Emergent vortices in populations of colloidal rollers

Coherent vortical motion has been reported in a wide variety of populations including living organisms (bacteria, fishes, human crowds) and synthetic active matter (shaken grains, mixtures of biopolymers), yet a unified description of the formation and structure of this pattern remains lacking. Here we report the self-organization of motile colloids into a macroscopic steadily rotating vortex. Combining physical experiments and numerical simulations, we elucidate this collective behavior. We demonstrate that the emergent-vortex structure lives on the verge of a phase separation, and single out the very constituents responsible for this state of polar active matter. Building on this observation, we establish a continuum theory and lay out a strong foundation for the description of vortical collective motion in a broad class of motile populations constrained by geometrical boundaries

Continuous Cold-atom Inertial Sensor with 1 nrad.s−11\ \text{nrad.s}^{-1} Rotation Stability

We report the operation of a cold-atom inertial sensor which continuously captures the rotation signal. Using a joint interrogation scheme, where we simultaneously prepare a cold-atom source and operate an atom interferometer (AI) enables us to eliminate the dead times. We show that such continuous operation improves the short-term sensitivity of AIs, and demonstrate a rotation sensitivity of $100\ \text{nrad.s}^{-1}.\text{Hz}^{-1/2}$ in a cold-atom gyroscope of $11 \ \text{cm}^2$ Sagnac area. We also demonstrate a rotation stability of $1 \ \text{nrad.s}^{-1}$ at $10^4$ s of integration time, which establishes the record for atomic gyroscopes. The continuous operation of cold-atom inertial sensors will enable to benefit from the full sensitivity potential of large area AIs, determined by the quantum noise limit.Comment: 4 pages, 3 figure

Metrology with Atom Interferometry: Inertial Sensors from Laboratory to Field Applications

Developments in atom interferometry have led to atomic inertial sensors with extremely high sensitivity. Their performances are for the moment limited by the ground vibrations, the impact of which is exacerbated by the sequential operation, resulting in aliasing and dead time. We discuss several experiments performed at LNE-SYRTE in order to reduce these problems and achieve the intrinsic limit of atomic inertial sensors. These techniques have resulted in transportable and high-performance instruments that participate in gravity measurements, and pave the way to applications in inertial navigation.Comment: 7 pages, 5 figure

Maths Express au carrefour des cultures

International audienceCette brochure a été réalisée à l'occasion de la quinzième édition du Salon Culture et Jeux mathématiques. Elle a pour objectif de montrer que les mathématiques sont au coeur de toutes les activités humaines qu'elles soient sociales, techniques, scientifiques, artistiques ou ludiques. Elles sont nées et se sont développées au rythme des sociétés humaines

A exceptionnal sundial

International audienc

Compte-rendu du livre Monks, Manuscripts & Sundials : The Navicula in Medieval England de C. Eagleton

International audienc

Le cadran solaire grec d’Aï Khanoum : la question de l’exactitude des cadrans antiques

Savoie Denis. Le cadran solaire grec d’Aï Khanoum : la question de l’exactitude des cadrans antiques. In: Comptes rendus des séances de l'Académie des Inscriptions et Belles-Lettres, 151ᵉ année, N. 2, 2007. pp. 1161-1190

Problèmes de datation d'une occultation observée par Aristote / Problems of dating an occultation observed by Aristote

SUMMARY. — Using the latest developments in modern planetary theories, I propose in this paper to date an occultation, observed by Aristotle, of Mars by the Moon. Firstly, I shall focus on the result obtained by Kepler, who was the first astronomer to calculate a date for this occultation. Secondly, I shall analytically and numerically show the difficulty of this kind of calculations, by proposing four different possible solutions to the problem at hand, each of which depends upon the chosen value of the coefficient accounting for the slowing down of the Earth's rotation.RÉSUMÉ. — Le but de cet article est de dater une occultation de Mars par la Lune observée par Aristote, en utilisant les développements les plus récents des théories planétaires modernes. Dans un premier temps, on s'intéresse au résultat que Kepler, le premier, a obtenu au début du XVIIe siècle. Puis dans un deuxième temps, on montre la difficulté de ce genre d'exercice de datation, en proposant quatre solutions possibles qui dépendent toutes du coefficient que l'on adopte pour le ralentissement de la rotation de la Terre.Savoie Denis. Problèmes de datation d'une occultation observée par Aristote / Problems of dating an occultation observed by Aristote. In: Revue d'histoire des sciences, tome 56, n°2, 2003. pp. 493-504

Gnomonique et mesure du temps en astronomie

National audienc

Yasukatsu Maeyama, Astronomy in Orient and Occident: Selected papers on its cultural and scientific history

Savoie Denis. Yasukatsu Maeyama, Astronomy in Orient and Occident: Selected papers on its cultural and scientific history. In: Revue d'histoire des sciences, tome 59, n°2, 2006. p. 363