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arXiv · 0912.0195

Quantum computations without definite causal structure

G. Chiribella·G. M. D'Ariano·P. Perinotti·B. Valiron
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Abstract

We show that quantum theory allows for transformations of black boxes that cannot be realized by inserting the input black boxes within a circuit in a pre-defined causal order. The simplest example of such a transformation is the classical switch of black boxes, where two input black boxes are arranged in two different orders conditionally on the value of a classical bit. The quantum version of this transformation-the quantum switch-produces an output circuit where the order of the connections is controlled by a quantum bit, which becomes entangled with the circuit structure. Simulating these transformations in a circuit with fixed causal structure requires either postselection, or an extra query to the input black boxes.

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G. Chiribella, G. M. D'Ariano, P. Perinotti, B. Valiron. 2013-10-27. Quantum computations without definite causal structure. https://doi.org/10.1103/physreva.88.022318

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