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# Optical-to-microwave frequency comparison with fractional uncertainty of 10-15

Published

### Author(s)

Jason Stalnaker, Scott A. Diddams, Tara M. Fortier, K Kim, Leo W. Hollberg, James C. Bergquist, Wayne M. Itano, Marie Delaney, Luca Lorini, Windell Oskay, Thomas P. Heavner, Steven R. Jefferts, Filippo Levi, Thomas E. Parker, Jon H. Shirley

### Abstract

We report the technical aspects of the optical-to-microwave comparison for our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. Over the course of six years, these measurements have resulted in a determination of the mercury single-ion frequency with a fractional uncertainty less than $7 \times 10^{-16}$ making it the most accurately measured optical frequency to date. In this paper, we focus on the details of the comparison techniques used in the experiment and discuss the uncertainties associated with the optical-to-microwave synthesis based on a femtosecond laser frequency comb. We also present our most recent results in the context of the previous measurements of the mercury single-ion frequency and arrive at a final determination of the mercury single ion optical frequency: $f({\rm Hg}^+) = 1 \, 064 \, 721 \, 609 \, 899 \, 145.30(69) \: {\rm Hz}$.
Citation
Applied Physics B

### Keywords

cesium frequency standard, femtosecond frequency comb, mercury ion clock, optical clock, optical frequency measurement, optical-to-microwave conversion

## Citation

Stalnaker, J. , Diddams, S. , Fortier, T. , Kim, K. , Hollberg, L. , Bergquist, J. , Itano, W. , Delaney, M. , Lorini, L. , Oskay, W. , Heavner, T. , Jefferts, S. , Levi, F. , Parker, T. and Shirley, J. (2007), Optical-to-microwave frequency comparison with fractional uncertainty of 10<sup>-15</sup>, Applied Physics B, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=50555 (Accessed May 11, 2021)
Created October 1, 2007, Updated November 29, 2016