A Machine learning approach to POS tagging

We have applied inductive learning of statistical decision trees and relaxation labelling to the Natural Language Processing (NLP) task of morphosyntactic disambiguation (Part Of Speech Tagging). The learning process is supervised and obtains a language model oriented to resolve POS ambiguities. This model consists of a set of statistical decision trees expressing distribution of tags and words in some relevant contexts. The acquired language models are complete enough to be directly used as sets of POS disambiguation rules, and include more complex contextual information than simple collections of n-grams usually used in statistical taggers. We have implemented a quite simple and fast tagger that has been tested and evaluated on the Wall Street Journal (WSJ) corpus with a remarkable accuracy. However, better results can be obtained by translating the trees into rules to feed a flexible relaxation labelling based tagger. In this direction we describe a tagger which is able to use information of any kind (n-grams, automatically acquired constraints, linguistically motivated manually written constraints, etc.), and in particular to incorporate the machine learned decision trees. Simultaneously, we address the problem of tagging when only small training material is available, which is crucial in any process of constructing, from scratch, an annotated corpus. We show that quite high accuracy can be achieved with our system in this situation.Postprint (published version

Saturated absorption frequency locking and second harmonic generation for narrowband entangled photon pair generation

En col·laboració amb la Universitat Autònoma de Barcelona (UAB) i la Universitat de Barcelona (UB).The design of two locking systems for light sources desired to generate narrowband photon pairs is under study. First a DBR laser will be locked to the D1 transition frequency of Rb 87 using a Doppler-free Saturated Absorption Spectroscopy. Afterwards the frequency-locked light will be doubled in a nonlinear PPKTP crystal resulting in a frequency stabilized source at 397.5 nm

Study of the development and verification of an integrated code for UAV design

L'objectiu d'aquest estudi és desenvolupar una eina de disseny d'aeronaus utilitzant algoritmes d'optimització per a facilitar el procés. Es pretén incorporar el codi d'estudi i simulació de les actuacions d'un UAV desenvolupat per l'equip Trencalòs Team en un software de disseny aerodinàmic ja existent, ja sigui XFLR5 o AVL. Les funcions objectiu incorporades seran les que l'equip considera per a la participació en el concurs internacional Air Cargo Challenge, amb la intenció de desenvolupar una eina de treball per a Trencalòs que permeti fer un disseny òptim dins del marc de la competició. El treball es dividirà en tres etapes: 1. Incorporació del codi desenvolupat per Trencalòs al software de disseny aerodinàmic2. Fer ús dels algoritmes d'optimització de funcions objectiu per a facilitar el procés de disseny3. Verificació els resultats obtinguts.

Radiation-pressure Waves and Multiphase Quasar Outflows

We report on quasar outflow properties revealed by analyzing more than 60 composite outflow spectra built from $\sim 60\,000$ CIV absorption troughs in the SDSS-III/BOSS DR12QBAL catalog. We assess the dependences of the equivalent widths of many outflow metal absorption features on outflow velocity, trough width and position, and quasar magnitude and redshift. The evolution of the equivalent widths of the OVI and NV lines with outflow velocity correlates with that of the mean absorption-line width, the outflow electron density, and the strength of lines arising from collisionally-excited meta-stable states. None of these correlations is found for the other high- or low-ionization species, and different behaviors with trough width are also suggested. We find no dependence on quasar magnitude or redshift in any case. All the observed trends can be reconciled by considering a multiphase stratified outflow structure, where inner regions are colder, denser and host lower-ionization species. Given the prevalence of radiative acceleration in quasar outflows found by Mas-Ribas & Mauland (2019), we suggest that radiation pressure sweeps up and compresses the outflowing gas outwards, creating waves or filaments where the multiphase stratified structure could take form. This scenario is supported by the suggested correlation between electron density and outflow velocity, and the similar behavior observed for the line and line-locking components of the absorption features. We show that this outflow structure is also consistent with other X-ray, radiative transfer, and polarization results, and discuss the implications of our findings for future observational and numerical quasar outflow studies.Comment: Main results Figs. 3 and 7. ApJ accepte

Improvements on stochastic vector quantization of images

A novel nonadaptive fixed-rate vector quantizer encoding scheme is presented, and preliminary results are shown. The design of the codebook has been based on a stochastic approach in order to match a previously defined model for the image to be encoded. Following this approach, the generation of the codebook is made extremely simple in terms of computational load. Good visual results are shown in the range of 0.5-0.8 bit/pixel. Much better performance is expected for adaptive schemes.Peer ReviewedPostprint (published version