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arXiv quant-ph/0611133quant-ph
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量子信息理论中两种密度矩阵的非对称态判别

Unambiguous State Discrimination of two density matrices in Quantum Information Theory

Philippe Raynal

摘要:

在本论文中,我们研究了明确区分两个混合量子态的问题。我们首先提出了两个一般的密度矩阵的最佳明确的歧视约化定理。我们表明,这个问题可以减少到两个密度矩阵,具有相同的秩$r$在2 $r$维希尔伯特空间。这些减少定理也使我们能够减少USD问题的解决方案可能是已知的更简单的。作为应用,我们考虑了具有简单对称性的$n$线性无关纯态的无歧义比较。此外,最优失效概率的下界已被推导出来。对于两个混合态,它们是根据保真度给出的。在这里,我们给出了更严格的界限,以及必要和充分条件,两个混合态达到这些界限。我们还构造了相应的最优度量。有了这个结果,我们提供了明确区分一类通用的混合状态的解析解。这超出了已知的结果,这些结果都可以简化为一些纯态的情况。然而,我们表明,存在的例子中的界限不能达到。接下来,我们推导出最佳USD测量的秩和谱的性质。这最终导致第二类精确解。事实上,我们提出了最佳的故障概率,以及明确区分任何一对几何均匀的混合状态在四维的最佳测量。这类问题包括例如在具有相干状态的BB84 QKD协议中区分基和比特值混合状态。

英文摘要:

In this thesis we study the problem of unambiguously discriminating two mixed quantum states. We first present reduction theorems for optimal unambiguous discrimination of two generic density matrices. We show that this problem can be reduced to that of two density matrices that have the same rank $r$ in a 2$r$-dimensional Hilbert space. These reduction theorems also allow us to reduce USD problems to simpler ones for which the solution might be known. As an application, we consider the unambiguous comparison of $n$ linearly independent pure states with a simple symmetry. Moreover, lower bounds on the optimal failure probability have been derived. For two mixed states they are given in terms of the fidelity. Here we give tighter bounds as well as necessary and sufficient conditions for two mixed states to reach these bounds. We also construct the corresponding optimal measurement. With this result, we provide analytical solutions for unambiguously discriminating a class of generic mixed states. This goes beyond known results which are all reducible to some pure state case. We however show that examples exist where the bounds cannot be reached. Next, we derive properties on the rank and the spectrum of an optimal USD measurement. This finally leads to a second class of exact solutions. Indeed we present the optimal failure probability as well as the optimal measurement for unambiguously discriminating any pair of geometrically uniform mixed states in four dimensions. This class of problems includes for example the discrimination of both the basis and the bit value mixed states in the BB84 QKD protocol with coherent states.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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