title: Single-atom optical clock with high accuracy.
abstract: For the past 50 years, atomic standards based on the frequency of the cesium ground-state hyperfine transition have been the most accurate time pieces in the world. We now report a comparison between the cesium fountain standard NIST-F1, which has been evaluated with an inaccuracy of about 4 x 10(-16), and an optical frequency standard based on an ultraviolet transition in a single, laser-cooled mercury ion for which the fractional systematic frequency uncertainty was below 7.2 x 10(-17). The absolute frequency of the transition was measured versus cesium to be 1,064,721,609,899,144.94 (97) Hz, with a statistically limited total fractional uncertainty of 9.1 x 10(-16) the most accurate absolute measurement of an optical frequency to date.
authors.affiliation: Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA.
authors.forename: W H
authors.initials: WH
authors.lastname: Oskay
authors.forename: S A
authors.initials: SA
authors.lastname: Diddams
authors.forename: E A
authors.initials: EA
authors.lastname: Donley
authors.forename: T M
authors.initials: TM
authors.lastname: Fortier
authors.forename: T P
authors.initials: TP
authors.lastname: Heavner
authors.forename: L
authors.initials: L
authors.lastname: Hollberg
authors.forename: W M
authors.initials: WM
authors.lastname: Itano
authors.forename: S R
authors.initials: SR
authors.lastname: Jefferts
authors.forename: M J
authors.initials: MJ
authors.lastname: Delaney
authors.forename: K
authors.initials: K
authors.lastname: Kim
authors.forename: F
authors.initials: F
authors.lastname: Levi
authors.forename: T E
authors.initials: TE
authors.lastname: Parker
authors.forename: J C
authors.initials: JC
authors.lastname: Bergquist