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Chinese Physics Letters

Proposal of a Novel Compact Cold Atomic Fountain Clock

Rong Wei, Yong Qian, Yu Zhang, Yuzhu Wang

Published Jan 1, 2004• 5 citations
Publisher

Abstract

We propose a novel atomic fountain clock which is compact and transportable. The clock is 60-cm in height. Its linewidth is expected to be 1.75 Hz, with accuracy 10 −14 , stability 10 −14 and signal–noise ratio 10 3 . A hollow beam is applied as atomic wave-guide, which can increase the stability and the signal–noise ratio by one order. The hollow beam will not affect the accuracy of the clock, if the frequency shift due to the hollow beam is amended and the fluctuation of the hollow beam intensity is controlled to be smaller than 0.6%.

DOI
10.1088/0256-307x/21/1/017

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Abstract : The grant as in support of a research program to explore the possibility of a microwave atomic clock based on an atomic fountain of cesium atoms. During the two year granting period, the following work was done: A large magneto-optic trap in a vapor cell was constructed where over 4x 10(exp 10) atoms were trapped at a density on the order of 10(exp 11) atoms/cu cm. This work increased the number of atoms optically trapped by a factor of -400 compared to previous work. The optical density of the atoms in this trap has a peak attenuation of e(-175). In an invited review article we examined the prospects for slow atom frequency standards. Most of the systematic effects that limit the current accuracy today's frequency standards decrease as the velocity of the atoms is reduced. We identified the one possible exception: collisions between ultra-cold atom in an atomic fountain could induce a phase shift that would ultimately limit the accuracy of the fountain clock. We brought the problem to the attention of B. Verhaar, who began a series of calculations on the effects of low energy collisions. jg p.4

•Jul 7, 1994•0 citations
We propose a novel atomic fountain clock which is compact and transportable. The clock is 60-cm in height. Its linewidth is expected to be 1.75 Hz, with accuracy 10 −14 , stability 10 −14 and signal–noise ratio 10 3 . A hollow beam is applied as atomic wave-guide, which can increase the stability and the signal–noise ratio by one order. The hollow beam will not affect the accuracy of the clock, if the frequency shift due to the hollow beam is amended and the fluctuation of the hollow beam intensity is controlled to be smaller than 0.6%.
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 $~\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$.
We present a novel cesium beam physics package, which uses optical techniques to combine the high signal-to-noise ratios achievable by optical pumping and beam brightening with the narrow linewidth allowed by a slow atomic beam. Our design can be realized in a compact, robust package suitable for space applications. Our approach differs from other laser-cooled clocks in that we use a cold-atom source that provides a continuous beam. We estimate the shot noise-limited stability that can be achieved with this physics package to be two to three orders of magnitude better than that of current compact cesium beam atomic clocks.
Herein, we report the scheme of an optically pumped atomic clock based on a cold cesium atomic beam source. We propose the laser system and physical mechanism of this atomic clock, wherein the atomic beam travels in an upper parabolic trajectory, thereby eliminating the light shift effect. In the experiments, when the length of the free evolution region was 167 mm, the line width of the Ramsey fringe was 37 Hz. When the expected signal-to-noise ratio of the Ramsey fringe that can be achieved is 36,000, the expected short-term frequency stability is about 3.6 × 10–14/√τ, which is significantly higher than that of a conventional optically pumped cesium clock of similar volume.
Cold atom fountain clocks provide exceptional long term stability as they increase interrogation time at the expense of a larger size. We present a compact cold atom fountain using a grating magneto-optical trap to laser cool and launch the atoms in a simplified optical setup. The fountain is evaluated using coherent population trapping and demonstrates improved single-shot stability from the launch. Ramsey times up to 100 ms were measured with a corresponding fringe linewidth of 5 Hz. This technique could improve both short- and long-term stabilities of cold atom clocks while remaining compact for portable applications.
Abstract : The grant as in support of a research program to explore the possibility of a microwave atomic clock based on an atomic fountain of cesium atoms. During the two year granting period, the following work was done: A large magneto-optic trap in a vapor cell was constructed where over 4x 10(exp 10) atoms were trapped at a density on the order of 10(exp 11) atoms/cu cm. This work increased the number of atoms optically trapped by a factor of -400 compared to previous work. The optical density of the atoms in this trap has a peak attenuation of e(-175). In an invited review article we examined the prospects for slow atom frequency standards. Most of the systematic effects that limit the current accuracy today's frequency standards decrease as the velocity of the atoms is reduced. We identified the one possible exception: collisions between ultra-cold atom in an atomic fountain could induce a phase shift that would ultimately limit the accuracy of the fountain clock. We brought the problem to the attention of B. Verhaar, who began a series of calculations on the effects of low energy collisions. jg p.4