Dual roles of quantum discord in a nondemolition probing task

We present a nondemolition quantum information processing task of probing the information of a class of quantum state. In this task, the information is extracted by some unitary evolution with the introduced probing qubit assisted, but the probed quantum state (density matrix) is undisturbed at any time and independent of the choice of the initial probing state. We give a sufficient and necessary condition on the Hamiltonian that can lead to the successful realization of such a task. We prove that, for any feasible scheme, the probed plus probing system will always stay at a disentangled state with one side's quantum discord absent and the other side's inevitably produced in the probing process. An explicit example is given for the demonstration, which shows that the ratio of quantum discord to the total correlation will have to reduce to zero for the maximal accessible information. In this sense, we say that quantum discord plays a dual role in this case. DOI: 10.1103/PhysRevA.87.022113

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