Characterizing multipartite symmetric Dicke states under the effects of noise

We study genuine multipartite entanglement (GME) in a system of n qubits prepared in symmetric Dicke states and subjected to the influences of noise. We provide general, setup-independent expressions for experimentally favorable tools such as fidelity- and collective spin-based entanglement witnesses, as well as entangled-class discriminators and multi-point correlation functions. Besides highlighting the effects of the environment on large qubit registers, we also discuss strategies for the robust detection of GME. Our work provides techniques and results for the experimental communities interested in investigating and characterizing multipartite entangled states by introducing realistic milestones for setup design and associated predictions.

Experimental Realization of Dicke States of up to Six Qubits for Multiparty Quantum Networking

We report the first experimental generation and characterization of a six-photon Dicke state. The produced state shows a fidelity of F=0.56 +/- 0.02 with respect to an ideal Dicke state and violates a witness detecting genuine six-qubit entanglement by 4 standard deviations. We confirm characteristic Dicke properties of our resource and demonstrate its versatility by projecting out four- and five-photon Dicke states, as well as four-photon Greenberger-Horne-Zeilinger and W states. We also show that Dicke states have interesting applications in multiparty quantum networking protocols such as open-destination teleportation, telecloning, and quantum secret sharing.

Experimental Quantum Networking Protocols via Four-Qubit Hyperentangled Dicke States

We report the experimental demonstration of two quantum networking protocols, namely quantum 1 -> 3 telecloning and open-destination teleportation, implemented using a four-qubit register whose state is encoded in a high-quality two-photon hyperentangled Dicke state. The state resource is characterized using criteria based on multipartite entanglement witnesses. We explore the characteristic entanglement-sharing structure of a Dicke state by implementing high-fidelity projections of the four-qubit resource onto lower-dimensional states. Our work demonstrates for the first time the usefulness of Dicke states for quantum information processing.

Dynamics of interacting Dicke model in a coupled-cavity array

We consider the dynamics of an array of mutually interacting cavities, each containing an ensemble of N two-level atoms. By exploring the possibilities offered by ensembles of various dimensions and a range of atom-light and photon-hopping values, we investigate the generation of multisite entanglement, as well as the performance of excitation transfer across the array, resulting from the competition between on-site nonlinearities of the matter-light interaction and intersite photon hopping. In particular, for a three-cavity interacting system it is observed that the initial excitation in the first cavity completely transfers to the ensemble in the third cavity through the hopping of photons between the adjacent cavities. Probabilities of the transfer of excitation of the cavity modes and ensembles exhibit characteristics of fast and slow oscillations governed by coupling and hopping parameters, respectively. In the large-hopping case, by seeding an initial excitation in the cavity at the center of the array, a tripartite W state, as well as a bipartite maximally entangled state, is obtained, depending on the interaction time. Population of the ensemble in a cavity has a positive impact on the rate of excitation transfer between the ensembles and their local cavity modes. In particular, for ensembles of five to seven atoms, tripartite W states can be produced even when the hopping rate is comparable to the cavity-atom coupling rate. A similar behavior of the transfer of excitation is observed for a four-coupled-cavity system with two initial excitations.

Work Extraction and Energy Storage in the Dicke Model

We study work extraction from the Dicke model achieved using simple unitary cyclic transformations keeping into account both a non optimal unitary protocol, and the energetic cost of creating the initial state. By analyzing the role of entanglement, we find that highly entangled states can be inefficient for energy storage when considering the energetic cost of creating the state. Such surprising result holds notwithstanding the fact that the criticality of the model at hand can sensibly improve the extraction of work. While showing the advantages of using a many-body system for work extraction, our results demonstrate that entanglement is not necessarily advantageous for energy storage purposes, when non optimal processes are considered. Our work shows the importance of better understanding the complex interconnections between non-equilibrium thermodynamics of quantum systems and correlations among their subparts.

