Identical charged macromolecules in a solvent may exhibit counterintuitive interactions, such as attractive behaviour in a certain parameter range. After a brief introduction, we will explain in simple terms the phenomenon, which occurs under strong coulombic couplings. We will next review its practical consequences and the controversies it has given rise to in the last 80 years. Recent progress allowing for an analytical description will
then be presented, together with open problems.

In this seminar, I will review recent advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical media [1] and will present interesting perspectives for the future.

[1] I. Carusotto, C. Ciuti, Rev. Mod. Phys. 85, 299 (2013)

On a coarse-grained level, colloids often interact via so-called ultrasoft potentials, which assume at short interparticle distances values in the order of a few k$_{\rm B}T$; thus, these particles are able to overlap at the cost of a relatively small energy penalty. Under equilibrium conditions such ultrasoft particles are able to form aggregates (clusters) of overlapping particles which can then either form a disordered or an ordered cluster phase. In the latter case, these aggregates populate the positions of a regular fcc or bcc lattice. Cluster crystals display rather unconventional properties, such as a density-independent lattice constant [1]. Also under out-of-equilibrium conditions, ultrasoft systems show unexpected features. Exposing a cluster crystal to shear leads – with increasing shear rate $\dot \gamma$ – to the following novel response-scenario: for small $\dot \gamma$-values the crystal melts; then gradually strings parallel to the flow direction form which are arranged in a hexagonal grid in the gradientvorticity plane. Upon further increasing $\dot \gamma$ this lattice eventually melts [2]. Exposing a cluster crystal to Poiseuille flow the emergence of a quantized flow pattern is observed where the height and the width of the fluid stream display well-defined plateaus, indicating a successive fluidization of crystal layers adjacent to the channel walls [3].
[1] B.M. Mladek et al., Phys. Rev. Lett. 96, 045701 (2006).
[2] A. Nikoubashman et al., Phys. Rev. Lett. 107, 068302 (2011).
[3] A. Nikoubashman et al., Soft Matter 8, 4121 (2012).

Quantum cryptography is the most mature quantum information technology and there exist several companies selling quantum cryptography products. In recent years, several attacks have hacked quantum cryptography implementations. These attacks have proven that implementations of quantum cryptography are subtler than expected and that any mismatch between theory and experiment may open important security loopholes. To close this gap, we introduce the device-independent approach to quantum information theory in which devices are just seen as quantum black boxes processing classical information. This level of abstraction makes the protocols defined in this scenario insensitive to the recent hacking attacks.

We then discuss the device-independent scenario from a more fundamental point of view and introduce the set of almost quantum correlations. We prove that this set (i) is supra-quantum, as it predicts correlations that are impossible in the quantum formalism but (ii) is operationally equivalent to the quantum set for many information tasks. These results then demonstrate that it is possible to modify the structure of quantum correlations without running into any strong operational contradiction with quantum predictions. Intriguingly, the set of almost quantum correlations can also be derived within the history approach introduced in the context of quantum gravity.

In this seminar I will review a family of models that couple Maxwell equations for the classical description of an electromagnetic field induced by a laser and Bloch equations for the quantum description of matter, which will be of various type, isotropic gases, crystals, quantum boxes. The models are obtained either in the Schrödinger or the Heisenberg picture of quantum mechanics. Various phenomena can be taken into account such as nonlinear effects, Coulomb interaction, electron-phonon interactions… Numerical methods have been developed to show the numerous situations that can be described by these models. They are however very expensive to simulate and a few hints on derived asymptotic models will also be given.

It is generally accepted that the plasticity of amorphous material comes from localized reorganizations, which may produce form large structures of deformation. This description is supposed to describe the plasticity and the failure in numerous experimental systems (emulsions, dense colloidal suspension, metallic glasses,..). However the experimental observations of such behavior are missing. In this seminar, we will discuss an experimental study of the plastic deformation of a granular material. The material is compressed and the deformations are measured using a dynamic light scattering set-up. A intermittent structure of the plastic flow appears. The anisotropy of the deformation are given by the mechanical stress redistributions of isolated events. This structure of the plastic flow is more intense when the failure is approached. However the final failure of the material seems to be driven by the internal solid friction of the material.

Quantum physics rules electrical and heat transport across low dimensional conductors. Despite thirty years of research in mesoscopic physics, the field of quantum heat transport remains mostly unexplored experimentally, essentially because heat currents are much harder to probe than electrical currents.
In this talk, I will present how heat transport measurements allow us to investigate not only fundamental quantities, such as the quantum of thermal conductance1,2, but also provides a new way to explore the rich physics of the integer and fractional quantum Hall effect. We have developed a very straightforward approach to measure the heat current in quantum conductors realized in two dimensional electron gases at the interface between GaAs and AlGaAs layers. It relies on the measurement of current fluctuations using an extremely sensitive voltage amplifier operating at cryogenic temperatures, based on homegrown high- electron-mobility transistors.

Understanding how materials respond to external mechanical perturbation is a central problem of science and engineering. While for most practical purposes it is useful to idealize the mechanical response of a material as a deterministic function of the externally applied perturbation, disorder and fluctuations are unavoidable, leading to sample-to-sample variations and non-trivial size effects. The size dependence of strength is a well known but still unresolved issue in the fracture of materials and structures. The difficulty in addressing this problem stems from the complex interplay between microstructual heterogeneity and long-range elastic interactions. Furthermore, in micro and nanoscale samples, the plastic yield strength displays size effects and strain bursts, features that are not present in macroscopic samples where plasticity is a smooth process. Large fluctuations both in fracture processes and in microscale plasticity make the use of conventional continuum mechanics problematic and calls instead for a statistically based approach. In this talk, I will review recent results obtained from idelized models of disordered fracture and from more realistic simulations of defected graphene. Finally, I will discuss the size dependence of strain burst statistics as revealed by statistical models for crystal and amorphous plasticity.

I will discuss the rheology and mechanical properties of wet granular materials, and show why the behavior can be very subtle. Once one understands the mechanical properties, I will show that one can use this knowledge to construct the perfect sandcastle, or to understand why the ancient Egyptians wetted the desert sand with water before sliding heavy stones over it. I will then go on to show some new results on friction at the microscopic scale, between 2 grains. Amonton's famous friction law states that the friction force is proportional to the normal force since both are proportional to the area of contact. However for spherical grains the contact area is not proportional to the normal force, as shown by Hertz long ago. We use new fluorescence technique that allows us to probe the real area of contact between 2 rough surfaces. In our case, we conclude that important deviations from Amonton's law are observed.