Reliability of optical gate array for C-Sign operator

We study optical gate array of KLM model of quantum computer for C-Sign operator, which contains linear elements and two nonlinear phase shifts. Linear elements and photon counting are taken to be ideal, whereas nonlinear phase shifts are subject of realistic and unavoidable factors of decoherence: the immediate photon leak from cavities and mediated leakage with the atom, which flies out of the cavity and plays the role of ancilla. We use JC model with Lindblad master equation to estimate the dependence of the validity of C-Sign operator from the rate of photon leak and estimate the limitation, which this validity imposes to quantum computations.