arXiv subjects

David Sánchez

Publications and source records attributed to David Sánchez.

45 records · Page 3Linked to original sources

arXiv2016

Supplementary Materials for "How to Avoid Reidentification with Proper Anonymization"- Comment on "Unique in the shopping mall: on the reidentifiability of credit card metadata"

The study by De Montjoye et al. ("Science", 30 January 2015, p. 536) claimed that most individuals can be reidentified from a deidentified credit card transaction database and that anonymization mechanisms are not effective against reidentification. Such claims deserve detailed quantitative scrutiny, as they might seriously undermine the willingness of data owners and subjects to share data for research. In a recent Technical Comment published in "Science" (18 March 2016, p. 1274), we demonstrate that the reidentification risk reported by De Montjoye et al. was significantly overestimated (due to a misunderstanding of the reidentification attack) and that the alleged ineffectiveness of anonymization is due to the choice of poor and undocumented methods and to a general disregard of 40 years of anonymization literature. The technical comment also shows how to properly anonymize data, in order to reduce unequivocal reidentifications to zero while retaining even more analytical utility than with the poor anonymization mechanisms employed by De Montjoye et al. In conclusion, data owners, subjects and users can be reassured that sound privacy models and anonymization methods exist to produce safe and useful anonymized data. Supplementary materials detailing the data sets, algorithms and extended results of our study are available here. Moreover, unlike the De Montjoye et al.'s data set, which was never made available, our data, anonymized results, and anonymization algorithms can be freely downloaded from http://crises-deim.urv.cat/opendata/SPD_Science.zip

cs.CR↗

arXiv2016

The Intrinsic Shape of Sagittarius A* at 3.5-mm Wavelength

The radio emission from Sgr A$^\ast$ is thought to be powered by accretion onto a supermassive black hole of $\sim\! 4\times10^6~ \rm{M}_\odot$ at the Galactic Center. At millimeter wavelengths, Very Long Baseline Interferometry (VLBI) observations can directly resolve the bright innermost accretion region of Sgr A$^\ast$. Motivated by the addition of many sensitive, long baselines in the north-south direction, we developed a full VLBI capability at the Large Millimeter Telescope Alfonso Serrano (LMT). We successfully detected Sgr A$^\ast$ at 3.5~mm with an array consisting of 6 Very Long Baseline Array telescopes and the LMT. We model the source as an elliptical Gaussian brightness distribution and estimate the scattered size and orientation of the source from closure amplitude and self-calibration analysis, obtaining consistent results between methods and epochs. We then use the known scattering kernel to determine the intrinsic two dimensional source size at 3.5 mm: $(147\pm7~μ\rm{as}) \times (120\pm12~μ\rm{as})$, at position angle $88^\circ\pm7^\circ$ east of north. Finally, we detect non-zero closure phases on some baseline triangles, but we show that these are consistent with being introduced by refractive scattering in the interstellar medium and do not require intrinsic source asymmetry to explain.

astro-ph.GA↗

arXiv2015

Utility-Preserving Differentially Private Data Releases Via Individual Ranking Microaggregation

Being able to release and exploit open data gathered in information systems is crucial for researchers, enterprises and the overall society. Yet, these data must be anonymized before release to protect the privacy of the subjects to whom the records relate. Differential privacy is a privacy model for anonymization that offers more robust privacy guarantees than previous models, such as $k$-anonymity and its extensions. However, it is often disregarded that the utility of differentially private outputs is quite limited, either because of the amount of noise that needs to be added to obtain them or because utility is only preserved for a restricted type and/or a limited number of queries. On the contrary, $k$-anonymity-like data releases make no assumptions on the uses of the protected data and, thus, do not restrict the number and type of doable analyses. Recently, some authors have proposed mechanisms to offer general-purpose differentially private data releases. This paper extends such works with a specific focus on the preservation of the utility of the protected data. Our proposal builds on microaggregation-based anonymization, which is more flexible and utility-preserving than alternative anonymization methods used in the literature, in order to reduce the amount of noise needed to satisfy differential privacy. In this way, we improve the utility of differentially private data releases. Moreover, the noise reduction we achieve does not depend on the size of the data set, but just on the number of attributes to be protected, which is a more desirable behavior for large data sets. The utility benefits brought by our proposal are empirically evaluated and compared with related works for several data sets and metrics.

