these is the familiar polarization qubit, with basis $\{|H\rangle,|V\rangle\}$. The other we shall term the momentum qubit—because our physical simulation of the FPB probe will rely on the polarization-momentum hyperentangled photon pairs produced by type-II phase matched spontaneous parametric downconversion (SPDC)—although in the collimated configuration in which SPTQ is implemented its basis states are single-photon kets for right and left beam positions (spatial modes), denoted $\{|R\rangle, |L\rangle\}$. Unlike the gates proposed for linear optics quantum computing [@KLM], which are scalable but non-deterministic, SPTQ quantum logic is deterministic but not scalable. Nevertheless, SPTQ quantum logic suffices for a complete physical simulation of polarization-based BB84 being attacked with the FPB probe, as we shall show. Before doing so, however, we need to comment on the gates that have been demonstrated in SPTQ logic.

It is well known that single qubit rotations and CNOT gates form a universal set for quantum computation. In SPTQ quantum logic, polarization-qubit rotations are easily accomplished with wave plates, just as is done in linear optics quantum computing. Momentum-qubit rotations are realized by first performing a SWAP operation, to exchange the polarization and momentum qubits, then rotating the polarization qubit, and finally performing another SWAP. The SWAP operation 