  We consider a nonlinear sign gate implemented using a sequence of two beam
splitters, and consider the use of further sequences of beam splitters to
implement feed-forward so as to correct an error resulting from the first beam
splitter. We obtain similar results to Scheel et al. [Scheel et al., Phys. Rev.
A 73, 034301 (2006)], in that we also find that our feed-forward procedure is
only able to produce a very minor improvement in the success probability of the
original gate.
