Interplay between computable measures of entanglement and other quantum correlations

Composite quantum systems can be in generic states characterized not only by entanglement, but also by more general quantum correlations. The interplay between these two signatures of nonclassicality is still not completely understood. In this work we investigate this issue focusing on computable and observable measures of such correlations: entanglement is quantified by the negativity N, while general quantum correlations are measured by the (normalized) geometric quantum discord D_G. For two-qubit systems, we find that the geometric discord reduces to the squared negativity on pure states, while the relationship $D_G \geq N^2$ holds for arbitrary mixed states. The latter result is rigorously extended to pure, Werner and isotropic states of two-qudit systems for arbitrary d, and numerical evidence of its validity for arbitrary states of a qubit and a qutrit is provided as well. Our results establish an interesting hierarchy, that we conjecture to be universal, between two relevant and experimentally friendly nonclassicality indicators. This ties in with the intuition that general quantum correlations should at least contain and in general exceed entanglement on mixed states of composite quantum systems.Comment: 10 pages, 4 figure

The GRAAL high resolution BGO calorimeter and its energy calibration and monitoring system

We describe the electromagnetic calorimeter built for the GRAAL apparatus at the ESRF. Its monitoring system is presented in detail. Results from tests and the performance obtained during the first GRAAL experiments are given. The energy calibration accuracy and stability reached is a small fraction of the intrinsic detector resolution.Comment: 19 pages, 14 figures, submitted to Nuclear Instruments and Method

Deuteron photo-disintegration with polarised photons in the energy range 30 - 50 MeV

The reaction d(\vec\gamma,np) has been studied using the tagged and polarised LADON gamma ray beam at an energy 30 - 50 MeV to investigate the existence of narrow dibaryonic resonances recently suggested from the experimental measurements in a different laboratory. The beam was obtained by Compton back-scattering of laser light on the electrons of the storage ring ADONE. Photo-neutron yields were measured at five neutron angle \vartheta_n = 22, 55.5, 90, 125 and 157 degrees in the center of mass system.Our results do not support the existence of such resonances.Comment: 16 pages, Latex, 22 figures, 1 table. Nucl. Phys. A to appea

Large-Scale Distributed Bayesian Matrix Factorization using Stochastic Gradient MCMC

Despite having various attractive qualities such as high prediction accuracy and the ability to quantify uncertainty and avoid over-fitting, Bayesian Matrix Factorization has not been widely adopted because of the prohibitive cost of inference. In this paper, we propose a scalable distributed Bayesian matrix factorization algorithm using stochastic gradient MCMC. Our algorithm, based on Distributed Stochastic Gradient Langevin Dynamics, can not only match the prediction accuracy of standard MCMC methods like Gibbs sampling, but at the same time is as fast and simple as stochastic gradient descent. In our experiments, we show that our algorithm can achieve the same level of prediction accuracy as Gibbs sampling an order of magnitude faster. We also show that our method reduces the prediction error as fast as distributed stochastic gradient descent, achieving a 4.1% improvement in RMSE for the Netflix dataset and an 1.8% for the Yahoo music dataset

An efficient and principled method for detecting communities in networks

A fundamental problem in the analysis of network data is the detection of network communities, groups of densely interconnected nodes, which may be overlapping or disjoint. Here we describe a method for finding overlapping communities based on a principled statistical approach using generative network models. We show how the method can be implemented using a fast, closed-form expectation-maximization algorithm that allows us to analyze networks of millions of nodes in reasonable running times. We test the method both on real-world networks and on synthetic benchmarks and find that it gives results competitive with previous methods. We also show that the same approach can be used to extract nonoverlapping community divisions via a relaxation method, and demonstrate that the algorithm is competitively fast and accurate for the nonoverlapping problem.Comment: 14 pages, 5 figures, 1 tabl

