1S–2S Transition-Frequency Calibration
Recent experiments on atomic hydrogen have shown that it is now possible to make measurements with a reproducibility far exceeding the accepted absolute precision of the available visible frequency standards. For example, the recent work of ALLEGRINI et al.  has shown that the overall systematic error of their measurements of the Rydberg states in hydrogen is about 4 parts in 1011. Their measurements have had to be referred to the visible iodine stabilised HeNe laser which has an internationally accepted absolute accuracy of only 1.6 parts in 1010. If any progress is to be made in making precision measurements of fundamental atomic constants and in tests of QED, new visible frequency standards and new ways of comparing optical frequencies will have to be developed.
KeywordsMethane Urea Lithium Iodine Borate
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