École normale supérieure de Cachan

About: École normale supérieure de Cachan is a education organization based out in Cachan, Île-de-France, France. It is known for research contribution in the topics: Decidability & Finite element method. The organization has 2717 authors who have published 5585 publications receiving 175925 citations.

Trace equivalence decision: negative tests and non-determinism

Abstract: We consider security properties of cryptographic protocols that can be modeled using the notion of trace equivalence. The notion of equivalence is crucial when specifying privacy-type properties, like anonymity, vote-privacy, and unlinkability.In this paper, we give a calculus that is close to the applied pi calculus and that allows one to capture most existing protocols that rely on classical cryptographic primitives. First, we propose a symbolic semantics for our calculus relying on constraint systems to represent infinite sets of possible traces, and we reduce the decidability of trace equivalence to deciding a notion of symbolic equivalence between sets of constraint systems. Second, we develop an algorithm allowing us to decide whether two sets of constraint systems are in symbolic equivalence or not. Altogether, this yields the first decidability result of trace equivalence for a general class of processes that may involve else branches and/or private channels (for a bounded number of sessions).

Reactivity of the charge transfer excited state of sodium decatungstate at the nanosecond time scale

Abstract: Previous studies have put into evidence the key role of a charge transfer excited state, noted X, in the primary steps of laser photolysis of sodium decatungstate solutions. In this work, we report a thorough study of the reactivity of the X transient towards different organic and inorganic substrates in acetonitrile and in water. The bimolecular reaction between X and the substrates is shown to occur on the nanosecond time-scale, either by H atom or electron transfer, and leads to the formation of the reduced form Y of decatungstate.We find that H atom transfer reactions are ruled by Z–H (Z=C, N, O) binding energies and lead to Y formation quantum yields that may reach the X formation quantum yield, whereas electron transfer reactions are ruled by the substrates' oxidation potential (Marcus reorganisation energy λ=1.0–1.3 eV for aromatics in acetonitrile) and lead to Y formation quantum yields similar to that obtained in the pure solvent.

Asymptotic Properties of Robust Complex Covariance Matrix Estimates

Abstract: In many statistical signal processing applications, the estimation of nuisance parameters and parameters of interest is strongly linked to the resulting performance. Generally, these applications deal with complex data. This paper focuses on covariance matrix estimation problems in non-Gaussian environments, and particularly the M -estimators in the context of elliptical distributions. First, this paper extends to the complex case the results of Tyler in [D. Tyler, “Robustness and Efficiency Properties of Scatter Matrices,” Biometrika, vol. 70, no. 2, p. 411, 1983]. More precisely, the asymptotic distribution of these estimators as well as the asymptotic distribution of any homogeneous function of degree 0 of the M -estimates are derived. On the other hand, we show the improvement of such results on two applications: directions of arrival (DOA) estimation using the MUltiple SIgnal Classification (MUSIC) algorithm and adaptive radar detection based on the Adaptive Normalized Matched Filter (ANMF) test.

Unprecedented Stability of a Photochromic Bisthienylethene Based on Benzobisthiadiazole as an Ethene Bridge

