Quantum repeaters with individual rare-earth ions at telecommunication wavelengths

We present a quantum repeater scheme that is based on individual erbium and europium ions. Erbium ions are attractive because they emit photons at telecommunication wavelength, while europium ions offer exceptional spin coherence for long-term storage. Entanglement between distant erbium ions is created by photon detection. The photon emission rate of each erbium ion is enhanced by a microcavity with high Purcell factor, as has recently been demonstrated. Entanglement is then transferred to nearby europium ions for storage. Gate operations between nearby ions are performed using dynamically controlled electric-dipole coupling. These gate operations allow entanglement swapping to be employed in order to extend the distance over which entanglement is distributed. The deterministic character of the gate operations allows improved entanglement distribution rates in comparison to atomic ensemble-based protocols. We also propose an approach that utilizes multiplexing in order to enhance the entanglement distribution rate.Comment: 13 pages, 4 figure

Numerical Analysis of Quasiholes of the Moore-Read Wavefunction

We demonstrate numerically that non-Abelian quasihole excitations of the $\nu = 5/2$ fractional quantum Hall state have some of the key properties necessary to support quantum computation. We find that as the quasihole spacing is increased, the unitary transformation which describes winding two quasiholes around each other converges exponentially to its asymptotic limit and that the two orthogonal wavefunctions describing a system with four quasiholes become exponentially degenerate. We calculate the length scales for these two decays to be $\xi_{U} \approx 2.7 \ell_0$ and $\xi_{E} \approx 2.3 \ell_0$ respectively. Additionally we determine which fusion channel is lower in energy when two quasiholes are brought close together.Comment: 4 pages, 3 figure

Complex networks created by aggregation

We study aggregation as a mechanism for the creation of complex networks. In this evolution process vertices merge together, which increases the number of highly connected hubs. We study a range of complex network architectures produced by the aggregation. Fat-tailed (in particular, scale-free) distributions of connections are obtained both for networks with a finite number of vertices and growing networks. We observe a strong variation of a network structure with growing density of connections and find the phase transition of the condensation of edges. Finally, we demonstrate the importance of structural correlations in these networks.Comment: 12 pages, 13 figure

The Schwinger Representation of a Group: Concept and Applications

The concept of the Schwinger Representation of a finite or compact simple Lie group is set up as a multiplicity-free direct sum of all the unitary irreducible representations of the group. This is abstracted from the properties of the Schwinger oscillator construction for SU(2), and its relevance in several quantum mechanical contexts is highlighted. The Schwinger representations for $SU(2), SO(3)$ and SU(n) for all $n$ are constructed via specific carrier spaces and group actions. In the SU(2) case connections to the oscillator construction and to Majorana's theorem on pure states for any spin are worked out. The role of the Schwinger Representation in setting up the Wigner-Weyl isomorphism for quantum mechanics on a compact simple Lie group is brought out.Comment: Latex, 17 page

The Carbon content in the Galactic CygnusX/DR21 star forming region

Observations of Carbon bearing species are among the most important diagnostic probes of ongoing star formation. CO is a surrogate for H$_2$ and is found in the vicinity of star formation sites. There, [CI] emission is thought to outline the dense molecular cores and extend into the lower density regions, where the impinging interstellar UV radiation field plays a critical role for the dissociation and ionization processes. Emission of ionized carbon ([CII]) is found to be even more extended than [CI] and is linking up with the ionized medium. These different tracers emphasize the importance of multi-wavelength studies to draw a coherent picture of the processes driving and driven by high mass star formation. Until now, large scale surveys were only done with low resolution, such as the COBE full sky survey, or were biased to a few selected bright sources (e.g. Yamamoto et al. 2001, Schneider et al. 2003). A broader basis of unbiased, high-resolution observations of [CI], CO, and [CII] may play a key role to probe the material processed by UV radiation.Comment: 4 pages, 4 figure, to appear in "Proceedings of the 4th Cologne-Bonn-Zermatt-Symposium", ed. S. Pfalzner, C. Kramer, C. Straubmeier, and A. Heithausen (Springer Verlag

A generalized Pancharatnam geometric phase formula for three level systems

We describe a generalisation of the well known Pancharatnam geometric phase formula for two level systems, to evolution of a three-level system along a geodesic triangle in state space. This is achieved by using a recently developed generalisation of the Poincare sphere method, to represent pure states of a three-level quantum system in a convenient geometrical manner. The construction depends on the properties of the group SU(3)\/ and its generators in the defining representation, and uses geometrical objects and operations in an eight dimensional real Euclidean space. Implications for an n-level system are also discussed.Comment: 12 pages, Revtex, one figure, epsf used for figure insertio

Comment on "Scaling of the quasiparticle spectrum for d-wave superconductors"

