Skip to main content
U.S. flag

An official website of the United States government

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.


Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Frequency Measurements of Al+ and Hg+ Optical Standards



Wayne M. Itano, James C. Bergquist, Till P. Rosenband, David J. Wineland, David Hume, Chin-Wen Chou, Steven R. Jefferts, Thomas P. Heavner, Thomas E. Parker, Scott A. Diddams, Tara M. Fortier


Frequency standards based on narrow optical transitions in 27Al+ and 199Hg+ ions have been developed at NIST. Both standards have absolute reproducibilities of a few parts in 1017. This is about an order of magnitude better than the fractional uncertainty of the SI second, which is based on the 133Cs hyperfine frequency. Use of femtosecond laser frequency combs makes it possible to compare the optical frequency standards to microwave frequency standards or to each other. The ratio of the Al+ and Hg+ frequencies can be measured more accurately than the reproducibility of the primary cesium frequency standards. Frequency measurements made over time can be used to set limits on the time variation of fundamental constants, such as the fine structure constant {alpaha} or the quark masses.
Proceedings Title
Proc. 2009 ICOLS Conference
Conference Dates
June 7-13, 2009
Conference Location


aluminum ion, mercury ion, optical frequency standards, time variation of fundamental constants, trapped ion frequency standards
Created June 8, 2009, Updated February 19, 2017