Abstract: Photochromic systems are of increasing interest as a result of their applications in ophthalmic lenses and as smart molecular materials that act as photoresponsive self-assembling systems, molecular switches, logic gates, and information storage media. In particular, bisthienylethenes (BTEs) have become one of the most promising families because of their thermal irreversibility and outstanding fatigue resistance since the 1,3,5-hexatriene section can adopt an appropriate conformation to undergo a conrotatory 6p-electron photocyclization. To date, the rational design of symmetric or asymmetric BTEs has mainly been carried out by modifying the thiophene rings. In contrast, the central ethene bridges necessary for the versatility of the BTE architectures reported so far have been mostly limited to a cyclopentene unit or its strong electron-withdrawing analogues, such as perfluorocyclopentene, maleic anhydride, or maleic imide. As alternatives to this, several research groups have investigated the use of six-membered rings containing a C=C bond, such as 1,10-phenanthroline and cyclohexene obtained by a [4+2] cycloaddition of butadiene with a dienophile, as the central ethene bridge of BTEs. Recently, we incorporated highly polar and electron-withdrawing chromophores of 2,1,3benzothiadiazole and naphthalimide units as a novel family of six-membered rings that act as ethene bridges and result in good photochromic performance with moderate fatigue resistance in solution or in organogel systems. Moreover, their fluorescence can be modulated by solvatoand photochromism, eventually leading to a combined NOR and INHIBIT logic operation system. However, the two building blocks have a degree of aromaticity, which can facilitate the undesirable thermal back reaction in the dark as a result of the large loss of aromatic stabilization energy upon photocyclization from the open to the closed form. Accordingly, a lower aromaticity of the central ethene moiety in BTEs is expected to lead to a higher thermal stability of the closed form. Since the ethene bridge within a six-membered ring has its own advantages, such as a higher quantum yield in its closed-ring form and longer absorption wavelength, we envisioned that the use of a non-aromatic sixmembered ring with a C=C bond as the central ethene bridge might widely extend the diversity of the thermally irreversible photochromic systems. With this in mind, we report herein an original BTE photochromic system (BTTE, Figure 1) based on the benzo[1,2-c :3,4-c’]bis[1,2,5]thiadiazole (abbreviated as benzobisthiadiazole) chromophore as a novel six-membered ring containing a central bridging ethene unit, which is expected to have a low aromaticity, and yield a thermally stable closed form (c-BTTE). As depicted in Figure 1A, BTTE was easily prepared by conventional Suzuki cross-coupling with the corresponding boronic acid. The key intermediate of 4,5-dibromobenzo[1,2c :3,4-c’]bis[1,2,5]thiadiazole (BBT) was conveniently synthesized from benzothiadiazole according to the reported procedure. The Suzuki coupling of BBT and 2,5-dimethylthien-3-yl boronic acid in the presence of a [Pd(PPh3)4] catalyst in a mixture of aqueous Na2CO3 (2m) and 1,4-dioxane under reflux gave the target molecule BTTE, which was fully characterized by H NMR and C NMR spectroscopy as well as HRMS (see the Supporting Information). Interestingly, the H NMR spectrum of BTTE shows two well-resolved sets of signals for the methyl protons which correspond to the parallel (d = 2.09 and 2.42 ppm) and antiparallel (d = 1.96 and 2.46 ppm) conformations commonly found in BTE systems (Figure 1D). In general, the two conformations, parallel (photochromic inactive) and antiparallel (photochromic active) conformations, undergo very fast single-bond rotation in most BTEs, thereby resulting in only one set of time-averaged signals in the H NMR spectrum. Only in the cases of diarylethenes bearing substituents can the rotation of the aryl groups be slowed down sufficiently to give two sets of signals. As found by integration of the H NMR spectrum, the ratio between the parallel and antiparallel conformations of BTTE in CDCl3 is 55:45. A solution of BTTE in THF is colorless, with an intense absorption band at 288 nm and a moderate one at 361 nm (Table 1). As expected, irradiation of the solution of BTTE with light at a wavelength of 365 nm results in the solution [*] Prof. Dr. W. Zhu, Y. Yang, Dr. Q. Zhang, Prof. Dr. Y. Xie, Prof. Dr. H. Tian Shanghai Key Laboratory of Functional Materials Chemistry Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology Shanghai 200237 (China) E-mail: whzhu@ecust.edu.cn

A new concept of focusing antennas using plane-parallel Fabry-Perot cavities with nonuniform mirrors

Abstract: An original configuration of low-profile directive antennas is presented in V-band. The focusing effect is performed by a plane-parallel Fabry-Perot (FP) resonator illuminated by a printed antenna. Both reflecting mirrors are made of metal strip gratings. The dimensions of the strips and slots of the nonperiodic output mirror are much smaller than the working wavelength; they are computed locally so that this mirror behaves as a spherical equiphase surface. Theoretical and experimental results show that the radiation patterns are symmetric and have low sidelobes. The antenna directivity is controlled by the value of the synthesized radius of curvature, that is to say by the nonperiodic distribution of the metal strips. It typically varies between 15 and 23.5 dB at 60 GHz. This new radiating structure is much more compact than substrate lenses and is compatible with low-cost multilayer technologies at millimeter wave frequencies. This is a possible candidate for user mobile-stations of indoor broadband communication systems.