of academic interest were there no practical means to turn it into a true deterministic implementation in which Eve did *not need to invade Bob’s receiver. Quantum nondemolition measurement technology provides the key to creating this complete attack. As shown in the appendix, it is possible, in principle, to use cross-phase modulation between a strong coherent-state probe beam and an arbitrarily polarized signal beam to make a QND measurement of the signal beam’s total photon number while preserving its polarization state. Cross-phase modulation QND measurement of photon number has long been a topic of interest in quantum optics [@Imoto], and recent theory has shown that it provides an excellent new route to photonic quantum computation [@Nemoto]. Thus it is not unwarranted to presume that polarization-preserving QND measurement of total photon number may be developed. With such technology in hand, the FPB-probe attack shown in Fig. 9 becomes viable. Here, Eve imposes a momentum qubit on Alice’s polarization-encoded photon and performs a P-CNOT operation exactly as discussed in conjunction with Figs. 6 and 8. Now, however, Eve uses a SWAP-gate half-wave plate combination so that the $|d_+\rangle$ and $|d_-\rangle$ momentum qubit states emerging from her P-CNOT become $|V\rangle$ and $|H\rangle$ states 