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arXiv quant-ph/0408115quant-ph
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量子测量中的经典随机性

Classical randomness in quantum measurements

Giacomo Mauro D'Ariano, Paoloplacido Lo Presti, Paolo Perinotti

摘要:

类似于量子态,量子测量也可以是“混合的”,对应于测量设备的集合内的随机选择。这种混合相当于一种隐变量,它产生纯粹经典性质的噪声。然后,自然会问哪些设备是“不可分解的”,即。e.并不对应于任何随机选择的装置。这个问题是有趣的,不仅为基础,但也为应用程序,因为大多数优化策略给出了最佳的设备是不可分解的。数学上的问题是提出了描述每个测量装置的正算子值的措施(POVM),它给出了统计的结果,为任何输入状态。POVM形成了一个凸集,在这种语言中,不可分解的设备由极值点表示-类似于状态凸集中的“纯状态”。然而,与状态的情况不同,不可分解的POVM不一定是秩一的。G.冯·诺依曼测量在本文中,我们给出了一个完整的分类不可分解的仪器(离散谱),通过提供不同的必要条件和充分条件的POVM的极值,以及一个简单的通用算法分解的POVM到极值。作为一个有趣的应用,“信息完整”的测量分析在这方面。POVM的凸集的充分特征在于确定其边界方面的简单的代数性质的POVM的。

英文摘要:

Similarly to quantum states, also quantum measurements can be "mixed", corresponding to a random choice within an ensemble of measuring apparatuses. Such mixing is equivalent to a sort of hidden variable, which produces a noise of purely classical nature. It is then natural to ask which apparatuses are "indecomposable", i. e. do not correspond to any random choice of apparatuses. This problem is interesting not only for foundations, but also for applications, since most optimization strategies give optimal apparatuses that are indecomposable. Mathematically the problem is posed describing each measuring apparatus by a positive operator-valued measure (POVM), which gives the statistics of the outcomes for any input state. The POVM's form a convex set, and in this language the indecomposable apparatuses are represented by extremal points--the analogous of "pure states" in the convex set of states. Differently from the case of states, however, indecomposable POVM's are not necessarily rank-one, e. g. von Neumann measurements. In this paper we give a complete classification of indecomposable apparatuses (for discrete spectrum), by providing different necessary and sufficient conditions for extremality of POVM's, along with a simple general algorithm for the decomposition of a POVM into extremals. As an interesting application, "informationally complete" measurements are analyzed in this respect. The convex set of POVM's is fully characterized by determining its border in terms of simple algebraic properties of the corresponding POVM's.

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