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

Limitations to the short term frequency stability in a compact cold atom clock

S. Trémine, Stéphane Guérandel, David Holleville, J. Delporte, N. Dimarcq, Andr  Clairon

Published Jan 18, 2006• 15 citations
Publisher

Abstract

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.

DOI
10.1109/freq.2005.1573911

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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.
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.
Our aim is to use cold atoms not for reducing instability or inaccuracy in the range of 10/sup -16/ or better, but to use them for reducing the size of atomic clocks. The key idea of HORACE is to fold in a microwave cavity every zone of the atomic resonator of a standard clock: zones of cooling and atomic preparation, zone of interrogation and zone of detection. Then the cell of HORACE has to be a microwave cavity as well as an optical cavity. Thus cooling of cesium atoms can only be obtained by isotropic cooling light generated inside it. In this paper, we first expose the main features of HORACE and two different configurations. Metrological aspects as well as the way the atoms would be efficiently cooled are then developed. A preliminary validation of HORACE is also demonstrated.
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.
The combination of coherent population trapping (CPT) and laser cooled atoms is a promising platform for realizing the next generation of compact atomic frequency references. Towards this goal, we have developed an apparatus based on the grating magneto-optical trap (GMOT) and the high-contrast lin ⊥ lin CPT scheme in order to explore the performance that can be achieved. One important trade-off for cold-atom systems arises from the need to simultaneously maximize the number of cold atoms available for interrogation and the repetition rate of the system. This compromise can be mitigated by recapturing cold atoms from cycle to cycle. Here, we report a quantitative characterization of the cold atom number in the recapture regime for our system, which will enable us to optimize this trade-off. We also report recent measurements of the short-term frequency stability with a short-term Allan deviation of 3 × 10-11/τ up to an averaging time of τ = 10 s.