  A design is given for an optimized entangling probe attacking the BB84
(Bennett-Brassard 1984) protocol of quantum key distribution and yielding
maximum information to the probe for a full range of induced error rates. Probe
photon polarization states become optimally entangled with the signal states on
their way between the legitimate transmitter and receiver. Although standard
von-Neumann projective measurements of the probe yield maximum information on
the pre-privacy amplified key, if instead the probe measurements are performed
with a certain positive operator valued measure, then the measurement results
are conclusive, at least some of the time, for a full range of inconclusive
rates.
