Quantum Casual Unravelling

Complex processes often arise from sequences of simpler interactions involving a few particles at a time. These interactions, however, may not be directly accessible to experiments. Here we develop the first efficient method for unravelling the causal structure of the interactions in a multipartite quantum process, using only access to its input and output systems. Our method is based on a quantum algorithm whose complexity scales polynomially for all processes with bounded information loss. Under additional assumptions, we also provide a second algorithm that has lower complexity and requires only local state preparation and local measurements. Our algorithms can be used to identify processes that can be characterized efficiently with the technique of quantum process tomography. Similarly, they can be used to identify useful communication channels in a quantum networks, and to test the structure of uncharacterized quantum circuits.