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2018 Agency in the Physical World
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2008 Foundational Questions in Physics and Cosmology
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2006 Foundational Questions in Physics and Cosmology
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Kevin Resch
University of Waterloo

Robert Spekkens
Perimeter Institute for Theoretical Physics

Project Title

Experimental test of intrinsically quantum causal relations

Project Summary

If two variables are found to be statistically correlated, there are many different causal mechanisms that could explain this. It could be that one variable causally influences the other. Or, they could both be effects of a common cause. Or perhaps, with some probability, it is a cause-effect relation, and with some probability, it is a common-cause relation. Alternatively, there may be a cause-effect mechanism and a common-cause mechanism acting simultaneously. In a quantum world, the menu of possibilities for how two systems are causally related is broader still: it could be that there is a quantum coherent mixture of cause-effect and common-cause mechanisms. We propose a theoretical investigation of the circumstances under which such novel nonclassical causal structures can arise and how their presence can be detected, as well as an experiment using single photons that realizes and detects such a structure. Understanding the nature of causation in a quantum world is anticipated to shed light on foundational problems in quantum theory, such as Bell's theorem, as well as problems in quantum gravity, where space and time must be given a quantum description.

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