369 research outputs found
Quantum communication with photon-number entangled states and realistic photodetection
We address the effects of realistic photodetection, with nonunit quantum
efficiency and background noise (dark counts), on the performances of quantum
communication schemes based on photon-number entangled states. We consider
channels based on Gaussian twin-beam states and non-Gaussian two-mode coherent
states and evaluate the channel capacity by optimizing the bit discrimination
threshold. We found that TWB-based channels are more robust against noise than
TMC-based ones and that this result is almost independent on the statistics of
dark counts.Comment: 8 pages, 2 figures, to appear in Phys. Lett.
Multiphoton communication in lossy channels with photon-number entangled states
We address binary and quaternary communication channels based on correlated
multiphoton two-mode states of radiation in the presence of losses. The
protocol are based on photon number correlations and realized upon choosing a
shared set of thresholds to convert the outcome of a joint photon number
measurement into a symbol from a discrete alphabet. In particular, we focus on
channels build using feasible photon-number entangled states (PNES) as two-mode
coherently-correlated (TMC) or twin-beam (TWB) states and compare their
performances with that of channels built using feasible classically correlated
(separable) states. We found that PNES provide larger channel capacity in the
presence of loss, and that TWB-based channels may transmit a larger amount of
information than TMC-based ones at fixed energy and overall loss. Optimized bit
discrimination thresholds, as well as the corresponding maximized mutual
information, are explicitly evaluated as a function of the beam intensity and
the loss parameter. The propagation of TMC and TWB in lossy channels is
analyzed and the joint photon number distribution is evaluated, showing that
the beam statistics, either sub-Poissonian for TMC or super-Poissonian for TWB,
is not altered by losses. Although entanglement is not strictly needed to
establish the channels, which are based on photon-number correlations owned
also by separable mixed states, purity of the support state is relevant to
increase security. The joint requirement of correlation and purity individuates
PNES as a suitable choice to build effective channels. The effects of losses on
channel security are briefly discussed.Comment: 8 pages, 19 figure
Polaris B, an optical companion of Polaris (alpha UMi) system: atmospheric parameters, chemical composition, distance and mass
We present an analysis of high-resolution spectroscopic observations of
Polaris B, the optical companion of the Polaris Ab system. The star has a
radial velocity V_r of -16.6km/s to -18.9km/s, and a projected rotational
velocity vsini=110 km/s. The derived atmospheric parameters are: Teff=6900K;
logg=4.3; V_t=2.5km/s. Polaris B has elemental abundances generally similar to
those of the Cepheid Polaris A (Usenko et al. 2005a), although carbon, sodium
and magnesium are close to the solar values. At a spectral type of F3V Polaris
B has a luminosity of 3.868L_sun, an absolute magnitude of +3.30mag, and a
distance of 109.5pc. The mass of the star is estimated to be 1.39M_sun, close
to a mass of 1.38+/-0.61M_sun for the recently-resolved orbital periods
companion Polaris Ab observed by Evans et al. (2007).Comment: 6 pages, 3 figures, 1 tabl
SQUID developments for the gravitational wave antenna MiniGRAIL
We designed two different sensor SQUIDs for the readout of the resonant mass gravitational wave detector MiniGRAIL. Both designs have integrated input inductors in the order of 1.5 muH and are planned for operation in the mK temperature range. Cooling fins were added to the shunt resistors. The fabricated SQUIDs show a behavior that differs from standard DC-SQUIDs. We were able to operate a design with a parallel configuration of washers at reasonable sensitivities. The flux noise saturated to a value of 0.84 muPhi0/radicHz below a temperature of 200 mK. The equivalent noise referred to the current through the input coil is 155 fA/radicHz and the energy resolution yields 62 h
Ментальньıе основьı древнерусского монaрxизма
RésuméLes bases mentales du monarchisme dans la Russie ancienne (milieu xiiie-milieu xve siècle) : les couples fidélité / trahison et vassal / sujet.On entend ici par monarchisme l'ensemble des notions explicites concernant le monarque et son pouvoir partagées par les membres d’une communauté donnée. Dans la Russie médiévale, du milieu du xiiie au milieu du xve siècle, le monarchisme était fondé sur des liens contractuels (ou « équivalents ») ou quasi-contractuels (« non équivalents »). La plus importante, parmi les relations contractuelles, était la relation suzerain-vassal, tandis que la relation despote-sujet dominait le système des liens quasi-contractuels. Il faut remarquer toutefois qu’un prince pouvait être à la fois suzerain et despote, vassal et sujet. Tous les rapports décrits ci-dessus étaient interprétés par les contemporains à travers le couple fidélité / trahison. Cette opposition, à son tour, s'appuyait sur la distinction entre « sien » et « étranger », sans pour autant donner naissance à un patriotisme au sens moderne de ce mot.AbstractThe mental bases of monarchism in ancient Russia between the mid-thirteenth and the mid-fifteenth centuries : the fealty / treason and vassal / subject relationships.We call monarchism the set of explicit concepts a given community has of the monarch and his power. In medieval Russia, between the mid-thirteenth and the mid-fifteenth centuries, monarchism was based on contractual (“equivalent”) or quasi-contractual (“non equivalent”) bonds. The most important contractual bond was that involving the overlord and the vassal, and the most important quasi-contractual bond involved the despot and the subject. It must be noted that a prince could at the same time be overlord and despot, vassal and subject. All these relationships were interpreted by contemporaries through the fealty / treason opposition. This opposition in turn rested on a distinction between “We” and “They”, but did not become the basis of patriotism in the modern acception of the word
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