8 May 2012 Strictly discordant quantum probes of the qubit depolarizing channel
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Quantum mutual information defined in terms of von Neumann entropy captures and quantifies all correlations, quantum and classical, between the two parts of a bipartite quantum system. Within this framework entanglement is the most distinguished type of quantum correlation, and a rich body of theory and experiment establishes that entanglement is a potent and fungible resource for quantum information processing broadly. Bipartite systems can exhibit quantum correlations beyond entanglement. Such non-classical states are called discordant in general and strictly discordant (or dissonant) when the quantum state is separable. We show that strict discord can increase the amount of information available from probing a quantum channel. We focus in this study on the qubit depolarizing channel, using quantum Fisher information to measure the information available about the channel depolarizing probability. We consider channel probes prepared, along with an ancilla, in a separable two-qubit Bell-diagonal state. We prove for Bell-diagonal probes of the qubit depolarizing channel that, in the absence of entanglement and controlling for marginal purity and degree of classical correlation, any increase in strict discord between the probe and ancilla yields an accompanying increase in available statistical information about the channel depolarizing probability.
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Michael R. Frey, Michael R. Frey, Theodore J. Yoder, Theodore J. Yoder, } "Strictly discordant quantum probes of the qubit depolarizing channel", Proc. SPIE 8400, Quantum Information and Computation X, 84000R (8 May 2012); doi: 10.1117/12.918769; https://doi.org/10.1117/12.918769

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