an individual attack on single-photon BB84 QKD, Eve probes Alice’s photons one at a time. In a collective attack, Eve’s measurements probe groups of Alice’s photons. Less is known about collective attacks [@collective], so we will limit our consideration to individual attacks. Fuchs and Peres [@FP] described the most general way in which an individual attack could be performed. Eve supplies a probe photon and lets it interact with Alice’s photon in a unitary manner. Eve then sends Alice’s photon to Bob, and performs a probability operator-valued measurement (POVM) on the probe photon she has retained. Slutsky [*et al.*]{} [@Slutsky] demonstrated that the Fuchs-Peres construct—with the appropriate choice of probe state, interaction, and measurement—affords Eve the maximum amount of Rényi information about the error-free sifted bits that Bob receives for a given level of disturbance, i.e., for a given probability that a sifted bit will be received in error. Brandt [@FPB] extended the Slutsky [*et al.*]{} treatment by showing that the optimal probe could be realized with a single CNOT gate. Figure 1 shows an abstract diagram of the resulting Fuchs-Peres-Brandt probe. In what follows we give a brief review of its structure and performance—see [@FPB] for a more detailed 