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NIST Primary Frequency Standards and the Realization of the SI Second



Michael A. Lombardi, Thomas P. Heavner, Steven R. Jefferts


As the base unit of time interval, the second holds a place of preeminence in the world of metrology. Time interval, and its reciprocal, frequency, can be measured with more resolution and less uncertainty than any other physical quantities. The precise realization of the second was made possible by the development of frequency standards based on the oscillation of atoms, an event that not only revolutionized the science of timekeeping, but that also forever changed many other areas of metrology. The definitions of other units in the international system (SI), including the meter, candela, ampere, and volt, now depend upon the definition of the second. In the approximate half century since atomic frequency standards were first introduced, the progress in time and frequency metrology has been rapid and relentless. Today, the National Institute of Standards and Technology (NIST), maintains the world's most accurate primary frequency standard, a laser-cooled cesium fountain known as NIST-F1. Since its completion in 1998, NIST-F1 has been continuously improved and is currently accurate to within 4 x 10-16. NIST-F2, a second-generation atomic fountain standard now being built at NIST, promises even better accuracy. This paper explores both the history and the technology of cesium primary frequency standards, beginning in the pre-atomic era and continuing through the present day.
Measure: The Journal of Measurement Science


cesium, frequency, primary standard, second, time interval


Lombardi, M. , Heavner, T. and Jefferts, S. (2007), NIST Primary Frequency Standards and the Realization of the SI Second, Measure: The Journal of Measurement Science, [online], (Accessed June 21, 2024)


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Created December 1, 2007, Updated February 17, 2017