An atomic fountain, in which atomic resonance frequency is accurately measured by using the Ramsey resonance between microwave and launched cold atoms, has been employed as a primary frequency standard with uncertainty of the order of 10−16 [1]. In order to improve the uncertainty, a collisional shift due to spin exchange between atoms is one of the most important issues. A diluter atomic cloud makes the collisional shift smaller but frequency instability worse owing to decrease of a signal-to-noise ratio. In order to defeat the trade-off, a continuous atomic beam and juggling of atomic clouds have been tried to be used for the fountains [2, 3]. However, in both cases, apparatuses and systems are more complicated than those in a common fountain where one atomic cloud is used in one sequence. Especially, since cold atoms must be generated even when other cold atoms are in an interrogation region for the Ramsey resonance, it is necessary that a light shift induced by stray light from cooling laser beams is sufficiently suppressed.

© 2009 IEEE

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