title: Improved measurement of the hydrogen 1S-2S transition frequency.
abstract: We have measured the 1S-2S transition frequency in atomic hydrogen via two-photon spectroscopy on a 5.8 K atomic beam. We obtain f(1S-2S) = 2,466,061,413,187,035 (10)  Hz for the hyperfine centroid, in agreement with, but 3.3 times better than the previous result [M. Fischer et al., Phys. Rev. Lett. 92, 230802 (2004)]. The improvement to a fractional frequency uncertainty of 4.2 × 10(-15) arises mainly from an improved stability of the spectroscopy laser, and a better determination of the main systematic uncertainties, namely, the second order Doppler and ac and dc Stark shifts. The probe laser frequency was phase coherently linked to the mobile cesium fountain clock FOM via a frequency comb.
authors.affiliation: Max-Planck-Institut für Quantenoptik, Garching, Germany.
authors.forename: Christian G
authors.initials: CG
authors.lastname: Parthey
authors.forename: Arthur
authors.initials: A
authors.lastname: Matveev
authors.forename: Janis
authors.initials: J
authors.lastname: Alnis
authors.forename: Birgitta
authors.initials: B
authors.lastname: Bernhardt
authors.forename: Axel
authors.initials: A
authors.lastname: Beyer
authors.forename: Ronald
authors.initials: R
authors.lastname: Holzwarth
authors.forename: Aliaksei
authors.initials: A
authors.lastname: Maistrou
authors.forename: Randolf
authors.initials: R
authors.lastname: Pohl
authors.forename: Katharina
authors.initials: K
authors.lastname: Predehl
authors.forename: Thomas
authors.initials: T
authors.lastname: Udem
authors.forename: Tobias
authors.initials: T
authors.lastname: Wilken
authors.forename: Nikolai
authors.initials: N
authors.lastname: Kolachevsky
authors.forename: Michel
authors.initials: M
authors.lastname: Abgrall
authors.forename: Daniele
authors.initials: D
authors.lastname: Rovera
authors.forename: Christophe
authors.initials: C
authors.lastname: Salomon
authors.forename: Philippe
authors.initials: P
authors.lastname: Laurent
authors.forename: Theodor W
authors.initials: TW
authors.lastname: Hänsch