Department of Physics, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
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Abstract
Similarly to quantum states, quantum operations can also be transformed by means of quantum superchannels, also known as process matrices. Quantum superchannels with multiple slots are deterministic transformations which take independent quantum operations as inputs. While they are enforced to respect the laws of quantum mechanics, the use of input operations may lack a definite causal order, and characterizations of general superchannels in terms of quantum objects with a physical implementation have been missing. In this paper, we provide a mathematical characterization for pure superchannels with two slots (also known as bipartite pure processes), which are superchannels preserving the reversibility of quantum operations. We show that the reversibility preserving condition restricts all pure superchannels with two slots to be either a quantum circuit only consisting of unitary operations or a coherent superposition of two unitary quantum circuits where the two input operations are differently ordered. The latter may be seen as a generalization of the quantum switch, allowing a physical interpretation for pure two-slot superchannels. An immediate corollary is that purifiable bipartite processes cannot violate device-independent causal inequalities.
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Cited by
[1] Jonathan Barrett, Robin Lorenz, and Ognyan Oreshkov, “Cyclic quantum causal models”, Nature Communications 12, 885 (2021).
[2] Fabio Costa, “A no-go theorem for superpositions of causal orders”, arXiv:2008.06205.
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The above citations are from SAO/NASA ADS (last updated successfully 2021-04-26 13:04:53). The list may be incomplete as not all publishers provide suitable and complete citation data.
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