Abstract
Any evolution described by a completely positive trace-preserving linear map can be imagined as arising from the interaction of the evolving system with an initially uncorrelated ancilla. The interaction is given by a joint unitary operator, acting on the system and the ancilla. Here we study the properties such a unitary operator must have in order to force the choice of a physical—that is, positive—state for the ancilla if the end result is to be a physical—that is, completely positive—evolution of the system.
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Jiang, Z., Piani, M. & Caves, C.M. Ancilla models for quantum operations: for what unitaries does the ancilla state have to be physical?. Quantum Inf Process 12, 1999–2017 (2013). https://doi.org/10.1007/s11128-012-0500-x
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DOI: https://doi.org/10.1007/s11128-012-0500-x