We examine several proposed schemes by Franson et al. for quantum logic gat
es based on non-local exchange interactions between two photons in a medium
. In these schemes the presence of a single photon in a given mode is suppo
sed to induce a large phase shift on another photon propagating in the same
medium. We conclude that the schemes proposed so Far are not able to produ
ce the required conditional phase shift, even though the proposals contain
many stimulating and intriguing ideas.