# 2020-2021 ICFP seminar Program

Next student seminar :

Master ICFP first year Internship

News : ICFP Research seminars
November 15 - 19, 2021 :

Tél : 01 44 32 35 60
enseignement@phys.ens.fr

The seminar takes place once every two weeks (odd number weeks), on Tuesday at 5:15pm. On the even number weeks you are invited to follow the colloquium of the physics department.

### October 6: Jean-François Rupprecht et Hervé Rouault

Centre Physique Théorique (Université Aix-Marseille)
Theoretical Physics of Living Matter (JFR) and Brain representations of space (HR)

### October 20: Marco Cirelli

LPTHE, Sorbonne Université
À la recherche de la matière noire de l’Univers
La Matière Noire constitue plus de 80% du montant total de matière dans l’Univers: nous sommes sûrs qu’elle existe, nous pouvons deviner certaines de ses propriétés, mais nous n’avons aucune idée de ce qu’elle est vraiment. Il s’agit d’un des problèmes les plus urgents en cosmologie et physique de particules à nos jours. La recherche expérimentale procède sur plusieurs axes: détection dite directe dans des expérience souterraines, détection dite indirecte dans l’espace et recherche aux collisionneurs de particules. Les recherches théoriques cherchent à bâtir un modèle cohérente et capable d’expliquer toutes les propriétés observées. Cet exposé passera en revue les preuves connues à ce jour de l’existence de la Matière Noire et dressera un portrait rapide des recherches phénoménologiques et théoriques.

### November 3: Quentin Glorieux

Laboratoire Kastler Brossel, Sorbonne Université
Paraxial fluid of light in hot atomic vapors
In a nonlinear de-focusing medium, paraxial light will behave as a superfluid and forms one example of a broader class of so-called “quantum fluids of light”. These superfluids of light (or photon superfluids) have been theoretically shown to behave in the same way as, for example, a Bose-Einstein Condensate (BEC) of atoms with some experiments confirming effects such as the spontaneous nucleation of quantised vortices. Compared to their BEC counterparts, superfluids of light exhibit many advantages, including a immediate accessibility to quantities such as the fluid phase profile and in general simpler experimental requirements that therefore potentially allow access to a wider range of underlying physical phenomena. A key step in the characterization of these system is the observation of the Bogoliubov dispersion. In this presentation I will present our system based on hot atomic vapor and show two measurement of the dispersion i) based on group velocity measurement for small density perturbations and ii) using short-pulse Bragg spectroscopy adapted for photon fluids. Moreover, I will present the first experimental evidence of interference between Bogoliubov excitations in a photon superfluid and discuss why these interferences can also be interpreted as stimulated Sakharov oscillations, i.e. an analogue of fluctuations imprinted in the primordial Universe and visible as oscillations in the cosmic microwave background spectrum. Finally, by measuring the static structure factor of our quantum fluid after an interaction quench, we also evidenced for the first-time quantum depletion in a fluid of light. These results therefore open new avenues for the study quantum effects and analogue cosmological phenomena in the laboratory, moving beyond the traditional studies of Hawking radiation and black hole analogues and reaching out to primordial cosmological physics.

### November 17: Brigitte Leridon

Laboratoire de Physique et d’Etude des matériaux (ESPCI)
High-Tc superconducting cuprates: phase transitions and fluctuations

Laboratoire Kastler Brossel, Sorbonne Université
Atomic interferometry and determination of the fine structure constant
For three decades now, laser cooling of atoms has made it possible to study matter at low temperatures. The atomic interferometer, which is based on the wave nature of matter at these temperatures, makes it possible to measure the phase of an atomic wave, and therefore its energy, with very high precision. At the Kastler Brossel laboratory, we have built one of the world’s most accurate atomic interferometers, which has enabled us to measure the mass of an atom with unrivalled precision. Thanks to this measurement, it is possible to improve the determination of the fine structure constant α. This determination is crucial for performing test of the standard model based on quantum electrodynamics.

no seminar

### January 12: Benjamin Huard

Ecole Normale Supérieure de Lyon
Measuring the number of photons in a microwave mode
Counting the number of photons in an electromagnetic mode is an important tool for quantum information processing. In order to perform a single shot measurement, one usually encodes information about the photon number into a qubit state and read out the qubit. Repeating this procedure while varying the encoded single bit of information enables to pinpoint the number of photons. In this talk, I will present two experiments that address two main challenges in photocounting. First, I will show how one can avoid the sequential repetition of qubit measurements and instead use a single superconducting qubit in order to multiplex the measurement of the photon number in a stationary microwave mode. Second, time permits, I will show how we could convert a stationary mode counter into a photocounter of traveling wave packets using a quantum memory.

### February 2: Christophe Gissinger

LPENS
Entraînement magnétique des océans des lunes de Jupiter

Grâce aux données des missions spatiales réalisées au cours des dernières décennies, il est désormais indéniable que de vastes océans liquides se cachent sous la surface de plusieurs lunes et planètes du système solaire (Ganymède, Europe, Encelade, Cérès, Titan, etc.).

Bien que ce soit peut-être le type de monde océanique le plus courant, l’hydrodynamique de ces océans souterrains reste peu comprise. Il existe de nombreuses questions concernant les mouvements océaniques engendrés sous la surface glacée ou les mécanismes qui maintiennent l’eau salée (et donc conductrice) à l’état liquide.

