Transition to turbulence in slowly divergent pipe flow

The results of a combined experimental and numerical study of the flow in slowly diverging pipes are presented. Interestingly, an axisymmetric conical recirculation cell has been observed. The conditions for its existence and the length of the cell are simulated for a range of diverging angles and expansion ratios. There is a critical velocity for the appearance of this state. When the flow rate increases further, a subcritical transition for localized turbulence arises. The transition and relaminarization experiments described here quantify the extent of turbulence. The findings suggest that the transition scenario in slowly diverging pipes is a combination of stages similar to those observed in sudden expansions and in straight circular pipe flow.Comment: 8 pages, 5 figure

REAM intensity modulator-enabled 10Gb/s colorless upstream transmission of real-time optical OFDM signals in a single-fiber-based bidirectional PON architecture

Reflective electro-absorption modulation-intensity modulators (REAM-IMs) are utilized, for the first time, to experimentally demonstrate colorless ONUs in single-fiber-based, bidirectional, intensity-modulation and direct-detection (IMDD), optical OFDM PONs (OOFDM-PONs) incorporating 25km SSMFs and OLT-side-seeded CW optical signals. The colorlessness of the REAM-IMs is characterized, based on which optimum REAM-IM operating conditions are identified. In the aforementioned PON architecture, 10Gb/s colorless upstream transmissions of end-to-end realtime OOFDM signals are successfully achieved for various wavelengths within the entire C-band. Over such a wavelength window, corresponding minimum received optical powers at the FEC limit vary in a range as small as <0.5dB. In addition, experimental measurements also indicate that Rayleigh backscattering imposes a 2.8dB optical power penalty on the 10Gb/s over 25km upstream OOFDM signal transmission. Furthermore, making use of on-line adaptive bit and power loading, a linear trade-off between aggregated signal line rate and optical power budget is observed, which shows that, for the present PON system, a 10% reduction in signal line rate can improve the optical power budget by 2.6dB. © 2012 Optical Society of America

Emergence of thin shell structure during collapse in isotropic coordinates

Numerical studies of gravitational collapse in isotropic coordinates have recently shown an interesting connection between the gravitational Lagrangian and black hole thermodynamics. A study of the actual spacetime was not the main focus of this work and in particular, the rich and interesting structure of the interior has not been investigated in much detail and remains largely unknown. We elucidate its features by performing a numerical study of the spacetime in isotropic coordinates during gravitational collapse of a massless scalar field. The most salient feature to emerge is the formation of a thin shell of matter just inside the apparent horizon. The energy density and Ricci scalar peak at the shell and there is a jump discontinuity in the extrinsic curvature across the apparent horizon, the hallmark that a thin shell is present in its vicinity. At late stages of the collapse, the spacetime consists of two vacuum regions separated by the thin shell. The interior is described by an interesting collapsing isotropic universe. It tends towards a vacuum (never reaches a perfect vacuum) and there is a slight inhomogeneity in the interior that plays a crucial role in the collapse process as the areal radius tends to zero. The spacetime evolves towards a curvature (physical) singularity in the interior, both a Weyl and Ricci singularity. In the exterior, our numerical results match closely the analytical form of the Schwarzschild metric in isotropic coordinates, providing a strong test of our numerical code.Comment: 24 pages, 10 figures. version to appear in Phys. Rev.

Field-Trial of Machine Learning-Assisted Quantum Key Distribution (QKD) Networking with SDN

We demonstrated, for the first time, a machine-learning method to assist the coexistence between quantum and classical communication channels. Software-defined networking was used to successfully enable the key generation and transmission over a city and campus network

Quantum state preparation in semiconductor dots by adiabatic rapid passage

Preparation of a specific quantum state is a required step for a variety of proposed practical uses of quantum dynamics. We report an experimental demonstration of optical quantum state preparation in a semiconductor quantum dot with electrical readout, which contrasts with earlier work based on Rabi flopping in that the method is robust with respect to variation in the optical coupling. We use adiabatic rapid passage, which is capable of inverting single dots to a specified upper level. We demonstrate that when the pulse power exceeds a threshold for inversion, the final state is independent of power. This provides a new tool for preparing quantum states in semiconductor dots and has a wide range of potential uses.Comment: 4 pages, 4 figure

Microcavity quantum-dot systems for non-equilibrium Bose-Einstein condensation

We review the practical conditions required to achieve a non-equilibrium BEC driven by quantum dynamics in a system comprising a microcavity field mode and a distribution of localised two-level systems driven to a step-like population inversion profile. A candidate system based on eight 3.8nm layers of In(0.23)Ga(0.77)As in GaAs shows promising characteristics with regard to the total dipole strength which can be coupled to the field mode.Comment: 4 pages, 4 figures, to be published in J. Phys. Conf. Ser. for QD201

Absorbing states and elastic interfaces in random media: two equivalent descriptions of self-organized criticality

We elucidate a long-standing puzzle about the non-equilibrium universality classes describing self-organized criticality in sandpile models. We show that depinning transitions of linear interfaces in random media and absorbing phase transitions (with a conserved non-diffusive field) are two equivalent languages to describe sandpile criticality. This is so despite the fact that local roughening properties can be radically different in the two pictures, as explained here. Experimental implications of our work as well as promising paths for future theoretical investigations are also discussed.Comment: 4 pages. 2 Figure

Study Physicochemical of the Raw Palm Oils of the Republic of Gabon and Congo

The palm oil exists in several forms: palm oil raw, not refined, of red color because very rich carotenoids, called «red palm oil». In Africa, it is generally sold and consumed rough, i.e. not having undergoes the stages of refining. Taking into account the quantity and relevance of existing work on the palm tree with oil, we limited our study of Elaeis guinensis of Congo Basin to the analysis of the oils taken on the ground of the natural palm plantations and of the plantations of the palm plantation of M'vouna (Gabon) and of the Rural Campus of Loukoko (Congo) the study of our samples reveals the presence of carotenoids between 400 and 480 nm, and the presence of the chlorophyllian pigments with 600 and 670 nm for the great number of analyzed spectra. In the palm oil, the contents of carotenoids extend between 400 and 480 nm. The total content carotenoids varies from 897,1 µg/ml with 1229,3 µg/ml for the samples of the Palm plantation of M'vouna (Franceville, Gabon), whereas for the samples of Loukoko (Congo) that varies from 583,1 µg/ml with 1110,5 µg/ml on average. During precipitation partial of the raw palm oils of Gabon, three cases of figure ariseed (progressive, brutal and null solidification). The content total polyphenols varies from 439 to 2516 µg/ml. Of all the analyzed samples, the sample of Congo contains the maximum of phenolic compounds (2516 µg/ml). Minimum of 439 µg/ml, being obtained on the sample of Gabon. Their high content in carotenoids, fatty-acid unsaturates and the strong content total polyphenols, make our different oils more beneficial as well in the nutritional plan as therapeutic. The objective of work is the physicochemical characterization of the raw palm oils of the Republic of Gabon and Congo. The physicochemical characteristics of these oils are given according to the conventional normalized methods. They are the analyzes such as the spectroscopic evaluation of the color, the content carotene, partial solidification and the content polyphenols