更少的窃听损失导致更多的窃听信息增益
Less eavesdropping losses induce more eavesdropping information gain
摘要：
Wójcik提出的窃听方案[物理学。{ \bf 90}，157901（2003）]对Bostrom和Felbinger的量子通信协议[Phys. { \bf 89}，187902（2002）]通过构造一组新的攻击操作来改进。改进的方案仅引起Wójcik方案中一半的窃听损失，因此，在量子信道传输效率$η$的更大域中，即，[0，75%]，窃听者夏娃可以攻击所有传输的比特。与Wójcik方案相比，改进方案在$η$域[0，50%]中窃听（合法）信息增益不变，而在$η$域（50%，75%）中窃听损失越小，窃听信息增益越大，因为Eve可以攻击{ \it all}发送的比特，因此窃听信息增益{ \it not}减小.此外，对于Boström-Felbinger协议，本文推高了Wójcik提出的$η$的不安全上界，即根据Wójcik的窃听方案，Boström-Felbinger协议在传输效率低于60%时是不安全的，而根据改进方案，Boström-Felbinger协议在传输效率低于80%时是不安全的.
英文摘要：
The eavesdropping scheme proposed by Wójcik [Phys. Rev. Lett. {\bf 90},157901(2003)] on the quantum communication protocol of Boström and Felbinger [Phys. Rev. Lett. {\bf 89}, 187902(2002)] is improved by constituting a new set of attack operations. The improved scheme only induces half of the eavesdropping losses in Wójcik's scheme, therefore, in a larger domain of the quantum channel transmission efficiency $η$, i.e., [0,75%], the eavesdropper Eve can attack all the transmitted bits. Comparing to Wójcik's scheme, in the improved scheme the eavesdropping (legitimate) information gain does not vary in the $η$ domain of [0, 50%], while in the $η$ domain of (50%, 75%] the less eavesdropping losses induce more eavesdropping information gains, for Eve can attack {\it all} the transmitted bits and accordingly eavesdropping information gains do {\it not} decrease. Moreover, for the Boström-Felbinger protocol, the insecurity upper bound of $η$ presented by Wójcik is pushed up in the this paper, that is, according to Wójcik's eavesdropping scheme, the Boström-Felbinger protocol is not secure for transmission efficiencies lower than almost 60%, while according to the improved scheme, it is not secure for transmission efficiencies lower than almost 80%.
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