Curator's Take
This research introduces powerful new diagnostic tools for characterizing quantum channels, which are essential building blocks in quantum computing systems where information must be reliably transmitted between qubits or quantum processors. The work addresses a critical challenge in quantum computing: how to systematically measure and compare the quality of different quantum channels, both in terms of how well they preserve quantum information (fidelity) and their ability to create useful entanglement. By developing mathematically rigorous metrics for entangling power that properly distinguish between separable and non-separable channels, this framework provides quantum engineers with better tools to evaluate and optimize quantum communication protocols. These diagnostics could prove invaluable for benchmarking quantum hardware performance and designing more efficient quantum algorithms that rely on distributed quantum processing.
— Mark Eatherly
Summary
We study two complementary diagnostics of bipartite quantum channels, namely fidelity preservation across different classes of input states and entanglement generation from product inputs, given by properly defined entangling power for bipartite channels. We show that the fidelity averaged over any fixed Schmidt-coefficients local-unitary orbit for equal local dimensions is completely determined by average input-output fidelity and its restriction to product inputs. We also introduce concurrence- and negativity-based entangling power for 2-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. Examples of non-separable channels are investigated. Finally, we generalize our definitions of entangling power to non-product inputs, and provide an analytical lower bound for the concurrence-based entanglement variation, showing its effectiveness with specific examples.