Wikipedia on Atomic Clock - https://en.wikipedia.org/wiki/Atomic_clock New optical clock breaks accuracy record - http://physicsworld.com/cws/article/news/2010/feb/12/new-optical-clock-breaks-accuracy-record Optical Clock Technology Tested in Space for the First Time - http://www.osa.org/en-us/about_osa/newsroom/news_releases/2016/optical_clock_technology_tested_in_space_for_the_f/ Wikipedia on Frequency comb - https://en.wikipedia.org/wiki/Frequency_comb Realization of a timescale with an accurate optical lattice clock - https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-3-6-563&id=340980 Time and Frequency from A to Z, H - https://www.nist.gov/time-and-frequency-services/h