polarization basis that he has selected while she retains the photon counting results corresponding to her $\{|d_+\rangle, |d_-\rangle\}$ measurement. Clearly Bob would never knowingly permit Eve to intrude into his receiver box in this manner. Moreover, if Eve could do so, she would not bother with an FPB probe as she could directly observe Bob’s bit values.

If Bob employs passive basis selection (Fig. 7), then he uses a 50/50 beam splitter followed by static-HWP analysis in the $H$-$V$ and $\pm 45^\circ$ bases, with only the former being explicitly shown in Fig. 7. The rest of Eve’s attack mimics what was seen in Fig. 6, i.e., she gets inside Bob’s measurement boxes with SWAP gates, half-wave plates, and additional detectors so that she can perform her probe measurement while providing Bob with his BB84 polarization-measurement data. Because the Fig. 7 arrangement requires that twice as many SWAP gates, twice as many half-wave plates, and twice as many single-photon detectors be inserted into Bob’s receiver system, as compared to what is needed in the Fig. 6 setup, we shall limit the rest of our discussion to the case of active basis selection as it leads to a more parsimonious physical simulation 