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Entangling power and fidelity diagnostics for bipartite quantum channels

Marcin Rudziński1,2, Gianluigi Tartaglione3,4, and Karol Życzkowski1,5

  • 1Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, ul. Łojasiewicza 11, 30-348 Kraków, Poland
  • 2Doctoral School of Exact and Natural Sciences, Jagiellonian University, ul. Łojasiewicza 11, 30-348 Kraków, Poland
  • 3Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Salerno, Italy
  • 4Institute of Nanotechnology of the National Research Council of Italy, CNR-NANOTEC, Lecce Central Unit, c/o Campus Ecotekne, Via Monteroni, 73100 Lecce, Italy
  • 5Center for Theoretical Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland

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Phys. Rev. A 114, 032437 – Published 16 September, 2026

DOI: https://doi.org/10.1103/gkc7-tkbq

Abstract

We study two distinct diagnostics of bipartite quantum channels: the average fidelity between a desired state and noisy output across different classes of input states, and generation of entanglement from pure product input states, quantified by well-defined entangling power. These diagnostics are generally independent, but together they provide a more complete description of bipartite channels. We show that the fidelity averaged over any local-unitary orbit with fixed Schmidt coefficients for equal local dimensions is completely determined by the average input-output fidelity and its restriction to product inputs. We also introduce concurrence- and negativity-based entangling power for two-qubit channels, prove their convexity and monotonicity under local postprocessing, and show that, unlike the previously proposed linear-entropy quantity, they vanish for all separable channels. Representative separable and nonseparable channels are investigated. Finally, we generalize our definitions of entangling power to nonproduct inputs, and provide an analytical lower bound for the concurrence-based entanglement variation, showing its effectiveness with specific examples.

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Phys. Rev. A 114, 032437– Published 16 September, 2026
  • Received 2 June 2026
  • Revised 7 August 2026
  • Accepted 24 August 2026
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