  We investigate decoherence effects in the recently suggested quantum
computation scheme using weak nonlinearities, strong probe coherent fields,
detection and feedforward methods. It is shown that in the
weak-nonlinearity-based quantum gates, decoherence in nonlinear media it can be
made arbitrarily small simply by using arbitrarily strong probe fields, if
photon number resolving detection is used. On the contrary, we find that
homodyne detection with feedforward is not appropriate for this scheme because
in this case decoherence rapidly increases as the probe field gets larger.
