momentum qubit on the photon being sent to Bob, she can employ an additional stage of qubit rotation to restore this momentum qubit to the $|R\rangle$ state corresponding to Alice’s transmission. Also, should Alice try to defeat Eve’s FPB probe by augmenting her BB84 polarization qubit with a randomly-chosen momentum qubit, Eve can use a QND measurement setup like that shown in Fig. 9 to collapse the value of that momentum qubit to $|R\rangle$ or $|L\rangle$, and then rotate that momentum qubit into the $|R\rangle$-state spatial mode before applying the FPB-probe attack. At the conclusion of her attack, she can then randomize the momentum qubit on the photon that will be routed on to Bob without further impact—beyond that imposed by her P-CNOT gate—on that photon’s polarization qubit. So, unless Alice and Bob generalize their polarization-based BB84 protocol to include cooperative examination of the momentum qubit, Alice’s randomization of that qubit will neither affect Eve’s FPB attack, nor provide Alice and Bob with any additional evidence, beyond that obtained from the occurrence of errors on sifted bits, of Eve’s presence.*

Some concluding remarks are now in order. We have shown that a physical simulation of the Fuchs-Peres-Brandt attack on polarization-based 