  We study coherent excitation hopping in a spin chain realized using highly
excited individually addressable Rydberg atoms. The dynamics are fully
described in terms of an XY spin Hamiltonian with a long range resonant
dipole-dipole coupling that scales as the inverse third power of the lattice
spacing, $C_3/R^3$. The experimental data demonstrate the importance of next
neighbor interactions which are manifest as revivals in the excitation
dynamics. The results suggest that arrays of Rydberg atoms are ideally suited
to large scale, high-fidelity quantum simulation of spin dynamics.
