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The authors study partially coherent optical fields, for which polarization and the spatial degrees of freedom (DoF) are relevant, by exploring the rank of the associated coherence matrix. They show that rank-2 fields can always be rendered separable and can concentrate entropy entirely into one DoF, whereas all rank-3 fields are nonseparable and always have some locked entropy in one DoF.
Harling et al. (Thu,) studied this question.
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