Alastair ABBOTT

Causality is often treated as a classical notion but there have been growing calls for a fundamentally quantum notion of causal structure, motivated e.g. by the quest to develop a theory of quantum gravity or to explain various nonclassical features of quantum mechanics. This has led to the development of an abstract framework in which causally indefinite processes can be formulated, for example by superposing quantum processes with different causal orders. However, interpreting many of these processes is difficult.

In this talk I will give an overview of indefinite causal order in quantum mechanics and some of the recent efforts to understand it within quantum information theory. I will focus, in particular, on the question of which causally indefinite processes can be given physical interpretations. On one hand, information theoretic principles may allow certain types of causal indefiniteness to be ruled out as non-physical; while on the other hand, certain processes can be interpreted concretely as generalisations of quantum circuits with quantum control. This interpretation moreover raises questions about the physical limits of computation and whether causal indefiniteness can be exploited