The ⁸⁷Sr optical frequency standard at PTB

With ⁸⁷Sr atoms confined in a one-dimensional optical lattice, the frequency of the optical clock transition 5s^{2 1}S₀–5s5p ³P₀ has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help of a femtosecond-frequency comb against one of Physikalisch-Technische Bundesanstalt (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 the secondary representation of the second with the currently smallest uncertainty.