In order to develop an "ultimately accurate" frequency standard, a high resolution Ramsey resonance spectrometer has been developed by employing the technique of launching of laser- cooled Cs atoms (=atomic fountain), and the technical survey is being performed. The atomic fountain method has following advantages over the conventional thermal beam type. A) Ramsey fringe of higher resolutioninearly 100 timesjcan be detected. B) Influece of the 2nd order Doppler effect is greatly reduced. C) Microwave phase shift is eliminated.

Figure 1: Experimental apparatus for the atomic fountain Cs frequency standard.

As Figure 1 shows, magneto-optically trapped Cs atoms are cooled to a few micro K by optical molasses,and launched by moving molasses technique. In the present set up where the microwave cavity is located 52 cm above the trap point, time interval between the successive (upward and downward) interactions with the microwave field is assumed about 0.5s when the atoms of 3.9 m/s are launched. Hence,the spectral width of 1 Hz is expected. Figure 2 shows an example of Ramsey resonance signal obtained with the present apparatus, and agrees well with the expectation.

Figure 2: Observed Ramsey resonance signal.

Since the basic technology including sequential control of laser frequency/power as well as detection of Ramsey resonance signal is achieved, the next step is to improve S/N and arrange the frequency control sysytem, by which the necessary data for the development of a practical standard will be obtained.

Figure 3: Multipulse Cs launching of 100 ms interval.

With an attempt to improve the present atomic fountain method, basic study on multipulse launching mode operation has been carried out.Collisional frequncy shift can be reduced, and stability is also expected to be improved. In the present optical geometry, this method is applicable because Cs loading is able to start immediately after launching.Figure 3 shows the case of every 100 ms launching pulse. To prevent the stray light and the fluorescence light from entering the microwave interaction region will be a future task.