  Unitarity and the optical theorem are used to derive the reduced density
matrices of Compton scattering in the presence of a witness particle. Two
photons are initially entangled wherein one photon participates in Compton
scattering while the other is a witness, i.e. does not interact with the
electron. Unitarity is shown to require that the entanglement entropy of the
witness photon does not change after its entangled partner undergoes
scattering. The final mutual information of the electronic and witness
particle's polarization is nonzero for low energy Compton scattering. This
indicates that the two particles become correlated in spite of no direct
interaction. Assuming an initial maximally entangled state, the change in
entanglement entropy of the scattered photon's polarization is calculated in
terms of Stokes parameters. A common ratio of areas occurs in the final reduced
density matrix elements, von Neumann entropies, Stokes parameter, and mutual
information. This common ratio consists of the Thomson scattering cross-section
and an accessible regularized scattering area.
