A machine that purifies two quantum inputs of different rank with positive probability cannot be a linear positive map, ruling out universal probabilistic purification from finite copies; approximate strategies exhibit a dimension-dependent trade-off between pure-output and append-environment maps.
Implementing quantum control for unknown subroutines
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abstract
We present setups for the practical realization of adding control to unknown subroutines, supplementing the existing quantum optical scheme for black-box control with a counterpart for the quantum control of the ordering of sequences of operations. We also provide schemes to realize either task using trapped ions. These practical circumventions of recent no-go theorems are based on existing technologies. We argue that the possibility to add control to unknown operations in practice is a common feature of many physical systems. Based on the proposed implementations we discuss the apparent contradictions between theory and practice.
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quant-ph 2years
2026 2verdicts
UNVERDICTED 2roles
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A review of the process matrix formalism for indefinite causal order in quantum theory, covering methodology, key results, experiments, and recent advances.
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Probabilistic and approximate universal quantum purification machines
A machine that purifies two quantum inputs of different rank with positive probability cannot be a linear positive map, ruling out universal probabilistic purification from finite copies; approximate strategies exhibit a dimension-dependent trade-off between pure-output and append-environment maps.
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Indefinite Quantum Causality
A review of the process matrix formalism for indefinite causal order in quantum theory, covering methodology, key results, experiments, and recent advances.