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1994 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM

William D. Phillips, W. M. Goldingi, Anders Kastberg, S. L. Rolston, R. J. C. Spreeuw

Published Jan 1, 1994• 0 citations
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Abstract

Atomic fountain frequency standards require extremely low temperature atomic samples. Laser cooling of neutral atoms has achieved temperatures of a few microkelvin, but even such low temperatures still present difficulties. Here we report laser cooling in an optical lattice, along with cooling by adiabatic expansion, that gives a temperature of 0.7 pK. Such low temperatures should improve the performance of atomic fountain frequency standards.

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Atomic fountain frequency standards require extremely low temperature atomic samples. Laser cooling of neutral atoms has achieved temperatures of a few microkelvin, but even such low temperatures still present difficulties. Here we report laser cooling in an optical lattice, along with cooling by adiabatic expansion, that gives a temperature of 0.7 pK. Such low temperatures should improve the performance of atomic fountain frequency standards.
Experimental techniques of laser cooling and trapping, along with other cooling techniques, have produced gaseous samples of atoms so cold that they are, for many practical purposes, in the quantum ground state of their centre-of-mass motion. Such low velocities have virtually eliminated effects such as Doppler shifts, relativistic time dilation and observation-time broadening that previously limited the performance of atomic frequency standards. Today, the best laser-cooled, caesium atomic fountain, microwave frequency standards realize the International System of Units (SI) definition of the second to a relative accuracy of ≈3×10(-16). Optical frequency standards, which do not realize the SI second, have even better performance: cold neutral atoms trapped in optical lattices now yield relative systematic uncertainties of ≈1×10(-16), whereas cold-trapped ions have systematic uncertainties of 9×10(-18). We will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.
We report an experiment on the adiabatic cooling of 87 Rb atoms in an atomic fountain to a temperature as low as 1.5 μK, which is roughly twice the recoil temperature. The atomic fountain has the (1,1,1) optical geometry for cooling and launching of cold atoms. The atoms are first cooled in an optical molasses of 6 beams to 3.4 μK by polarization gradient geometry and then are adiabatically cooled by decreasing the intensity of laser from 1.8I s per beam to zero in 1 ms during the launching of cold atoms. We also study the dependences of atomic temperature on different laser parameters. The method we used is useful in any cold atom physics experiment.
We are developing a simple and robust laser and optics system suitable for a transportable laser-cooled rubidium (Rb) atomic fountain frequency standard. This system uses DFB (Distributed Feedback) diode lasers and a frequency offset-locking scheme to generate the optical frequencies needed for laser-cooling, launching, post-cooling, and detection of Rb atoms. Here, we describe the early results from this project and the anticipated performance of a compact, transportable Rb fountain frequency standard using this laser system. This system will also be useful for nontransportable laser-cooled atomic frequency standards.
We report progress toward a Cs-fountain frequency standard operating in the nanokelvin temperature range. We have implemented the Raman sideband cooling in a far-off-resonance optical lattice and observed subrecoil temperatures of 200 nK. We have studied the structure of the lattice and optimized the capture of atoms released from the magneto-optical trap into the lattice. In addition, the atoms, accumulated by the cooling scheme in the stretched magnetic state m/sub F/=3, have been transferred to the magnetically insensitive state m/sub F/=0 by using the adiabatic-passage technique.
We have measured the temperature of cesium atoms released from optical molasses. For a wide range of laser intensity and detuning from resonance, the temperature depends only on the intensity to detuning ratio. The lowest temperature achieved is 2.5±0.6 μK, which corresponds to an rms velocity of 12.5 mm/s or 3.6 times the single‐photon recoil velocity. This is, to our knowledge, the coldest kinetic temperature ever measured for three dimensional (3D) cooling. We then discuss the possibility of using such a cold sample of atoms for realizing a high performance atomic clock in a fountain geometry. In particular, a method for launching the atoms upwards, while maintaining the very cold temperature, is demonstrated.