We consider a spacetime generalization of the density operator – the “superdensity operator” – and show that its quantum information theoretic properties are meaningful and have operational physical significance. For instance, the superdensity operator furnishes spacetime generalizations of entanglement, subsystem entropies, and subsystem mutual information. As an application, we explore causality for local observers in a quantum system. We construct an information-theoretic diagnostic of one subsystem causally influencing another, and explore a variety of examples. The superdensity operator formalism is used to uncover interesting structural properties of local causal influence.
09.07.2018
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