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

Dick effect and cavity pulling on HORACE compact cold atom clock

Nicolas Rossetto, François-Xavier Esnault, David Holleville, J. Delporte, M. Lours, N. Dimarcq

Published May 1, 2011• 2 citations
Publisher

Abstract

HORACE is a compact cold caesium (Cs) atom clock using isotropic light cooling technique directly inside the spherical microwave cavity. It has been designed for onboard and space applications. Ultimate stability as low as 2.2 10−13τ−1/2had been demonstrated, limited by atomic shot noise and quantum projection noise. Influence of Dick effect has been studied and a compact and simple frequency synthesizer using off the shelf quartz has been realized and characterized. As atoms remain in the cavity during Ramsey sequence, long term stability evaluation focused on cavity pulling effect. This proceeding presents both the microwave synthesizer development and cavity pulling study using a tricky technique to tune the microwave cavity.

DOI
10.1109/fcs.2011.5977902

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HORACE is a compact cold caesium (Cs) atom clock using isotropic light cooling technique directly inside the spherical microwave cavity. It has been designed for onboard and space applications. Ultimate stability as low as 2.2 10−13τ−1/2had been demonstrated, limited by atomic shot noise and quantum projection noise. Influence of Dick effect has been studied and a compact and simple frequency synthesizer using off the shelf quartz has been realized and characterized. As atoms remain in the cavity during Ramsey sequence, long term stability evaluation focused on cavity pulling effect. This proceeding presents both the microwave synthesizer development and cavity pulling study using a tricky technique to tune the microwave cavity.
The HORACE device is a compact cold atom clock where about 10/sup 8/ cesium atoms are laser cooled at a few /spl mu/K, then interrogated and detected directly in a 20 cm/sup 3/ spherical microwave cavity. The optimization of the short term stability with the cooling and interrogation durations is presented, leading to an estimation of the Allan deviation of /spl sigma//sub y/(/spl tau/) -1 10/sup -13/ /spl tau/ /sup - 1/2 /on earth and /spl sigma//sub y/(/spl tau/) = 7 10/sup -14/ /spl tau//sup -frac12;/ in space. The quantum projection noise, the Dick effect and the detection laser noises are taken into account. The calculations are based on a preliminary model for the recapture of cold atoms, in reasonable agreement with measurements.
In the compact cold atoms clock HORACE caesium atoms are successively laser cooled, submitted to a Ramsey microwave interrogation scheme and then detected by linear absorption directly inside the microwave cavity. The whole physics package and optical bench have been entirely redesigned to optimize such an in-situ detection and to perform a normalization scheme. Main efforts have been concentrated on technical issues such as magnetic shielding, microwave cavity polishing and tuning, use of fast mechanical switches to realize the optical sequence.
We present a compact cold-atom clock configuration where isotropic laser cooling, microwave interrogation, and clock signal detection are successively performed inside a spherical microwave cavity. For ground operation, a typical Ramsey fringe width of 20 Hz has been demonstrated, limited by the atom cloud's free fall in the cavity. The isotropic cooling light's disordered properties provide a large and stable number of cold atoms, leading to a high signal-to-noise ratio limited by atomic shot noise. A relative frequency stability of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved, averaged down to $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ after $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ s of integration. Development of such a high-performance compact clock is of major relevance for on-board applications, such as satellite-positioning systems. As a cesium clock, it opens the door to a new generation of compact primary standards and timekeeping devices.