Abstract
In this paper we report the recent progresses on the time and frequency primary standards at the National Institute of Metrology (NIM) China: stabilized lasers for practical realization of the length unit (meter) of the International System of Units (SI); the cesium fountain clocks to realize the SI time unit (second) with an uncertainty of 2×10-15; the femto-second optical frequency combs, establishing the coherent connection between microwave frequency and optical frequency, to calibrate the optical wavelengths traceably to the microwave standard frequency with uncertainty better than 4×10-14; and the construction of a Sr lattice optical clock as a candidate of possible future redefinition of the second. We also describe: (1) fs optical combs are serve-locking systems and the specification to characterize their performance should be their tracking precision; (2) the estimated uncertainty limitations of sample cell - atom beam - cold atom fountain - trapped atom/ion frequency reference techniques.
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