  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.