Single-Element and Competitive Metal Mobility Measured with Column Infiltration and Batch Tests

The distribution coefficient, K-d, is often used to quantify heavy metal mobility in soils. Batch sorption or column infiltration tests may be used to measure K-d. The latter are closer to natural soil conditions, but are difficult to conduct in clays. This difficulty can be overcome by using a laboratory centrifuge. An acceleration of 2600 gravities was applied to columns of London Clay, an Eocene clay sub-stratum, and Cu, Ni, and Zn mobility was measured in centrifuge infiltration tests, both as single elements and in dual competition. Single-element K-d values were also obtained from batch sorption tests, and the results from the two techniques were compared. It was found that K-d values obtained by batch tests vary considerably depending on the metal concentration, while infiltration tests provided a single K-d value for each metal. This was typically in the lower end of the range of the batch test K-d values. For both tests, the order of mobility was Ni > Zn > Cu. Metals became more mobile in competition than when in single-element systems: Ni K-d decreased 3.3 times and Zn K-d 3.4 times when they competed with Cu, while Cu decreased only 1.2 times when in competition with either Ni or Zn. Our study showed that competitive sorption between metals increases the mobility of those metals less strongly bound more than it increases the mobility of more strongly bound metals.

The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611

An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters, NGC3293, NGC4755 and NGC6611. Non-LTE model atmosphere calculations have been used to estimate effective temperatures (from either the helium spectrum or the silicon ionization equilibrium) and gravities (from the hydrogen spectrum). Projected rotational velocities have been deduced from the helium spectrum (for fast and moderate rotators) or the metal line spectrum (for slow rotators). The origin of the low gravity estimates for apparently near main sequence objects is discussed and is related to the stellar rotational velocity. The atmospheric parameters have been used to estimate cluster distances (which are generally in good agreement with previous determinations) and these have been used to estimate stellar luminosities and evolutionary masses. The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed. Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters. Projected rotational velocities for targets in the older clusters, NGC3293 and NGC4755, have been found to be systematically larger than those for the field, confirming recent results in other similar age clusters. The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities. For the relatively unevolved targets in the older clusters, NGC3293 and NGC4755, the peak of the velocity distribution would be 250 km s(-1) with a full-width-half-maximum of approximately 180 km s(-1). For NGC6611, the sample size is relatively small but implies a lower mean rotational velocity. This may be evidence for the spin-down effect due to angular momentum loss through stellar winds, although our results are consistent with those found for very young high mass stars. For all three clusters we deduce present day mass functions with Gamma-values in the range of -1.5 to -1.8, which are similar to other young stellar clusters in the Milky Way.

VLT FORS spectra of blue supergiants in the Local Group galaxy NGC 6822

Half hour exposures using the ESO VLT/FORS1 combination at Paranal in Chile have allowed us to obtain spectra for three B supergiants in the dwarf irregular galaxy NGC 6822. The spectra have been analysed using non-LTE techniques and temperatures, gravities, helium content and abundances have been obtained. Overall the metallicity of NGC 6822 is found to lie between that of the LMC and of the SMC, in agreement with previous observations of H II regions and in contrast to the earlier findings of Massey et al. (1995). The analysis of H-alpha yields estimates of the mass-loss rates and wind momenta. These results demonstrate that significantly longer exposures with the same instruments will allow us to perform quantitative spectroscopy of blue supergiants in galaxies far beyond the Local Group.

The carbon abundance in main-sequence B-type stars towards the Galactic anti-centre

Differential carbon abundances (based on the C II doublet at 6580 Angstrom) are presented for eight early type stars, towards the Galactic anti-centre. All the stars have similar atmospheric parameters with effective temperatures in the range 25000-29000 K and surface gravities between log g = 3.9-4.3 dex. The derived photospheric abundances vary by up to 0.6 dex, and with the exception of one star, RLWT-41, the differential abundances are found to be closely correlated with those of nitrogen. This implies that both elements may have been formed by similar mechanisms and that the lack of correlation between the nitrogen and oxygen abundances previously found in this sample is not directly due to CNO-processed core material being mixed to the stellar surface.

The chemical composition towards the galactic anti-centre .2. Differential abundance analyses and distance determinations

In a previous paper we have published observational data for 6 early B-type stars having, galactocentric distances of between 10 and 18 kpc. Using LTE line-blanketed model at mosphere techniques we derived their atmospheric parameters, finding that all our targets had similar effective temperatures and surface gravities. In the following study we additionally include two stars which have been presented previously (Rolleston et al. 1993) and found also to have compatible atmospheric parameters to the original programme stars. The homogeneity of this sample allows quantitative line-by-line differential abundance analyses to be carried out which should reliably detect variations in the chemical compositions of the stellar photospheres. We present differential abundances for eight stars, in either young open clusters or the field, with respect to an arbitrarily chosen standard which shows a normal abundance pattern. Our method of calculating distances from the derived atmospheric parameters means that the relative distance scale should be accurate.

