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Standards of time and frequency at the outset of the 21st century



Scott Diddams, James C. Bergquist, Steven R. Jefferts, Christopher W. Oates


In this paper we review state-of-the-art atomic time and frequency standards and discuss some of their uses in science and technology. After fifty years of development, microwave atomic clocks based on cesium have achieved fractional uncertainties below 1 part in 1015; a level unequaled in all of metrology. The past five years have seen accelerated development of optical atomic clocks, which hold significant potential for still improved timekeeping. Time and frequency standards with various levels of performance are ubiquitous in our modern society, finding applications in many technological fields as well as in the continued exploration of the frontiers of basic science.


atomic clock, microwave and optical frequency standards, time standards


Diddams, S. , Bergquist, J. , Jefferts, S. and Oates, C. (2004), Standards of time and frequency at the outset of the 21st century, Science, [online], (Accessed June 15, 2024)


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Created November 18, 2004, Updated October 12, 2021