Entanglement of photons

It is argued that the title of this paper represents a misconception. Contrary to widespread beliefs it is electromagnetic field modes that are ``systems'' and can be entangled, not photons. The amount of entanglement in a given state is shown to depend on redefinitions of the modes; we calculate the minimum and maximum over all such redefinitions for several examples.Comment: 5 pages ReVTe

Experimental Proposal for Achieving Superadditive Communication Capacities with a Binary Quantum Alphabet

We demonstrate superadditivity in the communication capacity of a binary alphabet consisting of two nonorthogonal quantum states. For this scheme, collective decoding is performed two transmissions at a time. This improves upon the previous schemes of Sasaki et al. [Phys. Rev. A 58, 146 (1998)] where superadditivity was not achieved until a decoding of three or more transmissions at a time. This places superadditivity within the regime of a near-term laboratory demonstration. We propose an experimental test based upon an alphabet of low photon-number coherent states where the signal decoding is done with atomic state measurements on a single atom in a high-finesse optical cavity.Comment: 7 pages, 5 figure

Quantum Cryptography Using Single Particle Entanglement

A quantum cryptography scheme based on entanglement between a single particle state and a vacuum state is proposed. The scheme utilizes linear optics devices to detect the superposition of the vacuum and single particle states. Existence of an eavesdropper can be detected by using a variant of Bell's inequality.Comment: 4 pages, 3figures, revte

Strong subadditivity inequality for quantum entropies and four-particle entanglement

Strong subadditivity inequality for a three-particle composite system is an important inequality in quantum information theory which can be studied via a four-particle entangled state. We use two three-level atoms in $\Lambda$ configuration interacting with a two-mode cavity and the Raman adiabatic passage technique for the production of the four-particle entangled state. Using this four-particle entanglement, we study for the first time various aspects of the strong subadditivity inequality.Comment: 5 pages, 3 figures, RevTeX4, submitted to PR

Single-particle nonlocality and entanglement with the vacuum

We propose a single-particle experiment that is equivalent to the conventional two-particle experiment used to demonstrate a violation of Bell's inequalities. Hence, we argue that quantum mechanical nonlocality can be demonstrated by single-particle states. The validity of such a claim has been discussed in the literature, but without reaching a clear consensus. We show that the disagreement can be traced to what part of the total state of the experiment one assigns to the (macroscopic) measurement apparatus. However, with a conventional and legitimate interpretation of the measurement process one is led to the conclusion that even a single particle can show nonlocal properties.Comment: 6 pages, 5 figure

Output state in multiple entanglement swapping

The technique of quantum repeaters is a promising candidate for sending quantum states over long distances through a lossy channel. The usual discussions of this technique deals with only a finite dimensional Hilbert space. However the qubits with which one implements this procedure will "ride" on continuous degrees of freedom of the carrier particles. Here we analyze the action of quantum repeaters using a model based on pulsed parametric down conversion entanglement swapping. Our model contains some basic traits of a real experiment. We show that the state created, after the use of any number of parametric down converters in a series of entanglement swappings, is always an entangled (actually distillable) state, although of a different form than the one that is usually assumed. Furthermore, the output state always violates a Bell inequality.Comment: 11 pages, 6 figures, RevTeX

Many body physics from a quantum information perspective

The quantum information approach to many body physics has been very successful in giving new insight and novel numerical methods. In these lecture notes we take a vertical view of the subject, starting from general concepts and at each step delving into applications or consequences of a particular topic. We first review some general quantum information concepts like entanglement and entanglement measures, which leads us to entanglement area laws. We then continue with one of the most famous examples of area-law abiding states: matrix product states, and tensor product states in general. Of these, we choose one example (classical superposition states) to introduce recent developments on a novel quantum many body approach: quantum kinetic Ising models. We conclude with a brief outlook of the field.Comment: Lectures from the Les Houches School on "Modern theories of correlated electron systems". Improved version new references adde

Ecballocystis pulvinata Bohlin (Hormotilaceae, Chlorophyta): morfologia externa e ocorrência em ambientes lóticos da região Sudeste do Brasil

