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