Spectral decomposition of Bell's operators for qubits

The spectral decomposition is given for the N-qubit Bell operators with two observables per qubit. It is found that the eigenstates (when non-degenerate) are N-qubit GHZ states even for those operators that do not allow the maximal violation of the corresponding inequality. We present two applications of this analysis. In particular, we discuss the existence of pure entangled states that do not violate any Mermin-Klyshko inequality for $N\geq 3$.Comment: 12 pages, 1 figure

Interference due to Coherence Swapping

We propose a method called `coherence swapping' which enables us to create superposition of a particle in two distinct paths, which is fed with initially incoherent, independent radiations. This phenomenon is also present for the charged particles, and can be used to swap the effect of flux line due to Aharonov-Bohm effect. We propose an optical version of the experimental set-up to test the coherence swapping. The phenomenon, which is simpler than entanglement swapping or teleportation, raises some fundamental questions about true nature of wave-particle duality, and also opens up the possibility of studying the quantum erasure from a new angle.Comment: Latex file, 10 pages, Two figure

Keeping matter in the loop in dS3_3 quantum gravity

We propose a mechanism that couples matter fields to three-dimensional de Sitter quantum gravity. Our construction is based on the Chern-Simons formulation of three-dimensional Euclidean gravity, and it centers on a collection of Wilson loops winding around Euclidean de Sitter space. We coin this object a Wilson spool. To construct the spool, we build novel representations of $\mathfrak{su}(2)$. To evaluate the spool, we adapt and exploit several known exact results in Chern-Simons theory. Our proposal correctly reproduces the one-loop determinant of a free massive scalar field on $S^3$ as $G_N\to 0$. Moreover, allowing for quantum metric fluctuations, it can be systematically evaluated to any order in perturbation theory.Comment: 48 pages + appendices; v2: final version for JHEP, minor edit

Entanglement via Barut-Girardello coherent state for suq(1,1)su_{q}(1, 1) quantum algebra: bipartite composite system

Using noncocommutative coproduct properties of the quantum algebras, we introduce and obtain, in a bipartite composite system, the Barut-Girardello coherent state for the q-deformed $su_{q}(1,1)$ algebra. The quantum coproduct structure ensures this normalizable coherent state to be entangled. The entanglement disappears in the classical $q \to 1$ limit, giving rise to a factorizable state.Comment: 12 page

Nonsequential positive-operator-valued measurements on entangled mixed states do not always violate a Bell inequality

We present a local-hidden-variable model for positive-operator-valued measurements (an LHVPOV model) on a class of entangled generalized Werner states, thus demonstrating that such measurements do not always violate a Bell-type inequality. We also show that, in general, if the state $\rho'$ can be obtained from $\rho$ with certainty by local quantum operations without classical communication then an LHVPOV model for the state $\rho$ implies the existence of such a model for $\rho'$.Comment: 4 pages, no figures. Title changed to accord with Phys. Rev. A version. Journal reference adde

Nitrogen as Annihilation Centre for Point Defects in Implanted Silicon

The accumulation of radiation defects in silicon implanted with 150 keV N+ ions at high ion current density (20 A cm-2) and low density (0.05 A cm-2) was investigated by means of X-ray double-crystal spectrometer and EPR method. At high ion current density the radiation defects accumulate up to amorphization at the ion dose of 11015 cm-2. At low ion current density the curve of lattice parameter change on dose has oscillatory view and amorphization of the layer is not achieved at least up to ion dose of 1.41016 cm-2. The processes of the nitrogen atoms capture on the vacancy defects and Watkins displacement of them from the nodes work as additional channel of radiation defect annihilation. At high ion current densities and at high level of ionization in the implanted layer process of Watkins substitution is suppressed

Optimal approach to quantum communication using dynamic programming

Reliable preparation of entanglement between distant systems is an outstanding problem in quantum information science and quantum communication. In practice, this has to be accomplished via noisy channels (such as optical fibers) that generally result in exponential attenuation of quantum signals at large distances. A special class of quantum error correction protocols--quantum repeater protocols--can be used to overcome such losses. In this work, we introduce a method for systematically optimizing existing protocols and developing new, more efficient protocols. Our approach makes use of a dynamic programming-based searching algorithm, the complexity of which scales only polynomially with the communication distance, letting us efficiently determine near-optimal solutions. We find significant improvements in both the speed and the final state fidelity for preparing long distance entangled states.Comment: 9 pages, 6 figure