Entangled eavesdropping in quantum key distribution

Recent designs are summarized for some optimized quantum cryptographic entangling probes to be used in attacking the BB84 protocol of quantum key distribution. They are all based on alternative optimum unitary transformations, each yielding the same maximum information on the pre-privacy amplified key. One of the optimum unitary transformations, not previously addressed in terms of implementation, is shown, for a certain choice of free parameters, to again yield one of the recent designs. In each of the designs, the main quantum circuit consists of a single CNOT gate which produces the optimum entanglement between the BB84 signal states and the correlated probe states. For each design a different well-defined single-photon polarization state serves as the initial state of the probe, and in each case the probe is projectively measured.