Our current cosmological model tells us that 80 per cent of the mass in our Universe
must be in the form of dark matter. The hunt for the particle that constitutes this dark component of our Universe is one of the biggest open quests in current physics. The dynamics of dark matter particles shape the large-scale structure of our Universe and provide the building blocks for the formation of galaxies. I will give on overview over what we know about dark matter and how we model it in big computer simulations of the formation of structure in the universe, and thus how we hope to get closer to understanding its nature.

It is shown that the addition of small amounts of microscopic rods in a viscous fluid at low Reynolds number causes a significant increase of the flow resistance. Numerical simulations of the dynamics of the solution reveal that this phenomenon is associated to a transition from laminar to chaotic flow. Polymer stresses give rise to flow instabilities which, in turn, perturb the alignment of the rods. This coupled dynamics results in the activation of a wide range of scales, which enhances the mixing efficiency of viscous flows.

Imagine that we want to design a tiny machine capable of sensing an external chemical concentration and move, for instance, upgradient. Our machine is so small that it cannot detect concentration gradients and so primitive that it cannot regulate its speed, which remains constant. If we assume that the machine has memory, as it moves in one direction it can compare the chemical concentration at different locations, compute effectively the gradient, and switch direction if it moves downgradient or keep its current direction if it moves upgradient. This is in a nutshell the chemotactic strategy used by some bacteria as E. coli. Here, we propose a different strategy, arguably simpler: we equip the particles with a clock that regulates the switching direction of the particles and show that there are specific clock designs, whose period depends on the chemical concentration, that allow guiding our tiny machine upgradient. We argue that the use of clocks may prove key to design chemotactic robots and to understand some bacterial systems.

Face-to-face contacts between individuals play an important role in social interactions and can also determine the potential transmission routes of infectious diseases, in particular of respiratory pathogens. An accurate description of these patterns is therefore of interest for the fundamental knowledge and understanding of human behaviour and social networks as well as in epidemiology, in order to identify contagion pathways, to inform models of epidemic spread, and to design and evaluate control measures such as the targeting of specific groups of individuals with appropriate prevention strategies or interventions.

In this talk, I will present results obtained by the SocioPatterns collaboration, which focuses on measuring and modeling human contacts, and on using the gathered knowledge in fields such as social science or epidemiology. I will briefly describe the SocioPatterns sensing platform and some of the datasets collected in the last years. I will show how such datasets can be of use to investigate issues of interest in the epidemiology of infectious diseases, such as the relative efficiency of various mitigation measures such as the targeted closure of classes in schools or the closing of whole schools, in case of an epidemic spread. I will also discuss the issue of how incomplete data sets can bias the outcome of simulations and describe methods to compensate for such biases.

The problem of the interaction of N particles with a laser beam and vacuum modes can give rise to many interesting phenomena concerning the spontaneous emission of light and its propagation in the medium. The cooperative effects, for example, like super- and subradiance, are effects related to the coherence created between the particles when a photon is emitted spontaneously from a single excited particle. Superradiance can be defined as the enhancement of the spontaneous emission due to constructive interference of the scattered light. Its counterpart, subradiance, is the trapping of some remaining light due to destructive interference. In cold atoms, some previous theoretical works predict and characterize these two cooperative effects in a large and dilute cloud, in the regime of weak intensities and large detuning of the driving laser. The theoretical model is a coupled-dipole model for two-level atoms at single excitations. The experiment consists in measuring the super- and subradiant decay rates from the temporal emitted power after the switch-off of the laser in the steady state. In this work, we report the experimental observation of super and subradiance in a large cloud of cold atoms, published recently [1] [2]. For subradiance, the main result is the linear scaling with the optical thickness of the cloud and its independence with the detuning. For superradiance, off-axis superradiance is observed, i.e., the superradiance out of the forward direction. Finally, we check the validity of our measurements from the predictions of the coupled-dipole model.

[1] W. Guerin, M. O. Araújo, R. Kaiser. Phys. Rev. Lett. 116, 083601 (2016)

[2] M. O. Araújo, I. Krešić, R. Kaiser, W. Guerin. Phys. Rev. Lett. 117, 073002 (2016)

Cet exposé a pour but de présenter les problématiques associées à la manipulation spectro-temporelle d'impulsions lumineuses ultra-brèves ainsi que les possibilités offertes par les cristaux liquides dans ce domaine. Je présenterai quelques résultats récents obtenus dans le cadre du laboratoire commun SOFTLITE.