the design of a new cesium fountain clock NIM6, which is under construction in NIM. Besides some improvements on the vacuum system, Ramsey cavity and microwave synthesizer to reduce the Type B uncertainty. Another major improvement on NIM6 is to collect more atoms from a MOT loading optical molasses and optical pumping to get a better signal to noise ratio at the detection. The atom distribution will be more uniform compared with a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling to keep the collisional-induced frequency shift low. The atom numbers can be further increased by a new de-pumping - optical pumping procedure to pump atoms to the |F=3, mF=0> clock state directly. With a new cryogenic sapphire oscillator (CSO) based frequency synthesizer, NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty compared with NIM5.
National Physical Laboratory India (NPLI) has been developing a cesium atomic fountain primary frequency standard. We have succeeded in trapping about 107Cs atoms, cool them to about 5 μK by both magneto-optical trap (MOT) and polarization gradient cooling (PGC) and launch them up by moving molasses method. We have demonstrated the fountain action by launching the atoms up to 72.9 cm up and detection of the return signal by measuring the fluorescence. Work is underway to interrogate the launched atoms with microwaves and observe Ramsey fringes.