Hermann Kampermann

Linking quantum discord to entanglement in a measurement.

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.

Behavior of Quantum Correlations under Local Noise

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.

Measurement-device-independent quantum key distribution with quantum memories

(Dated: June 14, 2013)We generalize measurement-device-independent quantum key distribution [ H.-K. Lo, M. Curty,and B. Qi, Phys. Rev. Lett. 108, 130503 (2012) ] to the scenario where the Bell-state measurementstation contains also heralded quantum memories. We find analytical formulas, in terms of deviceimperfections, for all quantities entering in the secret key rates, i.e., the quantum bit error rateand the repeater rate. We assume either single-photon sources or weak coherent pulse sources plusdecoy states. We show that it is possible to significantly outperform the original proposal, even inpresence of decoherence of the quantum memory. Our protocol may represent the first natural stepfor implementing a two-segment quantum repeater.I. INTRODUCTION

Linking a distance measure of entanglement to its convex roof

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.

Multipartite entanglement detection via structure factors.

We establish a relation between entanglement of a many-body system and its diffractive properties, where the link is given by structure factors. Based on these, we provide a general analytical construction of multiqubit entanglement witnesses. The proposed witnesses contain two-point correlations. They could be either measured in a scattering experiment or via local measurements, depending on the underlying physical system. For some explicit examples of witnesses we analyze the properties of the states that are detected by them. We further study the robustness of these witnesses with respect to noise.

Optimal super dense coding over noisy quantum channels

We investigate super dense coding in the presence of noise, i.e. the subsystems of the entangled resource state have to pass a noisy unital quantum channel between the sender and the receiver. We discuss explicitly the case of Pauli channels in an arbitrary dimension and derive the super dense coding capacity (i.e. the optimal information transfer) for some given resource states. For the qubit depolarizing channel, we also optimize the super dense coding capacity with respect to the input state. We show that, below a threshold value of the noise parameter, the super dense coding protocol is optimized by a maximally entangled initial state, while above the threshold it is optimized by a product state. Finally, we provide an example of a noisy channel where non-unitary pre-processing increases the super dense coding capacity, as compared to only unitary encoding.

Asymptotically perfect discrimination in the LOCC paradigm

We revisit the problem of discriminating orthogonal quantum states within the local quantum operation and classical communication (LOCC) paradigm. Our particular focus is on the asymptotic situation where the parties have infinite resources and the protocol may become arbitrarily long. Our main result is a necessary condition for perfect asymptotic LOCC discrimination. As an application, we prove that for complete product bases, unlimited resources are of no advantage. On the other hand, we identify an example, for which it still remains undecided whether unlimited resources are superior.

Experimental generation of pseudo bound entanglement

Experimentelle Physik IIIa, Technische Universit¨at Dortmund, D-44221 Dortmund, Germany(Dated: September 15, 2009)We use Nuclear Magnetic Resonance (NMR) to experimentally generate a bound entangled (moreprecisely: pseudo bound entangled) state, i.e. a quantum state which is non-distillable but neverthe-less entangled. Our quantum system consists of three qubits. We characterize the produced statevia state tomography to show that the created state has a positive partial transposition with respectto any bipartite splitting, and we use a witness operator to prove its entanglement.

Finite key analysis for symmetric attacks in quantum key distribution

We introduce a constructive method to calculate the achievable secret key rate for a generic class of quantum key distribution protocols, when only a finite number

Quantum repeaters and quantum key distribution: The impact of entanglement distillation on the secret key rate

n