The source-to-field channel and retained-source back-action

For superposed source inputs, the joint evolution creates source–field correlations. Removing the retained source removes the off-diagonal source blocks from the radiation channel; it is not a claim that the joint unitary itself destroys coherence.

-channel boundary. The retained-source channel is not the complete complement of a selected packet channel. That complement would also contain unselected scalar modes, the future-horizon output, and a specified purification of the mixed background. No complementary distance is asserted from ${\mathcal{B}}_F$ alone.