  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.
