be directed toward attacking BB84 QKD, as to our knowledge no such experiments have been performed, although a variety of potentially practical approaches have been discussed [@attacks]. Our particular objective will be to show that current technology permits physical simulation of the Fuchs-Peres-Brandt (FPB) probe [@FPB], i.e., the most powerful individual attack on single-photon BB84, and that developments underway in quantum nondemolition (QND) detection may soon turn this physical simulation into a full implementation of the attack. Thus we believe it is of interest to construct the physical simulation and put BB84’s security to the test: how much information can Eve really derive about the key that Alice and Bob have distilled while keeping Alice and Bob oblivious to her presence.

The remainder of this paper is organized as follows. In Sec. II we review the FPB probe and its theoretical performance. In Sec. III we describe a complete physical simulation of this probe constructed from single-photon two-qubit (SPTQ) quantum logic. We conclude, in Sec. IV, by showing how the addition of polarization-preserving QND measurements of photon number can convert this physical simulation into a true deterministic realization of the FPB attack on polarization-based BB84.

The Fuchs-Peres-Brandt Probe
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In 