Université Paris-Saclay

About: Université Paris-Saclay is a education organization based out in Gif-sur-Yvette, France. It is known for research contribution in the topics: Population & Context (language use). The organization has 29307 authors who have published 43183 publications receiving 867404 citations.

Cleaning large correlation matrices: tools from random matrix theory

Abstract: This review covers recent results concerning the estimation of large covariance matrices using tools from Random Matrix Theory (RMT). We introduce several RMT methods and analytical techniques, such as the Replica formalism and Free Probability, with an emphasis on the Marchenko-Pastur equation that provides information on the resolvent of multiplicatively corrupted noisy matrices. Special care is devoted to the statistics of the eigenvectors of the empirical correlation matrix, which turn out to be crucial for many applications. We show in particular how these results can be used to build consistent "Rotationally Invariant" estimators (RIE) for large correlation matrices when there is no prior on the structure of the underlying process. The last part of this review is dedicated to some real-world applications within financial markets as a case in point. We establish empirically the efficacy of the RIE framework, which is found to be superior in this case to all previously proposed methods. The case of additively (rather than multiplicatively) corrupted noisy matrices is also dealt with in a special Appendix. Several open problems and interesting technical developments are discussed throughout the paper.

Production, detection, and signaling of singlet oxygen in photosynthetic organisms.

Abstract: Significance: In photosynthetic organisms, excited chlorophylls (Chl) can stimulate the formation of singlet oxygen (1O2), a highly toxic molecule that acts in addition to its damaging nature as an important signaling molecule. Thus, due to this dual role of 1O2, its production and detoxification have to be strictly controlled. Recent Advances: Regulation of pigment synthesis is essential to control 1O2 production, and several components of the Chl synthesis and pigment insertion machineries to assemble and disassemble protein/pigment complexes have recently been identified. Once produced, 1O2 activates a signaling cascade from the chloroplast to the nucleus that can involve multiple mechanisms and stimulate a specific gene expression response. Further, 1O2 signaling was shown to interact with signal cascades of other reactive oxygen species, oxidized carotenoids, and lipid hydroperoxide-derived reactive electrophile species. Critical Issues: Despite recent progresses, hardly anything is known ab...

Direct observation of bimolecular reactions of ultracold KRb molecules.

Abstract: Femtochemistry techniques have been instrumental in accessing the short time scales necessary to probe transient intermediates in chemical reactions. In this study, we took the contrasting approach of prolonging the lifetime of an intermediate by preparing reactant molecules in their lowest rovibronic quantum state at ultralow temperatures, thereby markedly reducing the number of exit channels accessible upon their mutual collision. Using ionization spectroscopy and velocity-map imaging of a trapped gas of potassium-rubidium (KRb) molecules at a temperature of 500 nanokelvin, we directly observed reactants, intermediates, and products of the reaction 40K87Rb + 40K87Rb → K2Rb2* → K2 + Rb2. Beyond observation of a long-lived, energy-rich intermediate complex, this technique opens the door to further studies of quantum-state–resolved reaction dynamics in the ultracold regime.

Fifth-order susceptibility unveils growth of thermodynamic amorphous order in glass-formers

Abstract: Glasses are often thought of as frozen liquids without long-range order. Albert et al. used fifth-order dielectric susceptibility measurements to show that the real reason behind the stiffness of glass is more complicated. Measuring the response of two traditional glass formers to very high electric fields is challenging, but reveals the growth of compact domains across the glass transition. The emerging amorphous order is only weakly dependent on the specific molecular properties, suggesting a more universal governing behavior for making glasses. Science , this issue p. [1308][1] [1]: /lookup/doi/10.1126/science.aaf3182