Two-dimensional flows of foam: drag exerted on circular obstacles and dissipation

A Stokes experiment for foams is proposed. It consists in a two-dimensional flow of a foam, confined between a water subphase and a top plate, around a fixed circular obstacle. We present systematic measurements of the drag exerted by the flowing foam on the obstacle, \emph{versus} various separately controlled parameters: flow rate, bubble volume, solution viscosity, obstacle size and boundary conditions. We separate the drag into two contributions, an elastic one (yield drag) at vanishing flow rate, and a fluid one (viscous coefficient) increasing with flow rate. We quantify the influence of each control parameter on the drag. The results exhibit in particular a power-law dependence of the drag as a function of the solution viscosity and the flow rate with two different exponents. Moreover, we show that the drag decreases with bubble size, increases with obstacle size, and that the effect of boundary conditions is small. Measurements of the streamwise pressure gradient, associated to the dissipation along the flow of foam, are also presented: they show no dependence on the presence of an obstacle, and pressure gradient depends on flow rate, bubble volume and solution viscosity with three independent power laws.Comment: 23 pages, 13 figures, proceeding of Eufoam 2004 conferenc

Observation of a phononic Mollow triplet in a hybrid spin-nanomechanical system

Reminiscent of the bound character of a qubit's dynamics confined on the Bloch sphere, the observation of a Mollow triplet in the resonantly driven qubit fluorescence spectrum represents one of the founding signatures of Quantum Electrodynamics. Here we report on its observation in a hybrid spin-nanomechanical system, where a Nitro-gen Vacancy spin qubit is magnetically coupled to the vibrations of a Silicon Carbide nanowire. A resonant microwave field turns the originally parametric hybrid interac-tion into a resonant process, where acoustic phonons are now able to induce transitions between the dressed qubit states, leading to synchronized spin-oscillator dynamics. We further explore the vectorial character of the hybrid coupling to the bidimensional de-formations of the nanowire. The demonstrated microwave assisted synchronization of the spin-oscillator dynamics opens novel perspectives for the exploration of spin-dependent forces, the key-ingredient for quantum state transfer

Nano-optomechanical measurement in the photon counting regime

Optically measuring in the photon counting regime is a recurrent challenge in modern physics and a guarantee to develop weakly invasive probes. Here we investigate this idea on a hybrid nano-optomechanical system composed of a nanowire hybridized to a single Nitrogen-Vacancy (NV) defect. The vibrations of the nanoresonator grant a spatial degree of freedom to the quantum emitter and the photon emission event can now vary in space and time. We investigate how the nanomotion is encoded on the detected photon statistics and explore their spatio-temporal correlation properties. This allows a quantitative measurement of the vibrations of the nanomechanical oscillator at unprecedentedly low light intensities in the photon counting regime when less than one photon is detected per oscillation period, where standard detectors are dark-noise-limited. These results have implications for probing weakly interacting nanoresonators, for low temperature experiments and for investigating single moving markers

Bogoliubov theory in the Gross-Pitaevskii limit: a simplified approach

We show that Bogoliubov theory correctly predicts the low-energy spectral properties of Bose gases in the Gross-Pitaevskii regime. We recover recent results from [6, 7]. While our main strategy is similar to the one developed in [6, 7], we combine it with new ideas, taken in part from [15, 25]; this makes our proof substantially simpler and shorter. As an important step towards the proof of Bogoliubov theory, we show that low-energy states exhibit complete Bose-Einstein condensation with optimal control over the number of orthogonal excitations

Bogoliubov theory in the Gross-Pitaevskii limit: a simplified approach

We show that Bogoliubov theory correctly predicts the low-energy spectral properties of Bose gases in the Gross-Pitaevskii regime. We recover recent results from [6, 7]. While our main strategy is similar to the one developed in [6, 7], we combine it with new ideas, taken in part from [15, 25]; this makes our proof substantially simpler and shorter. As an important step towards the proof of Bogoliubov theory, we show that low-energy states exhibit complete Bose-Einstein condensation with optimal control over the number of orthogonal excitations

Evolving Comparative Advantage and the Impact of Climate Change in Agricultural Markets: Evidence from 1.7 Million Fields around the World

A large agronomic literature models the implications of climate change for a variety of crops and locations around the world. The goal of the present paper is to quantify the macro-level consequences of these micro-level shocks. Using an extremely rich micro-level dataset that contains information about the productivity—both before and after climate change—of each of 10 crops for each of 1.7 million fields covering the surface of the Earth, we find that the impact of climate change on these agricultural markets would amount to a 0.26% reduction in global GDP when trade and production patterns are allowed to adjust.National Science Foundation (U.S.) (Grant SES-1227635

Paquet R pour l'estimation d'un mélange de lois de Student multivariées à échelles multiples

National audienceL'utilisation d'un modèle de mélange de lois est une approche statistique classique en classification non-supervisée. Un mélange fréquemment utilisé pour sa simplicité est le mélange gaussien. Cependant, un tel modèle est sensible aux données atypiques. Pour remédier à cela, nous présentons ici le mélange de lois de Student multivariées à échelles multiples, que nous sommes en train d'incorporer au sein d'un paquet R. Ces lois peuvent gérer des queues de lourdeurs différentes selon les directions alors que les lois gaussiennes et les lois de Student multivariées standards sont contraintes à être symétriques

Extending Coq with Imperative Features and its Application to SAT Verification

This work was supported in part by the french ANR DECERT initiativeInternational audienceCoq has within its logic a programming language that can be used to replace many deduction steps into a single computation, this is the so-called reflection. In this paper, we present two extensions of the evaluation mechanism that preserve its correctness and make it possible to deal with cpu-intensive tasks such as proof checking of SAT traces

Laser beam self-symmetrization in air in the multifilamentation regime

We show experimental and numerical evidence of spontaneous self-symmetrization of focused laser beams experiencing multi-filamentation in air. The symmetrization effect is observed as the multiple filaments generated prior to focus approach the focal volume. This phenomenon is attributed to the nonlinear interactions amongst the different parts of the beam mediated by the optical Kerr effect, which leads to a symmetric redistribution of the wave vectors even when the beam consists of a bundle of many filaments.Comment: 9 pages, 7 figure