  One of the most significant challenges facing the development of linear
optics quantum computing (LOQC) is mode-mismatch, whereby photon
distinguishability is introduced within circuits, undermining quantum
interference effects. We examine the effects of mode-mismatch on the parity (or
fusion) gate, the fundamental building block in several recent LOQC schemes. We
derive simple error models for the effects of mode-mismatch on its operation,
and relate these error models to current fault tolerant threshold estimates.
