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Optical clocks and frequency standards are now challenging the best microwave fountain clocks. This paper reviews the current state of the art in optical clocks. The comparative performance of various clock species is described, and consideration is made of the implications for a future redefinition of the second
Forbidden transitions in single laser-cooled trapped ions provide highly stable and accurate references for optical frequency standards. This paper describes recent progress on strontium and ytterbium ion optical frequency standards under development at NPL.
The emergence of microwave and optical frequency standards with accuracies close to those of caesium primary frequency standards has led to the concept of secondary representations of the second. The recently-formed Joint Working Group of the CCL / CCTF has now recommended as secondary representations to the CCTF / CIPM one microwave radiation in 87Rb and three optical radiations in 199Hg+, 88Sr+...
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