Surface plasmon polaritons (SPPs) are bosonic quasiparticles. As such, they show specific quantum properties, some of which have already been observed. Here we report experimental results from two sets fundamental tests on SPPs : a clear observation of the wave-particle duality of a single SPP, and a first direct experimental evidence of entanglement between a SPP and a single photon. Both our experimental setups are based on a plasmonic platform that can convert photon into plasmon, and separate or recombine them on a plasmonic beamsplitter. This platform is used as output beamsplitter of a Mach-Zehnder (MZ) interferometer. We use a source of photon pairs in order to shine the platform with a single photon α, heralded by the second photon of the pair, β. In the first experiment, the polarization of α is chosen so that it excites either one single arm, or both arms of the MZ with equivalent probabilities, thus exhibiting either antibunching (particle-like behaviour) or interferences (wave-like behaviour). In the second experiment, α and β are entangled. By carrying out a projective measurement of the polarization of β, we show that we can control the interference state of the single SPP on the plasmonic platform, and make fringes appear or disappear in the interferograms, thus demonstrating entanglement between both particles.

Two-dimensional materials are currently a topic of widespread research interest, in areas both fundamental and applied. These are crystalline materials consisting of a single layer of atoms, although very thin, few-layer materials are also considered 2D or quasi-2D. Since the isolation of graphene in 2004, a wide variety of 2D materials have been found, with electronic properties ranging from metallic to insulating, and a variety of applications have been suggested. In this talk I will discuss the basic properties of 2D materials (including graphene, silicene, hexagonal boron nitride, black phosphorus, and the transition metal di-chalcogenides (TMDCs) such as molybdenum disulfide), their electromagnetic models, and various applications, including interactions with quantum emitters and valley pumping scenarios.

Exceptionnally the colloquium is opened to all students of the Physics Department. Detecting gravitational waves required 4 decades of experimental effort to reach a sensitivity at the h~10-21 level, corresponding to mirror displacements below 10-18 m. I will review this "noise hunting" effort and give some details about the recent observation of 2 black hole mergers. Apart from classical noise (seismic noise, thermal noise...), it was realized as soon as in the late 70s that quantum fluctuations of the light field were responsible for the Standard Quantum Limit, a sensitivity limit that second-generation gravitational-wave interferometers will reach once they operate at their design sensitivity. A number of ideas have been considered to beat the SQL: squeezed states of the light field, tailoring the optical response function or taking advantage of the mirror mechanical response to radiation pressure. I will present the first experimental demonstrations of such ideas, either on suspended interferometers or table-top experiments. I will also discuss how recent progress in micro/nanomechanics has allowed the emergence of the new field of quantum optomechanics, which addresses the quantum aspects of both the light field and the mechanical resonator. This new field is very promising both for the study of the foundations og quantum mechanics (quantum states and decoherence of macroscopic objects...) and for quantum information. I will present a few typical recent experiments.

Ionic liquids are an emerging class of solvents with an appealing set of physical attributes. These include negligible vapor pressure, impressive chemical and thermal stability, tunable solvation properties, high ionic conductivity, and wide electrochemical windows. In particular, the non-volatility renders ionic liquids practical components of devices, but they require structure-directing agents to become functional materials. Block polymers provide a convenient platform for achieving desirable nanostructures by self-assembly, with lengthscales varying from a few nanometers up to several hundred nanometers. Furthermore, ionic liquids and polymer blocks can be selected to impart exquisitely tunable thermosensitivity, by exploiting either upper or lower critical solution transitions (UCSTs and LCSTs). In selected cases, it is also possible to prepare photoreversible and photopatternable systems. Overall, by combining designed block polymers and ionic liquids we have demonstrated materials with superior performance for a remarkably diverse set of applications. These include micelles for extraction, nanoreactors for catalysis, gate dielectrics in organic transistors, electrochromic and electroluminescent gels, and membranes for gas separation, ion batteries, and fuel cells.

The electric double layer is generally viewed as simply the boundary that interpolates between an electrolyte solution and a metal surface. Contrary to that view, molecular simulations show that the interface between ionic liquids and metallic electrodes can exhibit structures and fluctuations that are not simple reflections of surrounding bulk materials. This rich behavior is absent from a mean-field picture that averages over intraplanar structure. The charge of the electrode is screened by the interfacial fluid and induces subtle changes in its structure, which cannot be captured by the conventional Gouy-Chapman theory.

In recent years, we have performed molecular dynamics simulations on a variety of electrochemical systems  made of ionic liquids (either pure or mixed with acetonitrile), and electrodes of different geometries ranging from planar to nanoporous. A key aspect of our simulations is to use a realistic model for the electrodes, by allowing the local charges on the atoms to vary dynamically in response to the electrical potential caused by the ions and molecules in the electrolyte.

These simulations have allowed us to gain strong insight on the structure and dynamics of ionic liquids at electrified interfaces. From the comparison between graphite and porous CDC electrodes, we have elucidated the molecular mechanism at the origin of the increase of the capacitance enhancement in nanoporous carbons. The simulations also provide us the diffusion coefficients of the ions and the charging times for the full supercapacitor device. 

After launching the information age with the development of microelectronics and optoelectronics, semiconductors are expected to play a major role in another technological revolution, linked to the control of individual quantum objects. Among all the quantum systems based on these materials, deep point defects emerge as very promising due to their high versatility and their potential large scale on-chip integration. While being trapped in a crystal, they share strong similarities with vacuum isolated atoms, like optical excitation and fluorescence emission. Furthermore, some of them also possess a long-lived ground-state electron spin that can be controlled by a combination of optical and microwave magnetic fields. This additional quantum degree of freedom can be used on a broad range of applications, spanning from ultrasensitive nanoscale sensing to quantum information processing.

In this talk, I will give an overview of the potential of spin defects in semiconductors for quantum technologies. First I will explain what makes these systems so versatile and attractive by focusing on the current most prominent defect: the NV center in diamond. Then I will pick a few examples of the work I carried out on this system to illustrate its current abilities. At last, I will detail the research project I want to develop at L2C, linked to another semiconductor that could share the advantages of diamond without its drawbacks and that is still very little explored in the context of quantum technologies, namely silicon carbide (SiC).