Information-disturbance tradeoff in sending Direction information via   antiparallel quantum spin
  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. 
ShengLi Zhang, XuBo Zou, ChuanFeng Li, ChenHui Jin and GuangCan Guo
quant-ph