On Causal Explanations of Quantum Correlations



                                                 Robert W. Spekkens
                                      Perimeter Institute for Theoretical Physics
                                31 Caroline St. N, Waterloo, Ontario, Canada, N2L 2Y5




                                                      Abstract

    The framework of causal models is ideally suited to formalizing certain conceptual problems in quantum the-
    ory, and conversely, a variety of tools developed by physicists studying the foundations of quantum theory have
    applications for causal inference. This talk reviews some of the connections between the two fields. In partic-
    ular, it is shown that certain correlations predicted by quantum theory and observed experimentally cannot be
    explained by any causal model while respecting the core principles of causal discovery algorithms. Nonetheless,
    it is argued that by understanding quantum theory as an innovation to the theory of Bayesian inference, one can
    introduce a quantum generalization of the notion of a causal model and salvage a causal explanation of these cor-
    relations without fine-tuning. Furthermore, experiments exhibiting certain quantum features, namely, coherence
    and entanglement, enable solutions to causal inference problems that are intractable classically. In particular,
    while passive observation of a pair of variables cannot determine the causal relation that holds between them
    according to classical physics, this is not the case in quantum physics. In other words, according to quantum
    theory, certain kinds of correlation do imply causation. The results of a quantum-optical experiment confirming
    these predictions will be presented.
    This talk is based on the work described in Refs. [1] and [2].


References
[1] Christopher J. Wood and Robert W. Spekkens, The lesson of causal discovery algorithms for quantum correlations:
    Causal explanations of Bell-inequality violations require fine-tuning, preprint arXiv:1208.4119, (2012).

[2] Katja Ried, Megan Agnew, Lydia Vermeyden, Dominik Janzing, Robert W. Spekkens and Kevin J. Resch, Inferring
    causal structure: a quantum advantage, preprint arXiv:1406.5036, (2014).