Strict hierarchy between parallel, sequential, and indefinite-causal-order strategies for channel discrimination

Presented at:

Seminar: Chapman University, group of Prof. Matthew Leifer (Orange, USA, 2021; online)
Seminar: Ulm University, group of Prof. Martin Plenio (Ulm, Germany, 2021; online)
Conference: YQIS 2021 - VI International Conference for Young Quantum Information Scientists (Michigan, USA, 2021; online)

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Strict hierarchy between parallel, sequential, and indefinite-causal-order strategies for channel discrimination

References

Strict hierarchy between parallel, sequential, and indefinite-causal-order strategies for channel discrimination

Jessica Bavaresco, Mio Murao, Marco Túlio Quintino

Physical Review Letters 127, 200504 (2021)

arXiv Journal

We present an instance of a task of minimum-error discrimination of two qubit-qubit quantum channels for which a sequential strategy outperforms any parallel strategy. We then establish two new classes of strategies for channel discrimination that involve indefinite causal order and show that there exists a strict hierarchy among the performance of all four strategies. Our proof technique employs a general method of computer-assisted proofs. We also provide a systematic method for finding pairs of channels that showcase this phenomenon, demonstrating that the hierarchy between the strategies is not exclusive to our main example.