  The recently proposed idea to generate entanglement between photon states via
exchange interactions in an ensemble of atoms (J.D. Franson and T.B. Pitman,
Phys. Rev. A 60, 917 (1999) and J.D. Franson et al., (quant-ph/9912121)) is
discussed using an S-matix approach. It is shown that if the nonlinear response
of the atoms is negligible and no additional atom-atom interactions are
present, exchange interactions cannot produce entanglement between photons
states in a process that returns the atoms to their initial state. Entanglement
generation requires the presence of a nonlinear atomic response or atom-atom
interactions.
