Quantum channels and some absolute properties of quantum states

Abstract

Environmental interactions are ubiquitous in any real-world application of a quantum information processing protocol. Such interactions result in depletion of quantum resources. Two important figure of merits in the context of quantum information are the fully entangled fraction (FEF) and conditional entropy of a composite quantum system. FEF has a key role to play in tasks like teleportation. Conditional entropy on the other hand can be negative for certain quantum states and thus the negativity remains a resource for tasks like dense coding and state merging. FEF $>1/d$ for a $d\otimes d$ quantum system is a significant threshold, however for some quantum states it remains less than the threshold even with global unitary operations, consequently being known as states having absolute fully entangled fraction (AFEF). Pertaining to conditional von Neumann entropy, there are some states which retains the nonnegativity of the conditional entropy under global unitary action, to be called as states with absolute conditional von Neumann entropy nonnegative (ACVENN) property. In the present submission, we probe the action of some quantum channels in two qubits and two qudits and find that some quantum states move from the non-absolute regime to the absolute regime under the action. Since, global unitary operations are unable to retrieve them back to the non-absolute regime, we provide a prescription for the retrieval using an entanglement swapping network. Furthermore, we extend the notion of absoluteness to conditional Rényi entropies and find the required condition for a state to have absolute conditional Rényi entropy non-negative (ACRENN) property. We then extend the work to include the marginals of a tripartite system and provide for their characterization with respect to the aforementioned absolute properties.

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• Fidelity of entanglement and quantum entropies: unveiling their relationship in quantum states and channels
• Detection of nonabsolute separability in quantum states and channels through moments