Improvements in both the photonic confinement and the emitter design have led to a steady increase in the strength of the light-matter coupling in cavity quantum electrodynamics experiments, to the point that the light matter coupling, Ω, can become a non-negligible fraction of the bare dipole frequency ω0. In such an interaction-dominated regime, usually referred to as ultrastrong coupling regime, the state of the system is usually well described in terms of mixed light-matter excitations, called polaritons. I will show that, when the coupling between light and matter becomes strong enough, this picture breaks down. In particular, in the non-perturbative deep strong coupling regime, Ω > ω0, light and matter degrees of freedom totally decouple. A striking consequence of such a counterintuitive phenomenon is that the Purcell effect is reversed and the spontaneous emission rate, usually thought to increase with the light-matter coupling strength, plummets instead for large enough couplings. This phenomenon, which should become observable already for resonant normalized couplings, Ω /ω0, of the order of 1, that is less than a factor 2 from present day experiments, poses an intrinsic upper bound to the efficiency of strongly coupled light emitters. Nevertheless, it could be exploited to dynamically control the electromagnetic field distribution in optoelectronic devices, by modulating the intensity of the light-matter interaction by optical or electrical means.

Integrable non-linear Hamiltonian systems perturbed by additive noise develop a Lyapunov instability, and are hence chaotic, for any amplitude of the perturbation. The situation described here being `multi-resonance' -- by nature well beyond the Kolmogorov-Arnold-Moser and the Nekhoroshev regimes, it offers an analytic glimpse on the situation in which many near-integrable systems, such as some planetary systems, find themselves in practice.

I will discuss recent progress on the understanding of the quantum bound states of supersymmetric black holes. Two complementary descriptions exist: the Coulomb branch description in terms of a solution space of bound states, and the Higgs branch description in terms of the cohomology of quiver moduli spaces. I will explain how one can determine explicitly the number of quantum states (the BPS index) for arbitrary sets of constituents as function of the "single centered indices". A single centered index enumerates the quantum states of a single constituent. I will furthermore describe a generalized mutation symmetry which relates bound states with different sets of constituents and single centered indices.

Metamaterials are artificial materials which are not available in nature and in which the propagation of waves is non conventional. Building a metamaterial requires to control the physical quantities that drive this propagation. In 1964, Veselago [1] theoretically postulated the possibility to get materials exhibiting negative refractive index. For electromagnetic waves, this imposes simultaneous negative values for both the dielectric permittivity ε and the magnetic permeability μ at the same frequency. This concept was extended to acoustic waves in which wave propagation is here driven by the mass density (rho) and the compressibility (kappa) of the material. Pioneering works [2] showed that negative values of these quantities may be attained by dispersing small entities (microresonators) in a bulk phase. As the exciting wave frequency matches the one of the dispersed objects, rho and kappa may exhibit very large or negative values, thus leading for example to the expected negative values for the acoustic index. We recently showed [3] that large resonances could be obtained in the presence of micro-sized objects exhibiting a large contrast of sound speed velocity (i.e. a ratio of 3 between the dispersed objects and the matrix) with the surrounding matrix. Moreover, we showed that the acoustic spectrum can be tuned by an external magnetic field when replacing the emulsion oil droplets by ferrofluid droplets [4]. However, possible meta effects (i.e. negative refraction) can only be obtained by reaching larger values for this contrast. Good candidates to reach lower sound speed values are porous materials, which exhibit both a high compressibility and a non zero mass density thus leading to very small c ~ 1/(rho.kappa)^1/2. Thus, we did synthesize porous beads of controlled size and composition using a microfluidic approach. Submitted to an ultrasonic acoustic wave, the obtained composite materials are shown to exhibit very large resonance peaks. These results pave the way to the first realization of soft acoustic metamaterials [5]. [1] Veselago, Soviet Phusics Uspekhi 10, 509 (1968) [2] Liu et al., Science 289, 1734 (2000) [3] Brunet et al., Applied Physics Letters 101, 011913 (2012); Mascaro et al., Journal of the Acoustical Society of America 133, 1996 (2013) [4] Brunet et al., Physical Review Letters 111, 264301 (2013); Zimny et al., Journal of Materials Chemistry B 2, 1285 (2014) [5] Brunet, Leng, Mondain-Monval, Science 342, 323 (2013)

Progress has been made during the last decades in the development of a statistical theory for classical out-of-equilibrium system. In addition to celebrated fluctuation theorems, stochastic thermodynamic now provides both a nice unifying theoretical framework and a useful way for studying these systems experimentally. This success in understanding classical systems drew interest in similar problems for out-of-equilibrium quantum systems. A set of various fluctuation relations have then been obtained. Severe issues however appeared in the very definition of thermodynamic quantities for quantum systems, and a unified framework for driven out-of-equilibrium quantum open system is not yet available. We propose to fill this gap by constructing a quantum stochastic thermodynamics allowing for definitions of basic thermodynamic quantities such as heat transfer, work and entropy at the single trajectory level. This formalism can then be used to obtained “central” fluctuation theorems, from which follow more practical fluctuation relations such as for instance Jarzynski and Crooks fluctuation theorems.

The description of the flow of granular media still represents a challenge due to the difficulty in writing constitutive laws. However, recent advances have shown that a simple hydrodynamical description based on a frictional visco-plastic law may help in understanding the properties of granular flows. In this talk, the successes and limits of this continuum description will be discussed, before revisiting the classical problem of the drag and lift forces experienced by an object moving in a granular medium. It will be shown that the peculiar rheology of granular media leads to striking behaviours when a cylinder is dragged through a packing of grains.

The interaction between the electromagnetic radiation and an optical active material give rise to new eigenstates of the coupled system called exciton-polaritons. Such regime of light-matter interaction is called “strong coupling”, indicating a non-perturbative change of the energy dispersion of the coupled modes with respect to the bare ones. The unique features arising from the double nature of these quasi-particles has allowed the study of quantum fluids dynamics in solid state structures, and offer interesting possibilities for their implementation in optical devices. After reviewing their main properties in the strong coupling regime, the observation of exciton-polaritons in the ultra-strong coupling regime, where the interaction strength is brought above a considerable fraction of the exciton transition energy, will be presented and discussed.

Asymptotic safety is a scenario of quantum gravity conjecturing that the ultraviolet behaviour of gravity is governed by a non-trivial fixed point of the renormalization group, providing a self-completion of the effective field theory approach. In this talk I will review the basic ideas behind such scenario and the methods used to test it. I will also highlight some recent developments and open questions.