  We study the optically-induced coupling between spins mediated by polaritons
in a planar micro-cavity. In the strong coupling regime, the vacuum Rabi
splitting introduces anisotropies in the spin coupling. Moreover, due to their
photon-like mass, polaritons provide an extremely long spin coupling range.
This suggests the realization of two-qubit all-optical quantum operations
within tens of picoseconds with spins localized as far as hundreds of
nanometers apart.
