Quantum entanglement in states generated by bilocal group algebras

Given a finite group G with a bilocal representation, we investigate the bipartite entanglement in the state constructed from the group algebra of G acting on a separable reference state. We find an upper bound for the von Neumann entropy for a bipartition (A,B) of a quantum system and conditions to saturate it. We show that these states can be interpreted as ground states of generic Hamiltonians or as the physical states in a quantum gauge theory and that under specific conditions their geometric entropy satisfies the entropic area law. If G is a group of spin flips acting on a set of qubits, these states are locally equivalent to 2-colorable (i.e., bipartite) graph states and they include GHZ, cluster states etc. Examples include an application to qudits and a calculation of the n-tangle for 2-colorable graph states.Comment: 9 pages, no figs; updated to the published versio

Koszul-Tate Cohomology For an Sp(2)-Covariant Quantization of Gauge Theories with Linearly Dependent Generators

The anti-BRST transformation, in its Sp(2)-symmetric version, for the general case of any stage-reducible gauge theories is implemented in the usual BV approach. This task is accomplished not by duplicating the gauge symmetries but rather by duplicating all fields and antifields of the theory and by imposing the acyclicity of the Koszul-Tate differential. In this way the Sp(2)-covariant quantization can be realised in the standard BV approach and its equivalence with BLT quantization can be proven by a special gauge fixing procedure.Comment: 13 pages, Latex, To Be Published in International Journal of Modern Physics

Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators

We study the magnetization dynamics in a nanocontact magnetic vortex oscillators as function of temperature. Low temperature experiments reveal that the dynamics at low and high currents differ qualitatively. At low currents, we excite a temperature independent standard oscillation mode, consisting in the gyrotropic motion of a free layer vortex about the nanocontact. Above a critical current, a sudden jump of the frequency is observed, concomitant with a substantial increase of the frequency versus current slope factor. Using micromagnetic simulation and analytical modeling, we associate this new regime to the creation of a vortex-antivortex pair in the pinned layer of the spin valve. The vortex-antivortex distance depends on the Oersted field which favors a separation, and on the exchange bias field, which favors pair merging. The pair in the pinned layer provides an additional spin torque altering the dynamics of the free layer vortex, which can be quantitatively accounted for by an analytical model

On a Petrov-type D homogeneous solution

We present a new two-parameter family of solutions of Einstein gravity with negative cosmological constant in 2+1 dimensions. These solutions are obtained by squashing the anti-de Sitter geometry along one direction and posses four Killing vectors. Global properties as well as the four dimensional generalization are discussed, followed by the investigation of the geodesic motion. A simple global embedding of these spaces as the intersection of four quadratic surfaces in a seven dimensional space is obtained. We argue also that these geometries describe the boundary of a four dimensional nutty-bubble solution and are relevant in the context of AdS/CFT correspondence.Comment: 20 pages, TeX fil

Single spin measurement using spin-orbital entanglement

Single spin measurement represents a major challenge for spin-based quantum computation. In this article we propose a new method for measuring the spin of a single electron confined in a quantum dot (QD). Our strategy is based on entangling (using unitary gates) the spin and orbital degrees of freedom. An {\em orbital qubit}, defined by a second, empty QD, is used as an ancilla and is prepared in a known initial state. Measuring the orbital qubit will reveal the state of the (unknown) initial spin qubit, hence reducing the problem to the easier task of single charge measurement. Since spin-charge conversion is done with unit probability, single-shot measurement of an electronic spin can be, in principle, achieved. We evaluate the robustness of our method against various sources of error and discuss briefly possible implementations.Comment: RevTeX4, 4 pages, some figs; updated to the published versio

A Study on the Dependence of Water Tree Permittivity with Time

During the growth of water trees in the insulation of a cable the distribution of the electric field is modified because of the local change of the dielectric properties of the material. It results a local enhancement of the electric field which could increase the risk of breakdown. The key factor is the permittivity of the water tree and the aim of the work is to determine its possible values and, particularly, the law of its increase with time during the of the trees. The paper presents permittivity measurements in uniform field in MV and powe

Ab initio study of ultrafast spin dynamics in Gd_x(FeCo)_1−x alloys

Using an ultrashort laser pulse, we explore ab initio the spin dynamics of Gdx(FeCo)1-x at femtosecond time scales. Optical excitations are found to drive charges from Fe majority d-states to unoccupied Gd f-minority states with f-electron character excited occupation lagging behind that of the d-electron character, leading to substantial demagnetization of both species while leaving the global moment almost unchanged. For x > 0.33, this results in the creation of an ultrafast ferromagnetic (FM) transient by the end of the laser pulse with the Gd demagnetization rate slower than that of Fe. For all concentrations, the Gd moments begin to rotate from their ground state orientations developing in-plane moments of between 0.2 and 0.5 μB. Thus, the ultrafast spin dynamics of the material captures three important ingredients of all optical switching that occurs at much later (picosecond) times: (i) the development of a FM transient, (ii) the different rates of demagnetization of Fe and Gd, and (iii) the breaking of the collinear symmetry of the ground state. Furthermore, several predictions are made about the behavior of Fe-Gd alloys that can be experimentally tested and can lead to a spin-filtering device

Ciliary Neurotrophic Factor Induces Genes Associated with Inflammation and Gliosis in the Retina: A Gene Profiling Study of Flow-Sorted, Müller Cells

Ciliary neurotrophic factor (CNTF), a member of the interleukin-6 cytokine family, has been implicated in the development, differentiation and survival of retinal neurons. The mechanisms of CNTF action as well as its cellular targets in the retina are poorly understood. It has been postulated that some of the biological effects of CNTF are mediated through its action via retinal glial cells; however, molecular changes in retinal glia induced by CNTF have not been elucidated. We have, therefore, examined gene expression dynamics of purified Müller (glial) cells exposed to CNTF in vivo.Müller cells were flow-sorted from mgfap-egfp transgenic mice one or three days after intravitreal injection of CNTF. Microarray analysis using RNA from purified Müller cells showed differential expression of almost 1,000 transcripts with two- to seventeen-fold change in response to CNTF. A comparison of transcriptional profiles from Müller cells at one or three days after CNTF treatment showed an increase in the number of transcribed genes as well as a change in the expression pattern. Ingenuity Pathway Analysis showed that the differentially regulated genes belong to distinct functional types such as cytokines, growth factors, G-protein coupled receptors, transporters and ion channels. Interestingly, many genes induced by CNTF were also highly expressed in reactive Müller cells from mice with inherited or experimentally induced retinal degeneration. Further analysis of gene profiles revealed 20-30% overlap in the transcription pattern among Müller cells, astrocytes and the RPE.Our studies provide novel molecular insights into biological functions of Müller glial cells in mediating cytokine response. We suggest that CNTF remodels the gene expression profile of Müller cells leading to induction of networks associated with transcription, cell cycle regulation and inflammatory response. CNTF also appears to function as an inducer of gliosis in the retina