  For sending unknown direction information, antiparallel spins contains more
direction information than parallel spins(Gisin and Popescu, 1999,
\textit{Phys. Rev. Lett.} 83, 432).In this paper, the optimal
information-disturbance tradeoff bound for antiparallel spins is derived. The
quantum measurements which attain the optimal tradeoff bound are obtained. This
result can be of practical relevance for posing some general limits on Eve's
eavesdropping process. Finally, we also present a comparison between the bound
for antiparallel spins and the bound for parallel spins.
