Adding the maximally mixed state with some weight to the entanglement system leads to disentanglement of the latter. For each predefined entangled state there exists a minimal value of this weight for which the system loses its entanglement properties. These values were proposed to be used as a quantitative measure of entanglement called robustness [G. Vidal and R. Tarrach, Phys. Rev. A 59, 141 (1999)]. Using the concurrence, we propose the derivation of this measure for the system of two-qubit. Namely, for a two-qubit pure state, an exact expression of robustness is obtained. Finally, in the same way, the robustness of special cases of mixed two-qubit states is calculated.

中文翻译：

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从并发中推导鲁棒性

向纠缠系统添加具有一定权重的最大混合状态会导致后者解缠结。对于每个预定义的纠缠状态，都存在该权重的最小值，系统失去其纠缠属性。这些值被提议用作纠缠的定量度量，称为鲁棒性 [G. 维达尔和 R. Tarrach，物理。牧师 A 59, 141 (1999)]。利用并发性，我们提出了对双量子比特系统的这种度量的推导。即，对于两个量子比特的纯态，获得了鲁棒性的精确表达。最后，以同样的方式计算混合双量子比特状态特殊情况的鲁棒性。