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NIM5, the first Chinese cesium fountain clock to steer world time

NIM5, a cesium fountain clock of the National Institute of Metrology (NIM), was approved to be “included in the report and calculation of the International Atomic Time (TAI)” by the International Bureau of Weights and Measures (BIPM), the organization established to maintain the International System of Units (SI), on August 7. On the previous day, Dr. Steve Jefferts, lead designer of the US cesium fountain clock NIST-F2, and now the Chair of the Working Group on Primary Frequency Standards, Consultative Committee of Time and Frequency (CCTF), declared the approval for NIM5 contributing to TAI as a Primary Frequency Standard (PFS) after appraisal of the clock’s uncertainty evaluation report. Since then, China has become one of the few countries that have pragmatic impact on the world standard of time. 


According to the BIPM, the realization of a practical time scale for world-wide use has two essential elements: a realization of accurate time unit and a continuous temporal reference. International Atomic Time (TAI) serves as the reference. It is calculated at BIPM using data from about 400 atomic clocks in more than 70 national time-keeping laboratories around the world. The long-term stability of TAI is assured by a judicious way of weighting the participating clocks; while its scale unit is calculated and assured using data from national laboratories which maintain “the best primary cesium standards”. NIM5, according to the notification of BIPM, is going to participate as one of “the best primary cesium standards”, whose data will be included in calculation of TAI scale unit and be published in the BIPM monthly Circular T this August and September. China will then become the 8th country to steer world time in succession to France, the US, Germany, Italy, Japan, the UK and Russia.

A precision and reliable time and frequency system is of strategic importance to a modern economy. Many technologies needed by key sectors rely on the exquisite precision of the system. For instance, the synchronization required by the satellite navigation system is 1 billionth of a second, which can only be provided by the atomic clock based frequency system. Besides, the modern telecommunications, the electric power grid, the computer network system, the traffic system and the financial dealing system…all rely on precision timing and synchronization that is only possible with atomic clocks. Historically, improved timekeeping has consistently led to technology improvements and innovation.

The establishment of NIM5 was started in 2005 by NIM’s research group, under the framework of the National Key Technologies Research Program funded by Chinese Ministry of Science and Technology. “It is a second generation cesium fountain clock,” said Dr. Fang Fang, the project leader, “the present uncertainty of the clock is better than 1.5×10-15, meaning that the clock would neither gain nor lose 1 second in about 20 million years.” Together with the atomic time scale primary standard UTC(NIM),  NIM5 also serves as the National Primary Standard for Time and Frequency. It can be used to steer UTC(NIM) independently in case of any communication failure with the BIPM.

(Chen Hanghang, chenhh@nim.ac.cn)