Rethinking Causality in Quantum Mechanics pp 65-89 | Cite as

# Witnessing Causal Nonseparability: Theory and Experiment

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## Abstract

We develop a technique to witness a process that is not causally separable, in that it cannot be written as a fixed-order quantum circuit or a mixture of those. Using the process matrix formalism, we find a set of measurements within the process that proves causal non-separability. The problem of finding these measurements can be written as a SemiDefinite Program that can be solved efficiently. We apply our method in an experiment we performed in our labs, where we implemented a causally non-separable process and proved it to be so by performing unitary operations and final measurements.

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