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 6.8×10^-14 (τ/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.