The effect of component variations on the gate fidelity in linear   optical networks
  We investigate the effect of variations in beam splitter transmissions and path length differences in the nonlinear sign gate that is used for linear optical quantum computing. We identify two implementations of the gate, and show that the sensitivity to variations in their components differs significantly between them. Therefore, circuits that require a precision implementation may benefit from additional circuit analysis of component variations to identify the most practical implementation. We suggest possible routes to efficient circuit analysis in terms of quantum parameter estimation. 
Jonathan Crickmore, Jonathan Frazer, Scott Shaw, and Pieter Kok
quant-ph