Optical state selection process with optical pumping in a cesium atomic fountain clock*

We propose and realize a new optical state selection method on a cesium atomic fountain clock by applying a two-laser 3-3 ' optical pumping configuration to spin polarize atoms. The atoms are prepared in |F = 3, m(F) = 0 clock state with optical pumping directly after being launched up, followed by a pushing beam to push away the atoms remaining in the |F = 4 state. With a state selection efficiency exceeding 92%, this optical method can substitute the traditional microwave state selection, and helps to develop a more compact physical package. A Ramsey fringe has been achieved with this optical state selection method, and a contrast of 90% is obtained with a full width half maximum of 0.92 Hz. The short-term frequency stability of 6.8 x 10(-14) (tau/s)(-1/2) is acquired. In addition, the number of detected atoms is increased by a factor of 1.7 with the optical state selection.