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

3D speckle cooling in a microwave clock

Paul-Éric Pottie, S. Trémine, Stéphane Guérandel, N. Dimarcq, Andr  Clairon

Published Jul 8, 2004• 4 citations
Publisher

Abstract

Sub-Doppler laser cooling is studied in an optically polished microwave cavity used as a reflecting cell. Light potential wells due to the speckle field in the cell make Sisyphus cooling mechanisms possible. For a 1 s capture time, about 10/sup 8/ cesium atoms are cooled at temperatures in the microkelvin range from thermal vapor. These results are discussed and compared with the basic theoretical model involving two laser beams lin /spl perp/ lin polarized. For a microwave clock using such cold atoms, the pulsed operation is precised and the main frequency shifts are evaluated. The short term relative frequency stability is expected to be lower than 10/sup -12//spl tau//sup - 1/2 /.

DOI
10.1109/freq.2003.1275078

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Sub-Doppler laser cooling is studied in an optically polished microwave cavity used as a reflecting cell. Light potential wells due to the speckle field in the cell make Sisyphus cooling mechanisms possible. For a 1 s capture time, about 10/sup 8/ cesium atoms are cooled at temperatures in the microkelvin range from thermal vapor. These results are discussed and compared with the basic theoretical model involving two laser beams lin /spl perp/ lin polarized. For a microwave clock using such cold atoms, the pulsed operation is precised and the main frequency shifts are evaluated. The short term relative frequency stability is expected to be lower than 10/sup -12//spl tau//sup - 1/2 /.
In sub-Doppler laser cooling of atoms, the lowest temperatures are known to occur at low (but not too low) excitation, and the temperature increases roughly linearly with laser intensity. However, under conditions for which the lowest temperatures are obtained, the small velocity capture range and low optical pumping rate limit the number of atoms that can be collected into the cold sample. In this study, we present measurements for laser cooling of Rb and metastable He atoms for two counterpropagating laser beams with orthogonal linear polarization (lin\ensuremath{\perp}lin) over a wide range of saturation parameters and laser detunings, together with results of semiclassical (Fokker-Planck) and quantum density matrix calculations. We find that at higher laser intensity, a larger number of atoms can be collected into a final velocity distribution that is significantly narrower than that given by a linear extrapolation vs the square root of laser intensity. Two cooling mechanisms are at work: the sub-Doppler Sisyphus mechanism and also Doppler cooling. Under certain conditions that we discuss, Doppler cooling aids Sisyphus cooling by collecting high-velocity atoms beyond the Sisyphus capture range. He∗{\mathrm{He}}^{*}He∗ presents a rather unusual situation in that the minimum average kinetic energy (Eav){(E}_{\mathrm{av}})(Eav​) for purely Sisyphus cooling is comparable to the minimum value of Eav{E}_{\mathrm{av}}Eav​ with Doppler cooling alone.
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 report on measurements of frequency shifts in a microwave cesium vapor cell atomic clock based on coherent population trapping (CPT). The dependence of the clock frequency on numerous experimental parameters, such as the laser power, the laser frequency, the microwave power, the cell temperature, the static magnetic field, but also the temperature of some key components, or the translation and rotation of critical wave plates and optical elements, is investigated. The stability budget of the clock frequency at one day is reported and discussed. This study constitutes a solid database for the future demonstration of a CPT-based cell clock with enhanced mid- and long-term stability performances.
We present a scheme for a compact rubidium cold-atom clock which performs diffuse light cooling, microwave interrogation, and detection of the clock signal in a cylindrical microwave cavity. The diffuse light is produced by laser light reflection at the inner surface of the microwave cavity. The pattern of the injected laser beams is specially designed to accumulate the majority of the cold atoms in the center of the microwave cavity. Microwave interrogation of the cold atoms in the cavity leads to Ramsey fringes, which have a linewidth of 24.524.524.5 Hz with a contrast of 95.695.6%95.6 when the free evolution time is 20 ms. Recently, a frequency stability of $7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.28em}{0ex}}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved. The scheme of this physical package can largely reduce the cold-atom clock complexity and increase clock performance.
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.