$^{87}$Sr lattice clock with inaccuracy below 10$^{-15}$
  Aided by ultra-high resolution spectroscopy, the overall systematic uncertainty of the $^{1}S_{0}$-$^{3}P_{0}$ clock resonance for lattice-confined $^{87}$Sr has been characterized to $9\times10^{-16}$. This uncertainty is at a level similar to the Cs-fountain primary standard, while the potential stability for the lattice clocks exceeds that of Cs. The absolute frequency of the clock transition has been measured to be 429,228,004,229,874.0(1.1) Hz, where the $2.5\times10^{-15}$ fractional uncertainty represents the most accurate measurement of a neutral-atom-based optical transition frequency to date. 
Martin M. Boyd, Andrew D. Ludlow, Sebastian Blatt, Seth M. Foreman,   Tetsuya Ido, Tanya Zelevinsky, and Jun Ye