Performance Analysis for Brandt's Conclusive Entangling Probe

The Fuchs-Peres-Brandt (FPB) probe realizes the most powerful individual attack on Bennett-Brassard 1984 quantum key distribution by means of a single controlled-NOT gate in which Alice's transmitted qubit becomes the control-qubit input, Bob's received qubit is the control-qubit output, and Eve supplies the target-qubit input and measures the target-qubit output. The FPB probe uses the minimum-error-probability projective measurement for discriminating between the target-qubit output states that are perfectly correlated with Bob's sifted bit value when that bit is correctly received. This paper analyzes a recently proposed modification of the FPB attack in which Eve's projective measurement is replaced by a probability operator-valued measurement chosen to unambiguously discriminate between the same two target-qubit output states.