Entanglement distribution and quantum discord

Establishing entanglement between distant parties is one of the most important problems of quantum technology, since long-distance entanglement is an essential part of such fundamental tasks as quantum cryptography or quantum teleportation. In this lecture we review basic properties of entanglement and quantum discord, and discuss recent results on entanglement distribution and the role of quantum discord therein. We also review entanglement distribution with separable states, and discuss important problems which still remain open. One such open problem is a possible advantage of indirect entanglement distribution, when compared to direct distribution protocols.Comment: 7 pages, 2 figures, contribution to "Lectures on general quantum correlations and their applications", edited by Felipe Fanchini, Diogo Soares-Pinto, and Gerardo Adess

Polarization Asymmetry In The Photodisintegration Of The Deuteron

The reaction ²(γ,p)n has been studied using a monochromatic and polarized gamma ray beam at energies E(γ)=19.8, 29.0, 38.6, and 60.8 MeV. The beam of an intensity ∼4×10⁵ γ/sec was obtained by Compton back scattering of mode-locked laser light off electron bunches in the Adone storage ring. Photoneutron yields were measured at nine neutron angles thetan≃15, 30, 45, 60, 90, 120, 135, 150, and 165 deg in the center of mass (c.m.) for E(γ)=19.8, 29.0, and 38.6 MeV, and at thetan≃30, 60, 90, 120, and 150 deg c.m. for E(γ)=60.8 MeV. The polarization independent component Iₒ(theta) of the differential cross section and the polarization dependent component PI₁(theta) were deduced and the angular distribution of the azimuthal asymmetry factor Σ(theta)=I₁(theta)/Iₒ(theta) was obtained. An extensive comparison with theory has been carried out and the inclusion of corrections due to meson exchange currents and to Δ-isobar configurations have been shown to be mandatory at energies E(γ)≳40 MeV. Theoretical and experimental implications of intermediate energy deuteron photo- disintegration studies are discussed in some detail

Polarization Asymmetry In The Photodisintegration Of The Deuteron

The reaction ²(γ,p)n has been studied using a monochromatic and polarized gamma ray beam at energies E(γ)=19.8, 29.0, 38.6, and 60.8 MeV. The beam of an intensity ∼4×10⁵ γ/sec was obtained by Compton back scattering of mode-locked laser light off electron bunches in the Adone storage ring. Photoneutron yields were measured at nine neutron angles thetan≃15, 30, 45, 60, 90, 120, 135, 150, and 165 deg in the center of mass (c.m.) for E(γ)=19.8, 29.0, and 38.6 MeV, and at thetan≃30, 60, 90, 120, and 150 deg c.m. for E(γ)=60.8 MeV. The polarization independent component Iₒ(theta) of the differential cross section and the polarization dependent component PI₁(theta) were deduced and the angular distribution of the azimuthal asymmetry factor Σ(theta)=I₁(theta)/Iₒ(theta) was obtained. An extensive comparison with theory has been carried out and the inclusion of corrections due to meson exchange currents and to Δ-isobar configurations have been shown to be mandatory at energies E(γ)≳40 MeV. Theoretical and experimental implications of intermediate energy deuteron photo- disintegration studies are discussed in some detail

Eta photoproduction off the neutron at GRAAL: Evidence for a resonant structure at W=1.67 GeV

New (preliminary) data on eta photoproduction off the neutron are presented. These data reveal a resonant structure at W=1.67 GeV.Comment: 8 pages, 4 figures. Published in Proceedings of Workshop on the Physics of Excited Nucleons NSTAR2004, Grenoble, France, March 24 - 27, pg.19

Conditions for the freezing phenomena of geometric measure of quantum discord for arbitrary two-qubit X states under non-dissipative dephasing noises

We study the dynamics of geometric measure of quantum discord (GMQD) under the influences of two local phase damping noises. Consider the two qubits initially in arbitrary X-states, we find the necessary and sufficient conditions for which GMQD is unaffected for a finite period. It is further shown that such results also hold for the non-Markovian dephasing process.Comment: 4 pages, 2 figure