  We present a systematic comparison of different methods of fidelity
estimation of a linear optical quantum controlled-Z gate implemented by
two-photon interference on a partially polarizing beam splitter. We have
utilized a linear fidelity estimator based on the Monte Carlo sampling
technique as well as a non-linear estimator based on maximum likelihood
reconstruction of a full quantum process matrix. In addition, we have also
evaluated lower bound on quantum gate fidelity determined by average quantum
state fidelities for two mutually unbiased bases. In order to probe various
regimes of operation of the gate we have introduced a tunable delay line
between the two photons. This allowed us to move from high-fidelity operation
to a regime where the photons become distinguishable and the success
probability of the scheme significantly depends on input state. We discuss in
detail possible systematic effects that could influence the gate fidelity
estimation.
