  We discuss the feasibility of a quantum nondemolition measurement (QND) of
photon number based on cross phase modulation due to the Kerr effect in
Photonic Crystal Waveguides (PCWs). In particular, we derive the equations for
two modes propagating in PCWs and their coupling by a third order nonlinearity.
The reduced group velocity and small cross-sectional area of the PCW lead to an
enhancement of the interaction relative to bulk materials. We show that in
principle, such experiments may be feasible with current photonic technologies,
although they are limited by material properties. Our analysis of the
propagation equations is sufficiently general to be applicable to the study of
soliton formation, all-optical switching and can be extended to processes
involving other orders of the nonlinearity.
