Systematic study of the $^{87}$Sr clock transition in an optical lattice
  With ultracold $^{87}$Sr confined in a magic wavelength optical lattice, we present the most precise study (2.8 Hz statistical uncertainty) to-date of the $^1S_0$ - $^3P_0$ optical clock transition with a detailed analysis of systematic shifts (20 Hz uncertainty) in the absolute frequency measurement of 429 228 004 229 867 Hz. The high resolution permits an investigation of the optical lattice motional sideband structure. The local oscillator for this optical atomic clock is a stable diode laser with its Hz-level linewidth characterized across the optical spectrum using a femtosecond frequency comb. 
Andrew D. Ludlow, Martin M. Boyd, T. Zelevinsky, Seth M. Foreman,   Sebastian Blatt, Mark Notcutt, Tetsuya Ido, and Jun Ye