A fundamental question in trying to understand the world -- be it
classical or quantum -- is why things happen. We seek a causal account
of events, and merely noting correlations between them does not provide a
satisfactory answer. In classical statistics, a better alternative
exists: the framework of causal models has proven useful for studying
causal relations in a range of disciplines. We try to adapt this
formalism to allow for quantum variables, and in the process discover a
new perspective on how causality is different in the quantum world. One
of the peculiarities that arise in this context can be harnessed to
solve a task of causal inference -- inferring the causal relation
between variables based on observed statistics -- that is impossible for
classical variables. I will report on a recent experimental realization
of this scheme.
Time permitting, I will also discuss a more realistic approach to the problem of characterizing quantum processes in the presence of initial correlations with an environment, viz non-Markovian dynamics. Another application of quantum causal inference arises in the context of quantum field theory: if one couples two detectors to a quantum field at different points throughout space-time, this may allow one of them to causally influence the other, via the field. We explore how different variables of the model, such as the acceleration of the detectors and the ultraviolet cutoff of the field theory, are reflected in the strength and quality of the causal influence.
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