  We examine several proposed schemes by Franson et al. for quantum logic gates
based on non-local exchange interactions between two photons in a medium. In
these schemes the presence of a {\em single} photon in a given mode is supposed
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 produce the
required conditional phase shift, even though the proposals contain many
stimulating and intriguing ideas.
