Entanglement Detection in Optical Lattices of Bosonic Atoms with Collective Measurements

The minimum requirements for entanglement detection are discussed for a spin chain in which the spins cannot be individually accessed. The methods presented detect entangled states close to a cluster state and a many-body singlet state, and seem to be viable for experimental realization in optical lattices of two-state bosonic atoms. The entanglement criteria are based on entanglement witnesses and on the uncertainty of collective observables.Comment: 5 pages, revtex4, no figures; changes before publicatio

Centre-Periphery Analysis About the Hungarian Public Road System

Although the European Union prefers and supports mostly railways in terms of transport investments, the public road network – with a special regard to motorways – is rather underdeveloped in Hungary. Motorway construction has recovered after the change of regime. It is important not only to simply construct motorways but to work on the improvement of accessibility by validating the aspects of regional development, and thus the amounts invested have much more benefits. This view has been gaining ground mainly in the past period in Hungary. This paper analyses the accessibility conditions in 2005 by applying geographical information science methods, and examines if a favourable accessibility implies clearly a favourable level of development. The study also analyses which areas need new transport investments taking into consideration the circumstances of accessibility in 2005. By comparing the level of economic development and accessibility, the article seeks to answer at last how much favourable basis the present economic situation is for new public road investments.

Genuine three-partite entangled states with a local hidden variable model

We present a family of three-qubit quantum states with a basic local hidden variable model. Any von Neumann measurement can be described by a local model for these states. We show that some of these states are genuine three-partite entangled and also distillable. The generalization for larger dimensions or higher number of parties is also discussed. As a byproduct, we present symmetric extensions of two-qubit Werner states.Comment: 5 pages including 2 figures + 1 page appendix, revtex4; published versio

Two measurement settings can suffice to verify multipartite entanglement

We present entanglement witnesses for detecting genuine multi-qubit entanglement. Our constructions are robust against noise and require only two local measurement settings, independent of the number of qubits. Thus they allow to verify entanglement of many qubits in experiments while requiring only a small effort. In contrast, usual methods need an effort which increases exponentially with the number of qubits. The witnesses detect states close to GHZ states and cluster states.Comment: 4 pages including a figure, LaTeX; to appear in the conference proceedings of QCMC0

Evaluation of convex roof entanglement measures

We show a powerful method to compute entanglement measures based on convex roof constructions. In particular, our method is applicable to measures that, for pure states, can be written as low order polynomials of operator expectation values. We show how to compute the linear entropy of entanglement, the linear entanglement of assistance, and a bound on the dimension of the entanglement for bipartite systems. We discuss how to obtain the convex roof of the three-tangle for three-qubit states. We also show how to calculate the linear entropy of entanglement and the quantum Fisher information based on partial information or device independent information. We demonstrate the usefulness of our method by concrete examplesComment: 6 pages including 3 figures, 6-page supplement with 2 figures, revtex4; v2: typos corrected, presentation improved, title shortened. For the CoRoNa MATLAB package for convex roof numerical analysis, which has been used for the manuscript, see http://www.mathworks.com/matlabcentral/fileexchange/47823-corona-convex-roof-numerical-analysi

Partial transpose as a direct link between concurrence and negativity

Detection of entanglement in bipartite states is a fundamental task in quantum information. The first method to verify entanglement in mixed states was the partial-transpose criterion. Subsequently, numerous quantifiers for bipartite entanglement were introduced, among them concurrence and negativity. Surprisingly, these quantities are often treated as distinct or independent of each other. The aim of this contribution is to highlight the close relations between these concepts, to show the connections between seemingly independent results, and to present various estimates for the mixed-state concurrence within the same framework.Comment: 10 pages, 3 figure