  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.
