偏振扰频器真的能使光去偏振吗？
Can a polarization scrambler really depolarize light?
摘要：
根据量子理论，由同一密度算符描述的两个量子系综是不可区分的。例如，非偏振光可以通过两个正交纯态的非相干混合或通过从最大偏振纠缠光子对中追踪光子来获得。在这两种情况下，人们无法以大于50%的概率猜测任何偏振测量的结果，但原因在概念上是不同的：而第一种情况是经典的无知（光子是以一种确定但未知的方式制备的），第二种是量子无知类型的-如果一个人不能访问量子态的其他自由度，不存在可用于预测测量结果的信息。利用这些概念，从量子通信的角度讨论了部分偏振光的量子物理解释，以及偏振扰频器是否真的能使光去偏振。给出了一个不依赖于经典无知的单光子偏振度的新定义。
英文摘要：
According to quantum theory, two ensembles of quantum systems that are described by the same density operator are indistinguishable. For example, unpolarized light can be obtained either by an incoherent mixture of two orthogonal pure states or by tracing out a photon from a maximally polarization-entangled photon pair. In both cases, one is unable to guess with probability greater than 50% the outcome of any polarization measurement, but the reasons are conceptually different: whereas the first case is a matter of classical ignorance (the photons were prepared in a definite but unknown way), the second one is of the quantum ignorance kind - if one cannot access the other degrees of freedom of the quantum state, there is no information that could be used to predict a measurement result. We use these concepts to discuss the quantum-physical interpretation of partially polarized light from the point of view of quantum communication and whether a polarization scrambler can really depolarize light. A novel definition of the degree of polarization of a single photon which does not depend on classical ignorance is also provided.
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