Spectroscopy and frequency measurement of the $^{87}$Sr clock transition   by laser linewidth transfer using an optical frequency comb
  We perform spectroscopic observations of the 698-nm clock transition in $^{87}$Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition is prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum is observed for an 80-ms interrogating pulse. We determine that the absolute frequency of the 5s$^{2}$ $^{1}$S$_{0}$ - 5s5p $^{3}$P$_{0}$ clock transition in $^{87}$Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second. 
Daisuke Akamatsu, Hajime Inaba, Kazumoto Hosaka, Masami Yasuda,   Atsushi Onae, Tomonari Suzuyama, Masaki Amemiya, and Feng-...
physics.atom-ph