  Two-photon interference effects, such as the Hong-Ou-Mandel (HOM) effect, can
be used to characterize to what extent two photons are identical. Furthermore,
these interference effects underly linear optics quantum computation. We show
here how nonlinear optical effects, such as those mediated by atoms or quantum
dots in a cavity, degrade the interference. This implies that, on the one hand,
nonlinearities are to be avoided if one wishes to utilize the interference, but
on the other hand, one may be able to measure or detect nonlinearities by
observing the disappearance of the interference.