In a recent Letter Simon and Lee suggested a scaling law for thermodynamic and kinetic properties of superconductors with lines of gap nodes. However their crossover parameter between the bulk dominated regime and the vortex dominated regime is different from that found in our paper (N.B. Kopnin and G.E. Volovik, JETP Lett., {\bf 64}, 690 (1996); see also cond-mat/9702093). We discuss the origin of the disagreement.Comment: submitted to Physical Review Letters as "Comment" to the paper by S.H. Simon and P.A. Lee, Phys. Rev. Lett., 78 (1997) 1548 (cond-mat/9611133

Surface abundances of ON stars

Massive stars burn hydrogen through the CNO cycle during most of their evolution. When mixing is efficient, or when mass transfer in binary systems happens, chemically processed material is observed at the surface of O and B stars. ON stars show stronger lines of nitrogen than morphologically normal counterparts. Whether this corresponds to the presence of material processed through the CNO cycle or not is not known. Our goal is to answer this question. We perform a spectroscopic analysis of a sample of ON stars with atmosphere models. We determine the fundamental parameters as well as the He, C, N, and O surface abundances. We also measure the projected rotational velocities. We compare the properties of the ON stars to those of normal O stars. We show that ON stars are usually helium-rich. Their CNO surface abundances are fully consistent with predictions of nucleosynthesis. ON stars are more chemically evolved and rotate - on average - faster than normal O stars. Evolutionary models including rotation cannot account for the extreme enrichment observed among ON main sequence stars. Some ON stars are members of binary systems, but others are single stars as indicated by stable radial velocities. Hence, mass transfer is not a simple explanation for the observed chemical properties. We conclude that ON stars show extreme chemical enrichment at their surface, consistent with nucleosynthesis through the CNO cycle. Its origin is not clear at present.Comment: 18 pages, 10 figures (+ appendix). A&A accepte

Arboriculture Biologique : 11 années d'expérimentation en vergers de pêchers et pommiers

Un programme de recherche a été conduit en arboriculture biologique pendant 11 ans (1994-2004) à l'unité expérimentale INRA (Institut National de la Recherche Agronomique) de Gotheron dans le Sud-Est de la France, en partenariat avec Agribiodrôme (F-26150 Die), asssociation de développement de l'Agriculture Biologique (AB) en Drôme, et le GRAB (Groupe de Recherche en Agriculture Biologique, F-84911 Avignon). Les objectifs étaient : (i) acquérir des références en arboriculture biologique ; (ii) identifier et analyser les verrous techniques de la production de pêches et pommes biologiques, afin d'optimiser la conduite des vergers AB et/ou de proposer de nouvelles expérimentations ; (iii) évaluer à long terme l'impact du mode de production pour le sol et la communauté d'arthropodes du verger. Le suivi longitudinal réalisé dans les 3 vergers expérimentaux de pêchers et pommiers a permis de souligner certaines contraintes en culture pérenne AB, notamment pour la conduite du sol (entretien de la fertilité, enracinement limité par le travail du sol sur le rang pour limiter les adventices) et la protection du verger. Probablement en rapport avec une fertilisation modérée et une conduite de la protection favorable à des régulations naturelles, certains ravageurs ne se sont pas développés : acariens phytophages, pucerons du pêcher,... D'autres bio-agresseurs ont pu être contrôlés par l'utilisation de méthodes biotechnique ou microbiologique : confusion sexuelle (tordeuse orientale), virus de la granulose (carpocapse),... En revanche, la maîtrise de certains bio-agresseurs a nécessité l'utilisation d'importantes quantités de matières actives présentant des effets non intentionnels (cuivre contre la cloque du pêcher, soufre contre la tavelure du pommier) et/ou a été insuffisante (puceron cendré du pommier, maladies de conservation du pêcher). Le suivi de la teneur en nitrates de la solution du sol à différentes profondeurs a permis d'établir que le risque de lessivage, et donc de pollution par les nitrates, était nul pour notre stratégie de fertilisation et notre situation. Enfin, l'étude de la communauté des arthropodes dans le verger de pommiers AB (2001-2003) a mis en évidence une biomasse plus élevée et un cortège d'auxiliaires de structure différente (prédominance de prédateurs polyphages) par rapport à d'autres vergers conduits en conventionnel. En raison d'une efficacité limitée et/ou d'effets non intentionnels de la lutte directe contre les bio-agresseurs en verger, il est donc important de concevoir et de gérer le verger AB afin de limiter le recours aux intrants phytosanitaires : variétés, distances de plantation, architecture de l'arbre, aménagement de l'environnement du verger, prophylaxie,..

Phase-Conjugated Inputs Quantum Cloning Machines

A quantum cloning machine is introduced that yields $M$ identical optimal clones from $N$ replicas of a coherent state and $N'$ replicas of its phase conjugate. It also optimally produces $M'=M+N'-N$ phase-conjugated clones at no cost. For well chosen input asymmetries $N'/(N+N')$, this machine is shown to provide better cloning fidelities than the standard $(N+N') \to M$ cloner. The special cases of the optimal balanced cloner ($N=N'$) and the optimal measurement ($M=\infty$) are investigated.Comment: 4 pages (RevTex), 2 figure