The National Metrology Institute of Japan (NMIJ) at National Institute of Advanced Industrial Science and Technology (AIST) has successfully developed an “atomic fountain primary frequency standard (JF-1)”, which is one of the most accurate atomic clocks in the world. In the conventional method, heated cesium atoms are used to observe the microwave transition. In contrast, the atomic fountain method uses ultra-cold atoms whose motion is suppressed using a laser cooling technique. The atomic fountain has improved the measurement resolution significantly by increasing the interaction time between the atom and the microwave and the accuracy by reducing many shifts including the relativistic effect. Accuracy has been improved by more than one order of magnitude compared with that of the conventional method. It has been confirmed by an experiment that relative difference from the definition of second is within 1.4 × 10^-15, and the accuracy is less than a second per twenty million years. In the future, by using this atomic clock to regularly monitor the progress of the Coordinated Universal Time (UTC), the international standard time, AIST will contribute to the international society in the area of science and technology. In addition, this clock can be used in a broad range of areas including basic researches, as a source of accurate time or frequency.

* AIST started the research of an atomic clock first for the first time in Japan and has developed four models of the primary frequency standard with an aim to monitor the progress of the UTC. This time, AIST has succeeded to develop the first “atomic fountain primary frequency standard in the Japan (JF-1)”. The detailed studies have confirmed that relative difference from the definition of second is 1.4 × 10^-15 or less (a second or less per twenty million years), which means that the accuracy is improved by approximately twenty times from the conventional method.

The conventional method uses thermal atoms generated by an oven and controls the microwave frequency so that it coincides with the specific frequency of the cesium atom (i.e. the definition of second ). On the other hand, the newly adopted atomic fountain method uses atoms which are cooled down to around the absolute zero degree using the radiation pressure of a laser and has the following merits.

1. Interaction time of the cesium atom with the microwaves increases.
   → Improvement of the measurement resolution. (Lower figures than ever can be measured.)
    
2. Relativistic effect is almost eliminated.
   → Improvement of the accuracy. (“The definition of second” can be realized more accurately.)

To realize these merits, it is necessary to develop the techniques to capture, cool, and launch atoms and together with a ultra-high vacuum technology.

* By operating the “atomic fountain primary frequency standard (JF-1)” improvement of the accuracy of UTC is expected. In the future, AIST is going to not only to contribute to the international society in the field of science and technology by monitoring the UTC regularly, but also to expand its application as a source of accurate time and frequency including the satellite positioning technologies.