Causal Quantum Theory

James T. Cushing

Hans H. Diel

This article presents an analysis of the extent to which local causal models or local realistic models of quantum theory (QT), including quantum field theory (QFT), are theoretically possible and practically feasible in light of the present state of these theories. Quantum physicists consider Bells famous inequality and its violation in experiments to be a strong indication that local realistic or local causal models of QT are not possible and that quantum theory as a whole is therefore not a local realistic or local causal theory. Based on a proposed definition of a "formal causal model" for a theory of physics (such as QT), this paper investigates the possibility of having a local causal model for QT. Areas of QT are identified in which the construction of a causal model is impeded because of deficiencies in the state of the respective theory. It is shown that the removal of the deficiencies can be achieved by the provision of a causal model. Whereas the construction of a causal model of QT, including QFT, appears to be feasible after the removal of certain deficiencies, the construction of a local (causal) model does not appear to be possible. As a consequence of the conclusion that local (causal) models of QT/QFT are not possible, if a strong interpretation of locality is assumed, a locality model is proposed in which the non-localities are confined to "quantum objects".

Mirjam Weilenmann, Roger Colbeck

Causal structures give us a way to understand the origin of observed correlations. These were developed for classical scenarios, but quantum mechanical experiments necessitate their generalisation. Here we study causal structures in a broad range of theories, which include both quantum and classical theory as special cases. We propose a method for analysing differences between such theories based on the so-called measurement entropy. We apply this method to several causal structures, deriving new relations that separate classical, quantum and more general theories within these causal structures. The constraints we derive for the most general theories are in a sense minimal requirements of any causal explanation in these scenarios. In addition, we make several technical contributions that give insight for the entropic analysis of quantum causal structures. In particular, we prove that for any causal structure and for any generalised probabilistic theory, the set of achievable entropy vectors form a convex cone.

Sergey M. Korotaev, Evgeniy O. Kiktenko

Peter J. Riggs

Mirjam Weilenmann, Roger Colbeck

Causal structures give us a way to understand the origin of observed correlations. These were developed for classical scenarios, but quantum mechanical experiments necessitate their generalisation. Here we study causal structures in a broad range of theories, which include both quantum and classical theory as special cases. We propose a method for analysing differences between such theories based on the so-called measurement entropy. We apply this method to several causal structures, deriving new relations that separate classical, quantum and more general theories within these causal structures. The constraints we derive for the most general theories are in a sense minimal requirements of any causal explanation in these scenarios. In addition, we make several technical contributions that give insight for the entropic analysis of quantum causal structures. In particular, we prove that for any causal structure and for any generalised probabilistic theory, the set of achievable entropy vectors form a convex cone.