Instytut Podstawowych Problemów Techniki

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The resource theory of quantum coherence studies the off-diagonal elements of a density matrix in a
distinguished basis, whereas the resource theory of purity studies all deviations from the maximally
mixed state. We establish a direct connection between the two resource theories, by identifying purity as the maximal coherence which is achievable by unitary operations. The states that saturate this
maximum identify a universal family of maximally coherent mixed states. These states are optimal
resources under maximally incoherent operations, and thus independent of the way coherence is quantified. For all distance-based coherence quantifiers the maximal coherence can be evaluated exactly, and is shown to coincide with the corresponding distance-based purity quantifier. We further show that purity bounds the maximal amount of entanglement and discord that can be generated by unitary operations, thus demonstrating that purity is the most elementary resource for quantum information processing.

Słowa kluczowe:

quantum coherence, quantum entanglement, esource theories

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Entanglement distribution with separable states has recently attracted considerable attention. Recent results suggest that quantum discord, a measure for quantum correlations beyond entanglement, is responsible for this counterintuitive phenomenon. In this work we study this question from a different perspective and find minimal requirements for a separable state to be useful for entanglement distribution. Surprisingly, we find that the presence of quantum discord is not sufficient to ensure entanglement distribution: There exist states with nonzero quantum discord that nevertheless cannot be used for entanglement distribution. As a result, we show that entanglement distribution is not possible with rank-2 separable states. Our work sheds light on the task of entanglement distribution with separable states and reveals a classification of quantum states with respect to their usefulness for this task.

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Establishing quantum entanglement between two distant parties is an essential step of many protocols in quantum information processing. One possibility for providing long-distance entanglement is to create an entangled composite state within a lab and then physically send one subsystem to a distant lab. However, is this the ‘‘cheapest’’ way? Here, we investigate the minimal ‘‘cost’’ that is necessary for establishing a certain amount of entanglement between two distant parties. We prove that this cost is
intrinsically quantum, and is specified by quantum correlations. Our results provide an optimal protocol
for entanglement distribution and show that quantum correlations are the essential resource for this task.

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We characterize the behavior of quantum correlations under the influence of local noisy channels. Intuition suggests that such noise should be detrimental for quantumness. When considering qubit systems, we show for which channels this is indeed the case: The amount of quantum correlations can
only decrease under the action of unital channels. However, nonunital channels (e.g., such as dissipation) can create quantum correlations for some initially classical states. Furthermore, for higher-dimensional systems even unital channels may increase the amount of quantum correlations. Thus, counterintuitively, local decoherence can generate quantum correlations.

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We show that a von Neumann measurement on a part of a composite quantum system unavoidably creates distillable entanglement between the measurement apparatus and the system if the state has nonzero quantum discord. The minimal distillable entanglement is equal to the one-way information deficit. The quantum discord is shown to be equal to the minimal partial distillable entanglement that is the part of entanglement which is lost, when we ignore the subsystem which is not measured. We then show that any entanglement measure corresponds to some measure of quantum correlations. This powerful correspondence also yields necessary properties for quantum correlations. We generalize the results to multipartite measurements on a part of the system and on the total system.

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We present an easy implementable algorithm for approximating the geometric measure of entanglement from above. The algorithm can be applied to any multipartite mixed state. It involves only the solution of an eigenproblem and finding a singular value decomposition; no further numerical techniques are needed. To provide examples, the algorithm was applied to the isotropic states of three qubits and the three-qubit XX model with external magnetic field.

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An important problem in quantum information theory is the quantification of entanglement in multipartite mixed quantum states. In this work, a connection between the geometric measure of entanglement and a distance measure of entanglement is established. We present a new expression for the geometric measure of entanglement in terms of the maximal fidelity with a separable state. A direct application of this result provides a closed expression for the Bures measure of entanglementof two qubits. We also prove that the number of elements in an optimal decomposition w.r.t. the geometric measure of entanglement is bounded from above by the Caratheodory bound, and we find necessary conditions for the structure of an optimal decomposition.

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