in both polarization and momentum. The first polarizing beam splitter transmits a horizontally-polarized photon and reflects a vertically-polarized photon while preserving their momentum entanglement. Eve uses a SWAP gate and (half-wave plate plus polarizing beam splitter) polarization rotation so that her photodetector’s clicking will, by virtue of the momentum entanglement, herald the setting of the desired $|T_{\rm in}\rangle$ momentum-qubit state on the horizontally-polarized photon emerging from the first polarizing beam splitter. Alice’s electronically controllable half-wave plate sets the BB84 polarization qubit on this photon, and the rest of the Fig. 8 configuration is identical to that shown and explained in Fig. 6. Inasmuch as the SPDC source and SPTQ gates needed to realize the Fig. 8 setup have been demonstrated, we propose that such an experiment be performed. Simultaneous recording of Alice’s polarization choices, Bob’s polarization measurements and Eve’s $|d_+\rangle$ versus $|d_-\rangle$ results can then be processed through the BB84 protocol stack to study the degree to which the security proofs and eavesdropping analyses stand up to experimental scrutiny.

The Complete Attack
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Although the FPB attack’s physical simulation, as described in the preceding section, is both experimentally feasible and technically informative, any vulnerabilities it might reveal would only be 