Pairwise entanglement in the XX model with a magnetic impurity

For a 3-qubit Heisenberg model in a uniform magnetic field, the pairwise thermal entanglement of any two sites is identical due to the exchange symmetry of sites. In this paper we consider the effect of a non-uniform magnetic field on the Heisenberg model, modeling a magnetic impurity on one site. Since pairwise entanglement is calculated by tracing out one of the three sites, the entanglement clearly depends on which site the impurity is located. When the impurity is located on the site which is traced out, that is, when it acts as an external field of the pair, the entanglement can be enhanced to the maximal value 1; while when the field acts on a site of the pair the corresponding concurrence can only be increased from 1/3 to 2/3.Comment: 9 Pages, 4 EPS figures, LaTeX 2

Generation of maximum spin entanglement induced by cavity field in quantum-dot systems

Equivalent-neighbor interactions of the conduction-band electron spins of quantum dots in the model of Imamoglu et al. [Phys. Rev. Lett. 83, 4204 (1999)] are analyzed. Analytical solution and its Schmidt decomposition are found and applied to evaluate how much the initially excited dots can be entangled to the remaining dots if all of them are initially disentangled. It is demonstrated that the perfect maximally entangled states (MES) can only be generated in the systems of up to 6 dots with a single dot initially excited. It is also shown that highly entangled states, approximating the MES with a good accuracy, can still be generated in systems of odd number of dots with almost half of them being excited. A sudden decrease of entanglement is observed by increasing the total number of dots in a system with a fixed number of excitations.Comment: 6 pages, 7 figures, to appear in Phys. Rev.

Quantum walks: a comprehensive review

Quantum walks, the quantum mechanical counterpart of classical random walks, is an advanced tool for building quantum algorithms that has been recently shown to constitute a universal model of quantum computation. Quantum walks is now a solid field of research of quantum computation full of exciting open problems for physicists, computer scientists, mathematicians and engineers. In this paper we review theoretical advances on the foundations of both discrete- and continuous-time quantum walks, together with the role that randomness plays in quantum walks, the connections between the mathematical models of coined discrete quantum walks and continuous quantum walks, the quantumness of quantum walks, a summary of papers published on discrete quantum walks and entanglement as well as a succinct review of experimental proposals and realizations of discrete-time quantum walks. Furthermore, we have reviewed several algorithms based on both discrete- and continuous-time quantum walks as well as a most important result: the computational universality of both continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing Journa

Cortinarius section Bicolores and section Saturnini (Basidiomycota, Agaricales), a morphogenetic overview of European and North American species

Cortinarius is the largest genus of ectomycorrhizal fungi worldwide. Recent molecular studies have shown high levels of morphological homoplasy within the genus. Importantly, DNA phylogenies can reveal characteristics that have been either over- or underemphasized in taxonomic studies. Here we sequenced and phylogenetically analysed a large set of pan-European and North American collections taxonomically studied and placed in Cortinarius sect. Bicolores and sect. Saturnini, according to traditional morpho-anatomical criteria. Our goal was to circumscribe the evolutionary boundaries of the two sections, to stabilize both the limits and nomenclature of relevant species, and to identify described taxa which, according to our current understanding, belong to other lineages. Our analysis resolves two clades: /Bicolores, including 12 species, one of which is new to science, and /Saturnini, including 6 species. Fifteen binomials, traditionally treated in these two sections based on morphology, do not belong to the above two phylogenetic clades. Instead, six of these latter are clearly placed in other clades that represent sect. Bovini, sect. Sciophylli, sect. Duracini and sect. Brunneotincti. The presence or absence of blue pigments and the detection of specific odours emerge as clearly misleading taxonomic features, but more surprisingly, spore size and ecology can be misleading as well. A total of 63 type specimens were sequenced, 4 neotypes and 2 epitypes are proposed here, and 1 new combination is made.Peer reviewe

Elaboration and characterization of thin layers of SiP lamellar alloys

International audienceIn contrast to III-V or II-VI semiconductor alloys, group IV-V alloys have been the subject of much less attention up to now. Theoretical calculations have shown that bulk SiP is an indirect bandgap semiconductor which becomes direct in its 2D form. Moreover, bulk SiP is a lamellar alloy that can be exfoliated, which is of particular interest for new applications in optoelectronics. In this work, we investigate SiPx thin films prepared by evaporation under high vacuum. The films were prepared by co-evaporation of Si from an e-beam gun and P from a GaP decomposition source. The structural and optical properties were investigated by means of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), vibrational spectroscopies (Infrared and Raman) and photoluminescence spectroscopy. After annealing at 1100°C, the structural characterizations reveal the presence of lamellar SiP grains crystallizing in an orthorhombic structure which coexist with Si polycrystals. To further characterize the SiP alloy, DFT calculations have been carried out to get a better understanding of the features observed in both infrared and Raman spectra. The calculated spectra are found to be in excellent agreement with the experimental results. Moreover, temperature dependent photoluminescence measurements strongly suggest that SiP is an indirect bandgap semiconductor with a bandgap energy of 1.47 eV. The obtained value is in good agreement with theoretical values found in the literature

Formation de nanocristaux de SiP2 dans des films minces d'oxydes de silicium SiO1.5 riches en phosphore

