  We previously suggested that photon exchange interactions could be used to
produce nonlinear effects at the two-photon level, and similar effects have
been experimentally observed by Resch et al. (quant-ph/0306198). Here we note
that photon exchange interactions are not useful for quantum information
processing because they require the presence of substantial photon loss. This
dependence on loss is somewhat analogous to the postselection required in the
linear optics approach to quantum computing suggested by Knill, Laflamme, and
Milburn [Nature 409, 46 (2001)].
