  We evaluate the advantages of performing cross-phase modulation (XPM) on a
very-far-off-resonance atomic system. We consider a ladder system with a weak
(few-photon level) control coherent field imparting a conditional nonlinear
phase shift on a probe beam. We find that by coupling to an optical resonator
the optimal XPM is enhanced proportional to the finesse of the resonator by a
factor of $F/4\pi$. We present a semi-classical description of the system and
show that the phenomenon is optimal in the self-defined condition of
off-resonance-effective-cooperativity equal to one.
