title: Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion.
abstract: The frequency of the 5s 2S(1/2)-4d 2D(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.
authors.affiliation: National Physical Laboratory (NPL), Teddington, Middlesex TW11 0LW, UK. helen.margolis@npl.co.uk
authors.forename: H S
authors.initials: HS
authors.lastname: Margolis
authors.forename: G P
authors.initials: GP
authors.lastname: Barwood
authors.forename: G
authors.initials: G
authors.lastname: Huang
authors.forename: H A
authors.initials: HA
authors.lastname: Klein
authors.forename: S N
authors.initials: SN
authors.lastname: Lea
authors.forename: K
authors.initials: K
authors.lastname: Szymaniec
authors.forename: P
authors.initials: P
authors.lastname: Gill