cs.CR↗

arXiv2015

t-Closeness through Microaggregation: Strict Privacy with Enhanced Utility Preservation

Microaggregation is a technique for disclosure limitation aimed at protecting the privacy of data subjects in microdata releases. It has been used as an alternative to generalization and suppression to generate $k$-anonymous data sets, where the identity of each subject is hidden within a group of $k$ subjects. Unlike generalization, microaggregation perturbs the data and this additional masking freedom allows improving data utility in several ways, such as increasing data granularity, reducing the impact of outliers and avoiding discretization of numerical data. $k$-Anonymity, on the other side, does not protect against attribute disclosure, which occurs if the variability of the confidential values in a group of $k$ subjects is too small. To address this issue, several refinements of $k$-anonymity have been proposed, among which $t$-closeness stands out as providing one of the strictest privacy guarantees. Existing algorithms to generate $t$-close data sets are based on generalization and suppression (they are extensions of $k$-anonymization algorithms based on the same principles). This paper proposes and shows how to use microaggregation to generate $k$-anonymous $t$-close data sets. The advantages of microaggregation are analyzed, and then several microaggregation algorithms for $k$-anonymous $t$-closeness are presented and empirically evaluated.

cs.CR↗

arXiv2014

Crowdsourcing Dialect Characterization through Twitter

We perform a large-scale analysis of language diatopic variation using geotagged microblogging datasets. By collecting all Twitter messages written in Spanish over more than two years, we build a corpus from which a carefully selected list of concepts allows us to characterize Spanish varieties on a global scale. A cluster analysis proves the existence of well defined macroregions sharing common lexical properties. Remarkably enough, we find that Spanish language is split into two superdialects, namely, an urban speech used across major American and Spanish citites and a diverse form that encompasses rural areas and small towns. The latter can be further clustered into smaller varieties with a stronger regional character.

physics.soc-ph↗

arXiv2013

Magnetic-field asymmetry of nonlinear thermoelectric and heat transport

Nonlinear transport coefficients do not obey, in general, reciprocity relations. We here discuss the magnetic-field asymmetries that arise in thermoelectric and heat transport of mesoscopic systems. Based on a scattering theory of weakly nonlinear transport, we analyze the leading-order symmetry parameters in terms of the screening potential response to either voltage or temperature shifts. We apply our general results to a quantum Hall antidot system. Interestingly, we find that certain symmetry parameters show a dependence on the measurement configuration.

cond-mat.mes-hall↗

arXiv2013

Nonlinear thermovoltage and thermocurrent in quantum dots

Quantum dots are model systems for quantum thermoelectric behavior because of the ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion.

cond-mat.mes-hall↗

arXiv2012

Thermally driven ballistic rectifer

The response of electric devices to an applied thermal gradient has, so far, been studied almost exclusively in two-terminal devices. Here we present measurements of the response to a thermal bias of a four-terminal, quasi-ballistic junction with a central scattering site. We find a novel transverse thermovoltage measured across isothermal contacts. Using a multi-terminal scattering model extended to the weakly non-linear voltage regime, we show that the device's response to a thermal bias can be predicted from its nonlinear response to an electric bias. Our approach forms a foundation for the discovery and understanding of advanced, nonlocal, thermoelectric phenomena that in the future may lead to novel thermoelectric device concepts.

cond-mat.mes-hall↗

arXiv2010

Mesoscopic Coulomb drag, broken detailed balance and fluctuation relations

When a biased conductor is put in proximity with an unbiased conductor a drag current can be induced in the absence of detailed balance. This is known as the Coulomb drag effect. However, even in this situation far away from equilibrium where detailed balance is explicitly broken, theory predicts that fluctuation relations are satisfied. This surprising effect has, to date, not been confirmed experimentally. Here we propose a system consisting of a capacitively coupled double quantum dot where the nonlinear fluctuation relations are verified in the absence of detailed balance.

cond-mat.mes-hall↗