  The performance of nondeterministic nonlinear gates in linear optics relies
on the photon counting scheme being employed and the efficiencies of the
detectors in such schemes. We assess the performance of the nonlinear sign
gate, which is a critical component of linear optical quantum computing, for
two standard photon counting methods: the double detector array and the visible
light photon counter. Our analysis shows that the double detector array is
insufficient to provide the photon counting capability for effective
nondeterministic nonlinear transformations, and we determine the gate fidelity
for both photon counting methods as a function of detector efficiencies.
