Competition between Spin Echo and Spin Self-Rephasing in a Trapped Atom Interferometer

We perform Ramsey interferometry on an ultracold 87Rb ensemble confined in an optical dipoletrap. We use a \pi-pulse set at the middle of the interferometer to restore the coherence of the spinensemble by canceling out phase inhomogeneities and creating a spin echo in the contrast. However,for high atomic densities, we observe the opposite behavior: the \pi-pulse accelerates the dephasingof the spin ensemble leading to a faster contrast decay of the interferometer. We understand thisphenomenon as a competition between the spin-echo technique and an exchange-interaction drivenspin self-rephasing mechanism based on the identical spin rotation effect. Our experimental data iswell reproduced by a numerical model

Free entry equilibria with positive profits: A unified approach to quantity and price competition games

Free entry equilibria are usually characterized by the zero profit condition. We plead instead for a strict application of theNash equilibriumconcept to a symmetric simultaneous game played by actual and potential entrants, producing under decreasing average cost. Equilibrium is then typically indeterminate, with a number of active firms varying between an upper bound imposed by profitability and a lower bound required by sustainability. We use a canonical model with strategies represented by prices, although covering standard regimes of quantity and price competition, to show that in equilibrium the critical (profit maximizing) price must lie between the break-even and the limit prices

On stabilization policy in sunspot-driven oligopolistic economies

Economies with oligopolistic markets are prone to inefficient sunspot fluctuations triggered by autonomous changes in firms equilibrium conjectures. We show that a well designed taxation-subsidization scheme can eliminate these fluctuations by coordinating firms in each sector on a single efficient equilibrium. At the macroeconomic level, implementing this stabilization policy leads to significant welfare gains, attributable to a quantitatively dominant "efficient stabilization effect". This effect, while important, is typically ignored in the traditional computations of the welfare costs of aggregate fluctuations (e.g., Lucas, 2003)

Application of lasers to ultracold atoms and molecules

In this review, we discuss the impact of the development of lasers on ultracold atoms and molecules and their applications. After a brief historical review of laser cooling and Bose-Einstein condensation, we present important applications of ultra cold atoms, including time and frequency metrology, atom interferometry and inertial sensors, atom lasers, simulation of condensed matter systems, production and study of strongly correlated systems, and production of ultracold molecules.Comment: Review paper written in the name of IFRAF to celebrate 50 years of lasers and their applications to cold atom physics; 15 pages, 2 figures; to appear in Comptes Rendus de l'Academie des Sciences, Pari

Polymer brush collapse under shear flow

Shear responsive surfaces offer potential advances in a number of applications. Surface functionalisation using polymer brushes is one route to such properties, particularly in the case of entangled polymers. We report on neutron reflectometry measurements of polymer brushes in entangled polymer solutions performed under controlled shear, as well as coarse-grained computer simulations corresponding to these interfaces. Here we show a reversible and reproducible collapse of the brushes, increasing with the shear rate. Using two brushes of greatly different chain lengths and grafting densities, we demonstrate that the dynamics responsible for the structural change of the brush are governed by the free chains in solution rather than the brush itself, within the range of parameters examined. The phenomenon of the brush collapse could find applications in the tailoring of nanosensors, and as a way to dynamically control surface friction and adhesion

An intriguing correlation between the masses and periods of the transiting planets

We point out an intriguing relation between the masses of the transiting planets and their orbital periods. For the six currently known transiting planets, the data are consistent with a decreasing linear relation. The other known short-period planets, discovered through radial-velocity techniques, seem to agree with this relation. We briefly speculate about a tentative physical model to explain such a dependence.Comment: 4 pages, 2 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Societ

Efeito do escorregamento dos fluidos nas superfícies de medição do reómetro sobre a viscosidade

Neste trabalho procedeu-se à investigação do efeito do escorregamento dos fluidos nas superfícies de medição do reómetro sobre a viscosidade. Foi necessário realizar medições da viscosidade de diferentes fluidos em superfícies lisas e rugosas para assim comparar resultados e concluir sobre o efeito da rugosidade e portanto do escorregamento. Para ser possível realizar esta investigação foi necessário fabricar duas geometrias idênticas para o reómetro que apenas são diferentes em termos de rugosidade da superfície que fica em contacto com o fluido, ou seja, uma lisa e outra rugosa. Realizaram-se vários testes de viscosidade em diferentes tipos de fluidos com as duas superfícies, para assim averiguar a existência de escorregamento. Foi possível verificar se existe ou não escorregamento pela comparação das curvas características da viscosidade obtida no reómetro e quantificar o erro obtido para diferentes fluidos. This work aimed to investigate the effect of fluid slippage on the measurement surfaces of the rheometer on the viscosity. It was necessary to perform viscosity measurements of fluids with different surfaces, smooth and rough, to be able to compare results and get conclusions of the effect of the roughness and therefore the slip. To carry out this investigation was necessary to fabricate two identical geometries but different in terms of the roughness of the surface that is in contact with the fluid, one smoothe and other rough. Several tests were performed with different types of fluids in both surfaces, to investigate the existence of slippage. It was possible to verify whether there is slip by comparing the viscosity curves obtained from the rheometer measurements and quantify the error obtained for different fluids

Re-assessing the radial-velocity evidence for planets around CoRoT-7

CoRoT-7 is an 11th magnitude K-star whose light curve shows transits with depth of 0.3 mmag and a period of 0.854 d, superimposed on variability at the 1% level, due to the modulation of evolving active regions with the star's 23 d rotation period. In this paper, we revisit the published HARPS radial velocity measurements of the object, which were previously used to estimate the companion mass, but have been the subject of ongoing debate. We build a realistic model of the star's activity during the HARPS observations, by fitting simultaneously the line width and the line bisector, and use it to evaluate the contribution of activity to the RV variations. The data show clear evidence of errors above the level of the formal uncertainties, which are accounted for either by activity, nor by any plausible planet model, and which increase rapidly with decreasing signal-to-noise of the spectra. We cite evidence of similar systematics in mid-SNR spectra of other targets obtained with HARPS and other high-precision RV spectrographs, and discuss possible sources. Allowing for these, we re-evaluate the semi-amplitude of the CoRoT-7b signal, finding Kb=1.6 +-1.3 m/s, a tentative detection with a much reduced significance (1.2-sigma) compared to previous estimates. We also argue that the combined presence of activity and additional errors preclude a meaningful search for additional low-mass companions, despite previous claims to the contrary. Our analysis points to a lower density for CoRoT-7b, the 1-sigma mass range spanning 1-4 MEarth, allowing for a wide range of bulk compositions. In particular, an ice-rich composition is compatible with the RV constraints. This study highlights the importance of a realistic treatment of both activity and uncertainties, particularly in the medium signal-to-noise ratio regime, which applies to most small planet candidates from CoRoT and Kepler.Comment: 11 pages, accepted for publication in MNRAS, revised version with minor modifications and one additional figur

Une sémantique multi-paradigme pour simuler des modèles SysML avec SystemC-AMS

National audienceDans le contexte de la modélisation de systèmes, SysML apparait comme un langage pivot de spécification et de documentation. Ses diagrammes permettent la définition de la structure et du comportement de systèmes. La flexibilité de SysML a pour incon-vénient qu'il n'existe pas de méthode standard pour définir leur sémantique. Ce problème est flagrant dans la conception de systèmes hétérogènes, où différentes sémantiques opéra-tionnelles peuvent être utilisées. Cet article présente une manière de donner une sémantique opérationnelle aux éléments de SysML sous la forme de transformations vers le langage SystemC-AMS, permettant ainsi la simulation de modèles SysML