The chemical composition towards the galactic anti-centre .1. Observations and model atmosphere analyses

High resolution spectra of six early B-type main-sequence stars having galactocentric distances of between 10 and 18 kpc are presented. We List the equivalent widths for the metal lines and illustrate their hydrogen and helium line profiles. The stars are analysed using LTE line-blanketed model atmosphere techniques to derive atmospheric parameters and surface chemical compositions. All six stars have similar effective temperatures and surface gravities, allowing a reliable comparison of their metal abundances and distances. Significant variations in the photospheric abundances are evident and are discuss the need for a more detailed line-by-line differential analysis to exactly quantify the differences. This will be presented in a companion paper (Smartt et al. 1996).

Modelling the H Lyman lines in evolved late-type stars

The importance of partial redistribution (PRD) in the modelling of the Lyman a and Lyman ß emission lines of hydrogen in stellar atmospheres is examined using simple atmospheric models of a range of late-type stars. These models represent the subgiant Procyon (F5 IV-V), and the two giants ß Gem (K0 III) and a Tau (K5 III). These stars are selected to span a wide range of surface gravities: 1.25 <log g <4.00. The calculations are performed using the computer code MULTI with the modifications made by Hubeny & Lites. It is found that PRD effects are highly significant, both in the direct prediction of the Lyman line profiles and in the application of hydrostatic equilibrium to calculate the atmospheric electron density in static atmospheric models.

Tomographic characterisation of correlations in a photonic tripartite state

Starting from a four-partite photonic hyper-entangled Dicke resource, we report the full tomographic characterization of three-, two-, and one-qubit states obtained by projecting out part of the computational register. The reduced states thus obtained correspond to fidelities with the expected states larger than 87%, therefore certifying the faithfulness of the entanglement-sharing structure within the original four-qubit resource. The high quality of the reduced three-qubit state allows for the experimental verification of the Koashi-Winter relation for the monogamy of correlations within a tripartite state. We show that, by exploiting the symmetries of the three-qubit state obtained upon projection over the four-qubit Dicke resource, such relation can be experimentally fully characterized using only 5 measurement settings. We highlight the limitations of such approach and sketch an experimentally-oriented way to overcome them.

Stiffness Analysis and Experiment of a Novel 5-DoF Parallel Kinematic Machine Considering Gravitational Effects

In order to carry out high-precision machining of aerospace structural components with large size, thin wall and complex surface, this paper proposes a novel parallel kinematic machine (PKM) and formulates its semi-analytical theoretical stiffness model considering gravitational effects that is verified by stiffness experiments. From the viewpoint of topology structure, the novel PKM consists of two substructures in terms of the redundant and overconstrained parallel mechanisms that are connected by two interlinked revolute joints. The theoretical stiffness model of the novel PKM is established based upon the virtual work principle and deformation superposition principle after mapping the stiffness models of substructures from joint space to operated space by Jacobian matrices and considering the deformation contributions of interlinked revolute joints to two substructures. Meanwhile, the component gravities are treated as external payloads exerting on the end reference point of the novel PKM resorting to static equivalence principle. This approach is proved by comparing the theoretical stiffness values with experimental stiffness values in the same configurations, which also indicates equivalent gravity can be employed to describe the actual distributed gravities in an acceptable accuracy manner. Finally, on the basis of the verified theoretical stiffness model, the stiffness distributions of the novel PKM are illustrated and the contributions of component gravities to the stiffness of the novel PKM are discussed.

Experimental investigation of flow and segregation behaviour of bulk solids in silos under high gravity conditions

Many researchers have investigated the flow and segregation behaviour in model scale experimental silos at normal gravity conditions. However it is known that the stresses experienced by the bulk solid in industrial silos are high when compared to model silos. Therefore it is important to understand the effect of stress level on flow and segregation behaviour and establish the scaling laws governing this behaviour. The objective of this paper is to understand the effect of gravity on the flow and segregation behaviour of bulk solids in a silo centrifuge model. The materials used were two mixtures composed of Polyamide and glass beads. The discharge of two bi-disperse bulk solids in a silo centrifuge model were recorded under accelerations ranging from 1g to 15g. The velocity distribution during discharge was evaluated using Particle Image Velocimetry (PIV) techniques and the concentration distribution of large and small particles were obtained by imaging processing techniques. The flow and segregation behaviour at high gravities were then quantified and compared with the empirical equations available in the literature.