This paper is in response to a critique of the author’s earlier papers on the matter of a non-local communication system by Ghirardi. The setup has merit for not apparently falling for the usual pitfalls of putative communication schemes, as espoused by the No-communication theorem (NCT) - that of non-factorisability. The enquiry occurred from the investigation of two interferometer based communication systems: one two-photon entanglement, the other single-photon path entanglement. Both systems have two parties: a sender (“Alice”) who transmits or absorbs her particle and a receiver (“Bob”) who has an interferometer, which can discern a pure or mixed state, ahead of his detector. Ghirardi used the density matrix and found that the system wasn’t factorisable; this was seen as a fulfilment of the NCT. We revisit the analysis and say quite simply that Ghirardi is mistaken. The system is rendered factorisable by a Schmidt decomposition and entanglement swapping to “which path information” of the interferometer; also one must consider the joint evolution before taking the partial trace. Ghirardi’s misuse, by the inapplicability of the NCT in this situation, renders this general prohibitive bar incomplete or entirely wrong.
Inequalities regarding partial trace and partial determinant