O gênero Ecballocystis foi proposto por Bohlin, em 1897, a partir da descrição da espécie Ecballocystis pulvinata coletada no Brasil (Rio Grande do Sul). O gênero é considerado de rara ocorrência no mundo. No Brasil, E. pulvinata tem sido citada com maior frequência. Diversos espécimes de E. pulvinata provenientes de diferentes riachos foram avaliados taxonomicamente por meio de estudos morfométricos e verificou-se a ocorrência desta espécie em relação a algumas variáveis ambientais. Considerável amplitude morfológica foi constada nas populações avaliadas, provavelmente relacionada às variações do ambiente. A característica que mais variou foi o número de cloroplastos, sugerindo que este atributo não seja um caráter taxonômico confiável. A presença de E. pulvinata em riachos de leito rochoso e correnteza rápida (38,5-75,0 cm s-1), mostra que, provavelmente, este tipo de hábitat seja o mais adequado ao seu desenvolvimento.Genus Ecballocystis was proposed by Bohlin in 1897 starting from the description of type species Ecballocystis pulvinata, gathered in Brazil (Rio Grande do Sul State). Genus is considered of rare occurrence in the world. In Brazil, the species E. pulvinata has been cited more frequently. Several specimens identified as E. pulvinata gathered from different streams were taxonomically evaluated through morphometrical studies, and the occurrence of the species in relation to some environmental variables was verified. Considerable morphological amplitude was verified in the populations evaluated, probably related to the environmental variations. The characteristic that varied more was the chloroplasts number, suggesting that this attribute is not a reliable taxonomic character. All samples of E. pulvinata were gathered in rocky bed and fast current streams (38.5-75.0 cm s-1) indicating that these habitat conditions are the most appropriate to its development.Universidade Estadual Paulista Departamento de Zoologia e BotânicaUniversidade Estadual Paulista Departamento de Zoologia e Botânic

O gênero Vaucheria D.C. (Vaucheriaceae, Xanthophyceae) no sul do Brasil: aspectos taxonômicos e ecológicos

(O gênero Vaucheria D.C. [Vaucheriaceae, Xanthophyceae] no Sul do Brasil: aspectos taxonômicos e ecológicos). O levantamento taxonômico e ocorrência ambiental do gênero Vaucheria foram investigados em 105 riachos do sul do Brasil. As variáveis abióticas foram correlacionadas com a ocorrência das espécies. Três espécies foram identificadas de acordo com as características do gametângio: V. geminata (Vauch.) D.C., V. taylorii Blum e V. sessilis (Vauch.) D.C., este último sendo relatado pela primeira vez no Brasil. Os resultados deste estudo sugerem que Vaucheria é influenciada pelas características ambientais, ocorrendo principalmente em pH neutro a levemente ácido, baixa turbidez, velocidade da correnteza lenta e em riachos parcialmente sombreados. Esta combinação de variáveis parece ser a mais favorável para o estabelecimento da população de Vaucheria em ambientes lóticos, independentemente do bioma.(The genus Vaucheria D.C. [Vaucheriaceae, Xanthophyceae] in Southern Brazil: taxonomic and ecology aspects). Taxonomic survey and environmental occurrence of the genus Vaucheria were investigated in 105 streams from Southern Brazil. Abiotic variables were correlated to species occurrence. Three species were identified according to gametangia characteristics: V. geminata (Vauch.) D.C., V. taylorii Blum and V. sessilis (Vauch.) D.C.; the latter is reported for the first time in Brazil. The results of this survey suggest that Vaucheria is influenced by environmental characteristics, occurring mainly, in neutral to slightly acidic pH, low turbidity, slow current velocity and in partly shaded streams. This combination of variables seems to be the most favorable in the Vaucheria population establishment in lotic habitats independently of biome.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Universidade Estadual Paulista FCLUniversidade Federal da Integração Latino-AmericanaUniversidade Estadual Paulista FC

Microhabitat de Tolypothrix distorta Kütz. ex Bornet & Flahault (Microchaetaceae, Cyanophyta) em dois riachos da região Centro-Sul do Estado do Paraná, Brasil

This study aimed to evaluate some microhabitat characteristics of two populations of Tolypothrix distorta in streams from midsouthern region of Paraná State on a seasonal period. Each population was monthly investigated for the abundance of T. distorta and environmental variables such as depth, current velocity, irradiance, richness, and substrate diversity (H') using the quadrat technique for each sample unit (n = 7-10). For each segment the following physical and chemical water variables were also measured: temperature, oxygen saturation, specific conductance, pH, and turbidity. The seasonal period of higher abundance (percent cover) occurred during the months corresponding to summer/fall for both populations. The analyses also showed that populations had lower niche width and wide variation under the environmental conditions, characterizing the species as a generalist in the use of resources. Faster water flow and larger substrates favored the development of T. distorta, which may be related to the species morphological characteristics, thallus type that consists of tangled filaments formed by tuft and covered with thick gelatinous coats.Universidade Estadual do Paraná Departamento de BiologiaUniversidade Estadual Paulista Julio de Mesquita Filho Departamento de Ciências BiológicasUniversidade Federal da Integração Latino-AmericanaUniversidade Estadual Paulista Julio de Mesquita Filho Departamento de Ciências Biológica