Laboratoire Kastler Brossel, ENS
"Optique Adaptative Extrême : imagerie en milieux diffusants"
La diffusion de la lumière dans un milieu diffusant, par exemple la peau ou encore un verre de lait, est en général considéré comme une perturbation inévitable et néfaste. Ce phénomène détruit en apparence, via des diffusions et des interférences multiples, tout information spatiale ou de phase contenue dans une onde laser incidente. Spatialement, cela se manifeste par l’apparition de tavelures (le « speckle », en anglais) dues aux interférences. Dans le domaine temporel, une impulsion lumineuse courte entrant dans un milieu diffusant verra sa durée allongée à cause de la multiplicité de chemins longs ou courts que la lumière peut prendre avant de sortir du milieu. D’un point de vue pratique, la diffusion limite donc fortement les possibilités d’action dans un milieu diffusant, tant pour l’imagerie que pour la manipulation optique d’objets.
Néanmoins, les milieux diffusants les différents paramètres, de structures totalement désordonnées (comme des suspensions dans des liquides) à totalement ordonnées (comme les cristaux photoniques), de solide à granulaire ou liquide, de simple diffusion à diffusion résonante, de milieu absorbant à milieu amplificateur, ouvrent de nombreuses voies d’exploration et d’étude de phénomènes physiques nouveaux.
La diffusion multiple, phénomène hautement complexe, reste un phénomène déterministe : elle est donc en principe réversible. Le speckle est cohérent, et il est donc envisageable de le contrôler de manière cohérente. En « façonnant », ou en « adaptant » le champ incident, il est en principe possible de contrôler la propagation et de s’affranchir du processus de diffusion. Je montrerai comment il est possible est de contrôler le champ incident sur un milieu diffusant, et ainsi imager ou étudier ces milieux.
Groupe Ecoulements de Particules, IUSTI, Polytech Marseille
"Les milieux granulaires : entre fluide et solide"
Sable, sucre, riz, cailloux sont quelques exemples de milieux granulaires. Cette matière en grains qui est présente dans notre vie de tous les jours est d’une grande importance dans de nombreux procédés industriels ainsi que dans de nombreux phénomènes naturels (avalanches, éboulements de terrain, dunes…). Pourtant leur comportement à la frontière des fluides et des solides résiste encore sur bien des points à notre compréhension. Le séminaire sera l’occasion à travers quelques expériences de démonstration de discuter des avancées récentes dans le domaine et des questions encore ouvertes que posent le comportement des milieux divisés.
Laboratoire de physique théorique, ENS
"How physics makes biology work"
Living organisms have to obey the laws of physics. They can also exploit physical principles to function. Huge bird flocks move in a correlated manner, cells transmit information when responding to their ever changing environment and the immune system brings together many cells to protect us from threatening bugs. In the spirt of the condensed matter “more is different” maxime, I will show how we can explain the observed complex behavior in biological systems in terms of physical laws and maybe learn some new ones.
2 présentations de stage long par des anciens étudiants du M1
"Normal Modes of Soft-Sphere Packings : from High to Physical Dimensions"
"Development of Robust Quantum Control over Nitrogen Vacancy Centers in Diamond"
Nitrogen Vacancy centers in diamond (NV centers) are defects with interesting properties that make them relatively easy to manipulate. Their ground state forms an electronic spin S = 1 triplet which can be optically addressed (pumping) and read. The NV centers are sensitive to magnetic fields, electric fields and strain, and are already being used as nanoscale sensors with potential applications in research, industry and medicine. Transitions are addressed with RF pulses and can be isolated to form an effective two-level system (qubit) with long coherence times, making the NV center a good candidate for quantum computing. The goal of my project was to install and test a new Digital to Analog Converter (DAC) used as an Arbitrary Waveform Generator (AWG) for enhanced control over the NVs’ spin states. The DAC was implemented in the experimental setup, and was used to perform Dynamical Decoupling (DD), allowing a significative improvement over the coherence times of the qubits. Finally, the DAC and DD sequences were used to investigate the origin of decoherence of shallow NV centers by probing surface noise.
