is a cascade of three CNOTs, as shown in Fig. 5. For its implementation in SPTQ quantum logic the left and right CNOTs in Fig. 5 are momentum-controlled NOT gates (M-CNOTs) and the middle CNOT is a polarization-controlled NOT gate (P-CNOT). (An M-CNOT uses the momentum qubit of a single photon to perform the controlled-NOT operation on the polarization qubit of that same photon, and vice versa for the P-CNOT gate.) Experimental demonstrations of deterministic M-CNOT, P-CNOT, and SWAP gates are reported in [@SPTQ1; @SPTQ2].

Figure 6 shows a physical simulation of polarization-based BB84 under FPB attack when Alice has a single-photon source and Bob employs active basis selection; Fig. 7 shows the modification needed to accommodate Bob’s using passive basis selection. In either case, Alice uses a polarizing beam splitter and an electro-optic modulator, as a controllable half-wave plate (HWP), to set the randomly-selected BB84 polarization state for each photon she transmits. Moreover, she employs a single spatial mode, which we assume coincides with the $R$ beam position in Eve’s apparatus. Eve then begins her attack by imposing the probe state $|T_{\rm in}\rangle$ on the momentum qubit. She does this by applying a SWAP gate, to exchange the momentum 