Self-quenching of the semiconductor laser linewidth below the Schawlow-Townes limit by using optical feedback

We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow-Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 X 10^3 is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor

Trend analysis of the COVID-19 pandemic in China and the rest of the world

The recent epidemic of Coronavirus (COVID-19) that started in China has already been "exported" to more than 140 countries in all the continents, evolving in most of them by local spreading. In this contribution we analyze the trends of the cases reported in all the Chinese provinces, as well as in some countries that, until March 15th, 2020, have more than 500 cases reported. Notably and differently from other epidemics, the provinces did not show an exponential phase. The data available at the Johns Hopkins University site seem to fit well an algebraic sub-exponential growing behavior as was pointed out recently. All the provinces show a clear and consistent pattern of slowing down with growing exponent going nearly zero, so it can be said that the epidemic was contained in China. On the other side, the more recent spread in countries like, Italy, Iran, and Spain show a clear exponential growth, as well as other European countries. Even more recently, US -which was one of the first countries to have an individual infected outside China (Jan 21st, 2020)- seems to follow the same path. We calculate the exponential growth of the most affected countries, showing the evolution along time after the first local case. We identify clearly different patterns in the analyzed data and we give interpretations and possible explanations for them. The analysis and conclusions of our study can help countries that, after importing some cases, are not yet in the local spreading phase, or have just started

A STATISTICAL MODEL FOR STOCK ASSESSMENT OF SOUTHERN BLUEFIN TUNA WITH TEMPORAL CHANGES IN SELECTIVITY

Assessment of the status of southern bluefin tuna (SBT) by Australia and Japan has used a method (ADAPT) that imposes a number of structural restrictions, and is similar to methods used for a number of stocks worldwide. A flexible method for assessment of the SBT population is presented that is much less restrictive and has potentially wide applicability. The three key features are: (1) all fitting to data is within the context of maximum likelihood, (2) catch-at-age data are not assumed to be without error (as in existing methods), but rather to be random variables, while age-specific selectivity is allowed to change over time within the bounds of specific structure, and (3) autocorrelation in recruitment processes is considered within the likelihood framework of the model. While the results suggest the stock has been depleted considerably from its virgin biomass, and are generally consistent with previous assessments, they also indicate that it is not as much below the biomass that will produce maximum suitable yield as previously estimated and that the extent of stock rebuilding necessary may not be as large as has been argued. The available data are shown to provide little information on the steepness parameter of the stock-recruitment function, and hence on sustainable catch levels for the stock.Afr. J. mar. Sci. 25: 331–36

Stochastic modelling of blockchain consensus

Blockchain and general purpose distributed ledgers are foundational technologies which bring significant innovation in the infrastructures and other underpinnings of our socio-economic systems. These P2P technologies are able to securely diffuse information within and across networks, without need for trustees or central authorities to enforce consensus. In this contribution, we propose a minimalistic stochastic model to understand the dynamics of blockchain-based consensus. By leveraging on random-walk theory, we model block propagation delay on different network topologies and provide a classification of blockchain systems in terms of two emergent properties. Firstly, we identify two performing regimes: a functional regime corresponding to an optimal system function; and a non-functional regime characterised by a congested or branched state of sub-optimal blockchains. Secondly, we discover a phase transition during the emergence of consensus and numerically investigate the corresponding critical point. Our results provide important insights into the consensus mechanism and sub-optimal states in decentralised systems

Organotin(IV) derivatives with 5,7-disubstituted-1,2,4-triazolo[1,5-a]pyrimidine and their cytotoxic activities: the importance of being conformers.

The organotin(IV) compounds Me2SnCl2(dbtp)(1), Me2SnCl2(dbtp)2 (2), Et2SnCl2(dbtp) (3), Et2SnCl2(dbtp)2 (4), Et2SnCl2(dptp) (5), nBu2SnCl2(dbtp)2 (6), nBu2SnCl2(dptp) (7), Ph2SnCl2(dbtp) (8), Ph2SnCl2(dptp)2 (9), where dbtp = 5,7-di-tert-butyl-1,2,4-triazolo[1,5-a]pyrimidine and dptp = 5,7-diphenyl-1,2,4-triazolo[1,5-a]pyrimidine, have been tested towards their cytotoxic activity by MTT test on three tumor cell lines, HepG2 (human hepatocellular carcinoma), HeLa (human cervix adenocarcinoma) and MCF-7 (human breast cancer). Except for 1 and 2, which were ineffective, all compounds significantly showed a dose-dependent anti-proliferative effect toward the three cell lines. By calculated IC50 values, cytotoxicity of the complexes followed the order nBu > Ph > Et > Me for all the selected tumor cells. Organotin(IV) compounds (6-9)-induced cell death of HepG2 was considered to be apoptotic by measuring the exposure of phosphadytilserine to the outer membrane and observing the typical apoptotic morphological change by acridine orange/ethidium bromide staining. Flow cytometric analysis of propidium iodide-stained cells also demonstrated that organotin(IV) complexes caused apoptosis of HepG2 cells through cell arrest at G0-G1 phase. The crystal structure of 7 was investigated by X-ray diffraction analysis, exhibiting a distorted trigonal bipyramidal geometry with N, Cl as axial atoms and Cl and butyl groups in the equatorial plane. The triazolopyrimidine unit coordinates to the Sn atom through N(3) in a monodentate mode. Two conformational isomers (molecule A and B in the crystallographic independent unit) are cocrystallized in the solid state, a phenomenon that has been observed only occasionally. Conformational mobility of the cytotoxic complex 7 can sum up to the ligands ability to form Hbonds and π···π stacking, facilitating its intracellular uptake