  An ultrastable optical clock based on neutral atoms trapped in an optical
lattice is proposed. Complete control over the light shift is achieved by
employing the $5s^2 {}^1S_0 \to 5s5p {}^3P_0$ transition of ${}^{87}{\rm Sr}$
atoms as a "clock transition". Calculations of ac multipole polarizabilities
and dipole hyperpolarizabilities for the clock transition indicate that the
contribution of the higher-order light shifts can be reduced to less than 1
mHz, allowing for a projected accuracy of better than $ 10^{-17}$.

Atomic Physics