"A Beautiful Question : Finding Nature’s Deep Design"
Physicists often talk of the beauty of their picture of the world. But
what exactly do we mean ? Are we kidding ourselves ? What does this kind
of “beauty" have to do with beauty in art and music ? And if we truly
identify beauty in the fundamental laws of nature : Why does it happen ?
What does it mean ? Those are the kinds of questions I’ll explore, aided
by some striking images.
Laboratoire Interdisciplinaire de Physique, Univ. J. Fourier (Grenoble)
"Multiscale modelling : connecting statistical physics to engineering applications"
The development of nanosciences raises new challenges and opportunities for material sciences. One may broadly say that these new challenges fall in two different categories. Firstly, how do the properties of matter evolve as a function of the scale under consideration, and in particular when the dimensions of devices fall into the nanoscale range ? While this question is most often discussed for electronic properties, it also arises for many other usage properties of materials, in particular mechanical, flow or thermal properties. Second, can one make use of a better knowledge of the nanoscale properties to gain a better understanding of, and possibly improve, macroscopic properties of materials that involve internal structures at various scales ?
These questions are intimately related to the development of multiscale modelling approaches, a rather broad domain which is understood here as methods that allow one to establish relevant links between phenomena taking place at different scales of description, and assess the validity of each description at different length and time scales. In this spirit, the presentation will address problems that are of current interest to the materials science and engineering community, and involve particular challenges in bridging the length and time scales gaps, such as interfacial transfers or properties of nanostructured materials.
Université Paris-Sud, ENS
"Genetic Algorithms and Quasispecies"
We will present the basic genetic algorithm. We will explain the quasispecies model introduced by Eigen.
We will finally discuss possible ways to control the behavior of the genetic algorithm.
Laboratoire de radioastronomie, ENS
"The large ground-based instruments for 21st century radio-interferometry"
Radioastronomy, which was born in in the 1930s, is now entering a new era, with several large ground-based interferometric instruments opening to the astrophysical community, and several others currently under construction. In the submillimetre and millimetre range, the Atacama Large Millimeter Array (ALMA) has already provided revolutionary observational data that challenge our understanding in several fields of astrophysics. In the near future, the longer wavelength ranges will undergo a similar, if not even more drastic revolution, with the advent of the Low Frequency Array (LOFAR) and Square Kilometer Array (SKA) instruments.
I will give an introduction to the field of radio-interferometry, then present these new and upcoming observational facilities, with an emphasis on the exciting science that is expected from them.
UMR CNRS Gulliver 7083, ESPCI
"Waves and ship wakes"
Simply by looking at a duck swimming in a pond or a cargo ship moving on a calm sea, one can clearly tell that there is something common about their wake. Indeed, they both display a familiar V-shaped pattern which only differ from one another by their dimensions. In 1887, Lord Kelvin was able to provide a theory to explain the ship-wave pattern. His most popular achievement was to prove that the wake created by a disturbance moving at a uniform pace is always delimited by a straight wedge with half-angle 19.5 degrees, independent of the velocity of the disturbance. Recently, Kelvin’s century old and well accepted theory was challenged, by that drawing the attention of the fluid dynamics community…
Laboratoire Pierre Aigrain, ENS
"A carbon nanotube based single-photon source"
After a brief introduction on the optical properties of solids at the nanoscale, I will present our recent results about using carbon nanotubes as quantum sources of light. In particular, the use of an original micro Fabry-Perot cavity based on micro-engineered optical fibers allows us to tailor the emission properties of carbon nanotubes by means of cavity quantum electrodynamical (CQED) effects. Finally, the specificity of a solid-state embedded two-level system shows up in the coupling to vibrations (phonons) and provides an extra knob to tune the photon emitter on a very large spectral window.
Laboratoire Kastler Brossel, ENS
"Metastable solid helium 4 : a way to supersolidity ?"
After a quick presentation of the fascinating properties of condensed phases of helium -4 at low temperatures, I will present the effort of the experimental group "solid helium" at Laboratoire Kastler Brossel to achieve a metastable state of solid helium. This state is a candidate for a possible but not yet discovered supersolid state of matter.
RECRUTEMENT ACCES DIRECT
Le département de physique
Programme, cours et calendrier 2022 - 2023 