The 87-Sr optical frequency standard at PTB
  With 87-Sr atoms confined in a one dimensional optical lattice, the frequency of the optical clock transition 5s^2 ^1S_0 - 5s5p ^3P_0 has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help of a fs-frequency comb against one of PTB's H-masers whose frequency was measured simultaneously by the PTB Cs fountain clock CSF1. The Sr optical frequency standard contributes with a fractional uncertainty of 1.5 10^-16 to the total uncertainty. The agreement of the measured transition frequency with previous measurements at other institutes supports the status of this transition as secondary representation of the second with the currently smallest uncertainty. 
St. Falke, H. Schnatz, J.S.R. Vellore Winfred, Th. Middelmann, St.   Vogt, S. Weyers, B. Lipphardt, G. Grosche, F. Riehle, U...
physics.atom-ph