Quantum key distribution session with 16-dimensional photonic states

The secure transfer of information is an important problem in modern telecommunications. Quantum key distribution (QKD) provides a solution to this problem by using individual quantum systems to generate correlated bits between remote parties, that can be used to extract a secret key. QKD with D-dimensional quantum channels provides security advantages that grow with increasing D. However, the vast majority of QKD implementations has been restricted to two dimensions. Here we demonstrate the feasibility of using higher dimensions for real-world quantum cryptography by performing, for the first time, a fully automated QKD session based on the BB84 protocol with 16-dimensional quantum states. Information is encoded in the single-photon transverse momentum and the required states are dynamically generated with programmable spatial light modulators. Our setup paves the way for future developments in the field of experimental high-dimensional QKD.Comment: 8 pages, 3 figure

Finite dimensional quantizations of the (q,p) plane : new space and momentum inequalities

We present a N-dimensional quantization a la Berezin-Klauder or frame quantization of the complex plane based on overcomplete families of states (coherent states) generated by the N first harmonic oscillator eigenstates. The spectra of position and momentum operators are finite and eigenvalues are equal, up to a factor, to the zeros of Hermite polynomials. From numerical and theoretical studies of the large $N$ behavior of the product $\lambda\_m(N) \lambda\_M(N)$ of non null smallest positive and largest eigenvalues, we infer the inequality $\delta\_N(Q) \Delta\_N(Q) = \sigma\_N \overset{<}{\underset{N \to \infty}{\to}} 2 \pi$ (resp. $\delta\_N(P) \Delta\_N(P) = \sigma\_N \overset{<}{\underset{N \to \infty}{\to}} 2 \pi$) involving, in suitable units, the minimal ($\delta\_N(Q)$) and maximal ($\Delta\_N(Q)$) sizes of regions of space (resp. momentum) which are accessible to exploration within this finite-dimensional quantum framework. Interesting issues on the measurement process and connections with the finite Chern-Simons matrix model for the Quantum Hall effect are discussed

First-principles study of the electrooptic effect in ferroelectric oxides

We present a method to compute the electrooptic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. It allows us to study the non-linear optic behavior of three ferroelectric ABO_3 compounds : LiNbO_3, BaTiO_3 and PbTiO_3. Our calculations reveal the dominant contribution of the soft mode to the electrooptic coefficients in LiNbO_3 and BaTiO_3 and identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electrooptic response in these compounds.Comment: accepted for publication in Phys. Rev. Let

Tropical Fourier-Motzkin elimination, with an application to real-time verification

We introduce a generalization of tropical polyhedra able to express both strict and non-strict inequalities. Such inequalities are handled by means of a semiring of germs (encoding infinitesimal perturbations). We develop a tropical analogue of Fourier-Motzkin elimination from which we derive geometrical properties of these polyhedra. In particular, we show that they coincide with the tropically convex union of (non-necessarily closed) cells that are convex both classically and tropically. We also prove that the redundant inequalities produced when performing successive elimination steps can be dynamically deleted by reduction to mean payoff game problems. As a complement, we provide a coarser (polynomial time) deletion procedure which is enough to arrive at a simply exponential bound for the total execution time. These algorithms are illustrated by an application to real-time systems (reachability analysis of timed automata).Comment: 29 pages, 8 figure

The effect of thresholding on temporal avalanche statistics

We discuss intermittent time series consisting of discrete bursts or avalanches separated by waiting or silent times. The short time correlations can be understood to follow from the properties of individual avalanches, while longer time correlations often present in such signals reflect correlations between triggerings of different avalanches. As one possible source of the latter kind of correlations in experimental time series, we consider the effect of a finite detection threshold, due to e.g. experimental noise that needs to be removed. To this end, we study a simple toy model of an avalanche, a random walk returning to the origin or a Brownian bridge, in the presence and absence of superimposed delta-correlated noise. We discuss the properties after thresholding of artificial timeseries obtained by mixing toy avalanches and waiting times from a Poisson process. Most of the resulting scalings for individual avalanches and the composite timeseries can be understood via random walk theory, except for the waiting time distributions when strong additional noise is added. Then, to compare with a more complicated case we study the Manna sandpile model of self-organized criticality, where some further complications appear.Comment: 15 pages, 12 figures, submitted to J. Stat. Mech., special issue of the UPoN2008 conferenc

Biosynthesis, Characterization and Antimicrobial Activity of Silver Nanoparticles Using Cell Free Lysate of Bacillus Subtilis: A Biotechnology Approach

Present study was aimed at the bio-synthesis and characterization of silver nanoparticles (AgNPs) using bacterial cell free lysate of Bacillus specie isolated from iron-rust contaminated soil sample from the Niger Delta region of Nigeria. This method was cost effective, eco-friendly and an alternative to chemical synthesis which is hazardous and requiring tedious synthetic manipulation. This study also evaluated the antimicrobial effect of the synthesized silver nanoparticles. Silver nanoparticles produced by reacting cell-free lysate of Bacillus subtilis and 1 mM of aqueous silver nitrate solution were characterized by Ultraviolet-visible spectroscopy, Transmission electron microscopy (TEM) and Photon correlation microscopy (Zeta sizer). The UV-Visible spectrophotometric result revealed an absorption maxima corresponding to peaks near 428 nm, depicting reduction of ionic silver (Ag+) to silver atom (Ag0). It has already been reported that nitrate reductase enzymes are implicated in metal ion reduction reactions. The Transmission electron microscopy analysis revealed that the AgNPs size ranged between 58.24 ± 1.04 nm and 72.20 ± 2.10 nm complementing the result obtained from Photon correlation microscopy (76.86 ± 1.14 nm). The antibacterial activity of AgNPs gave highest inhibition zone diameter of 26 mm on Pseudomonas aeruginosa at the dose of 0.10 mg/ml and 14 mm on tested Candida albicans. The synthesized silver nanoparticles were found to produce a dose dependent antimicrobial inhibitory effect while surface adsorption and lysis were implicated as the mode of action

Boletim agrometeorológico 1997.

bitstream/item/113943/1/boletim-agrometeorologico-1997.pd

Stochastic Backgrounds of Gravitational Waves from Cosmological Sources: Techniques and Applications to Preheating

Several mechanisms exist for generating a stochastic background of gravitational waves in the period following inflation. These mechanisms are generally classical in nature, with the gravitational waves being produced from inhomogeneities in the fields that populate the early universe and not quantum fluctuations. The resulting stochastic background could be accessible to next generation gravitational wave detectors. We develop a framework for computing such a background analytically and computationally. As an application of our framework, we consider the stochastic background of gravitational waves generated in a simple model of preheating.Comment: Replaced with published version: Phys. Rev. D 78, 063541 (2008