International audienceLes nanocristaux à base de Si (NC-Si) insérés dans une matrice diélectrique ont fait l'objet de très nombreuses études durant ces vingt dernières années du fait de leurs propriétés électroniques et optiques originales. De nouvelles fonctionnalités peuvent être attendues s'il est possible de doper ces nanocristaux. Des études théoriques et expérimentales ont montré la possibilité de doper des NC-Si avec du bore (B) ou du phosphore (P). Toutefois, la localisation exacte des dopants et la concentration maximale possible restent encore des questions ouvertes. Dans ce travail, nous étudions les propriétés structurales et optiques de couches minces de SiO1.5 riches en phosphore. Elles sont obtenues par co-évaporation sous vide de SiO, de SiO2 et de phosphore provenant d'une cellule à décomposition de GaP. Les propriétés structurales et optiques ont été étudiées par microscopie électronique en transmission à balayage (STEM), spectroscopie d'absorption infrarouge à transformée de Fourier (FTIR) et Raman. Pour des concentrations en phosphore supérieures à 3 at.%, les caractérisations microstructurales mettent en évidence deux types de nano-objets. D'une part, nous observons la formation d'excroissances à l'interface film/substrat en épitaxie avec le substrat de Si. D'autre part, les films minces contiennent des nanoparticules quasi-sphériques de SiP2 cristallisant dans une structure orthorhombique. Leur taille croît de 15 à 35 nm lorsque la teneur en phosphore augmente de 3 à 10 at.%. L'analyse dispersive en énergie montre que les excroissances contiennent jusque 1 % de P. Par ailleurs, la stoechiométrie des nanoparticules est compatible avec le composé SiP2. La cartographie chimique obtenue par spectroscopie de perte d'énergie a permis de préciser la localisation de chaque élément présent dans nos films. Enfin, les caractérisations Raman combinées avec des calculs basés sur la fonctionnelle de la densité ont permis de donner une première caractérisation spectroscopique des nanoparticules de SiP2

Formation of SiP2 nanoparticles in heavily P-doped SiO1.5 thin films

International audienceSi nanocrystals (Si-NCs) embedded in dielectric matrices have been extensively studied due to potential applications in novel opto-or nanoelectronic devices. To reach this goal, a deep understanding of the electrical doping of Si-NCs is, however, mandatory. Both theoretical and experimental studies have shown the possibility to dope Si-NCs with either Boron (B) or Phosphorus (P). However, the exact location of the dopants in the heavy doping regime has received only little attention up to now. In this work, we investigate heavily phosphorus doped SiO1.5 thin films prepared by evaporation under high vacuum. The films were prepared by co-evaporation of both SiO and SiO2 from e-beam guns while phosphorus was supplied by a GaP decomposition source. The structural and optical properties were studied by means of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), Raman and Fourier transform infrared (FTIR) absorption spectroscopies. For thin films annealed at 1100°C, we provide clear experimental evidence of the formation of both SiP2 and SiP nanoparticles crystallizing in an orthorhombic structure. The overall nanoparticle size increases with the P doping from about 15 to 35 nm. STEM characterizations combined with energy dispersive X-ray spectroscopy measurements indicate that most of the nanoparticles consist of SiP2. This is consistent with Raman measurements and with density functional theory calculations of the SiP2 vibrational properties

Structural and optical properties of SiP thin films

International audienceIn contrast to III-V or II-VI semiconductor alloys, group IV-V alloys have been the subject of much less attention up to now. Very recently, however, alloyed SiPx thin films have gained an increasing interest. Indeed, it was predicted theoretically that a monolayer of SiP is a direct bandgap semiconductor which could be promising for the development of 2D blue light emitting diodes. In this work, we investigate SiPx thin films prepared by evaporation under high vacuum. The films were prepared by co-evaporation of Si from an e-beam gun and P from a GaP decomposition source. The structural and optical properties were investigated by means of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), Fourier transform infrared absorption spectroscopy (FTIR), Raman and photoluminescence spectroscopies. After annealing at 1100°C, the structural characterizations reveal the presence of SiP grains crystallizing in an orthorhombic structure which coexist with Si polycrystals. To further characterize the SiP alloy, DFT calculations have been carried out to get a better understanding of the features observed in both FTIR and Raman spectra. The calculated spectra are found to be in excellent agreement with the experimental results. Moreover, temperature dependent photoluminescence measurements strongly suggest that SiP is an indirect bandgap semiconductor with a bandgap energy of 1.47 eV. The obtained value is in good agreement with theoretical values found in the literature

Cortinarius subgenus Leprocybe in Europe: expanded Sanger and Next Generation Sequencing unveil unexpected diversity in the Mediterranean

Molecular phylogenies in the past decade have demonstrated that the described diversity of Cortinarius is still underestimated, especially outside continental and boreal ecoregions where the genus has been historically investigated. We tackled this issue by revisiting the so far unresolved subgenus Leprocybe, and focused on the largely unexplored Mediterranean hotspot of biodiversity. The sequencing and phylogenetic analysis of 161 vouchered collections from Austria, Cyprus, France, Germany, Italy and Spain, including 16 types, allowed for the delineation of 11 species in this lineage, three of them recognised as new to science and formally introduced as C. jimenezianus, C. selinolens and C. viridans spp. nov., respectively. Interestingly, the newly described species exhibit a strict Mediterranean distribution, and one of them is putatively endemic to the island of Cyprus, highlighting the remarkable potential of this neglected ecoregion to uncover further undescribed diversity of Cortinarius in the future. The present work also unveils 23 synonymies in this subgenus, as well as previously undetected crypticism within C. venetus. Next Generation Sequencing carried out on three old and contaminated holotypes, successfully decrypts their phylogenetic identity, including that of C. leproleptopus, finally settling the long-standing controversy over the taxonomic status of this species. A brief overview of each species in the subgenus is lastly provided and a key is proposed to facilitate the identification of presently known European taxa of Leprocybe in the field