Secondary Representations of the Second
Over the past few years, optical frequency standards based on cold atoms and cold trapped ions have benefited significantly from the advent of the self-referenced femtosecond comb, with the result that the most accurate optical frequency measurements are limited by the caesium primary standard itself. Optical frequency standards are also likely to demonstrate reproducibilities below that of the caesium standard, raising the prospect of an optical redefinition of the second in the future.
Optical standards are unable to demonstrate lower absolute uncertainty until such time as a redefinition might occur, even though they may be shown to be more reproducible, for example by direct comparisons between two optical standards. This situation has led to the establishment of secondary representations of the second. These secondary representations, whether optical or microwave standards, may be used to realise the SI second, albeit with uncertainty no better than that of the caesium primary standard.
There are currently five different standards which may be used as secondary representations of the second:
- The 6.8 GHz rubidium microwave fountain standard
- The 5d106s 2S1/2 (F=0) - 5d96s2 2D5/2 (F=2) quadrupole transition in 199Hg+ at 282 nm (1065 THz)
- The 5s 2S1/2 - 4d 2D5/2 transition in 88Sr+ at 674 nm (445 THz)
- The 6s 2S1/2 (F=0) - 5d 2D3/2 (F=2) transition in 171Yb+ at 436 nm (688 THz)
- The 5s2 1S0 - 5s5p 3P0 transition in 87Sr at 698 nm (429 THz)
It is considered that the establishment of these secondary representations will help with the detailed evaluation of reproducibility at the highest level, and will significantly aid the process of comparing different standards in the preparation of a future redefinition of the second.