En combinant des expériences de laboratoire sur des métaux liquides, et des simulations numériques de l’intérieur d’Europe, nous montrerons durant cette conférence que le champ magnétique de Jupiter a une influence majeure sur la dynamique globale de l’océan d’Europe et peut jouer un rôle important dans le bilan de chaleur des lunes joviennes :

Le champ magnétique de Jupiter, qui varie dans le temps, agit sur l’eau salée conductrice de l’électricité. Cela engendre à l’équateur d’Europe un puissant jet océanique similaire au Golf Stream sur Terre, et produit un faible chauffage par effet Joule aux pôles. Autrement dit, Europe se comporte comme un gigantesque moteur à induction! Ces résultats concordent avec la présence de ces étranges fractures de la glace aux basses latitudes, tandis que les panaches de vapeur d’eau sortant de la croûte de glace sont plutôt observés dans les régions polaires d’Europe.

### February 16: Raphaël Jeanneret

ENS
Un peu de physique relative au phytoplancton

En introduisant certaines caractéristiques de la vie microscopique aquatique, notamment du phytoplancton (micro-organismes photosynthétiques), je m’appliquerai tout d’abord à expliquer l’attrait du physicien pour de tels systèmes biologiques, attrait pas forcément récent mais qui a gagné en popularité ces 10-20 dernières années. J’exposerai ensuite certains de mes résultats expérimentaux et théoriques concernant les interactions hydrodynamiques entre micro-algues motiles (i.e. qui nagent) et particules passives suspendues dans le milieu, un problème important pour mieux aborder certains aspects de la vie océanique (predation, micro-plastiques, etc). Je finirai en évoquant des projets actuels visant à comprendre certains comportements collectifs qui émergent au sein de populations de phytoplancton soumises à différentes situations stressantes.

### March 2: Tarik Yefsah

LKB-ENS
Quantum Simulation of Many-Body Physics with Ultracold Fermi Gases

Strongly-correlated fermions are ubiquitous in Nature, from the quark-gluon plasma of the early universe to neutron stars found in the outer space, they lie as well at the heart of many modern materials such as high-temperature superconductors, colossal magneto-resistance devices or graphene. While being a pressing issue covering a wide fundamental and technological scope, the understanding. of strongly-correlated fermions constitutes a serious challenge of modern physics, which is often hindered by the complexity of the host systems themselves. The contribution of ultracold atom experiments in this outstanding quest resides in the ability to set fermions in a well-characterized environment, where one can add a single ingredient at a time (interactions, impurities, lattice, disorder, etc.) with a high degree of control. These experiments thus allow the realization of quantum many-body systems with incremental complexity and represent an ideal playground for a direct comparison to theories. In this talk, I will first discuss how ultracold fermion platforms can serve as efficient quantum simulators for a variety of many-body problems. In the second part, I will present the progress towards the understanding of the unitary Fermi gas, which is a topic of current research in our group as well as several groups worldwide.

### March 16: Anaëlle Maury

AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris
Embryons d’étoiles et graines de planètes: l’origine (physique) des mondes

Quelles conditions dans l’Univers permettent de former des systèmes solaires comme le nôtre ?
Pour comprendre les origines de notre Soleil et de son cortège de planètes, dont la Terre, il faudrait remonter dans le temps il y à 4.6 milliards d’années… Un défi impossible auquel s’attellent les astrophysiciens, en utilisant l’espace comme succédané de machine à remonter le temps. En effet, pour éclaircir ce mystère, nous utilisons les embryons d’étoiles de nurseries stellaires à des années lumières comme laboratoire à ciel ouvert, dans le but d’établir un scénario physique permettant d’expliquer la formation des milliards d’étoiles de la Voie Lactée, mais aussi pourquoi l’immense majorité d’entre elles semblent abriter des cortèges planétaires.
J’exposerai nos méthodes pour dépouiller l’émission de ces objets jeunes et les comparer à des modèles, et passerai en revue certains récents résultats marquants pour la formation stellaire.

MSC, Université de Paris
Critical properties of large interacting ecosystems through the prism of statistical physics

Many complex systems in Nature, from metabolic networks to ecosystems, appear to be poised at the edge of stability, hence displaying enormous responses to external perturbations. This feature, also known in physics as marginal stability, is often the consequence of the complexity of the underlying interaction network, which can induce large-scale collective dynamics and therefore critical behaviors.

In this seminar, I will present the problem of ecological complexity by focusing on some reference models in theoretical ecology, with a special emphasis on the high-dimensional Lotka-Volterra model with random interactions and finite demographic noise [1].

By using techniques rooted in mean-field spin-glass theory, I will show how to obtain a complete characterization of the phase diagrams. Notably, I will relate emerging collective behaviors and slow relaxation dynamics to the appearance of complex phases and rough energy landscapes akin to those occurring in other classes of disordered systems [2,3].

Finally, I will discuss the extension of our results: i) to the case of weakly asymmetric interactions; ii) to higher-order potentials in the dynamics of the species abundances, which turns out to be useful to model cooperative effects in ecological and biological communities.

[1] A. Altieri, F. Roy, C. Cammarota, G. Biroli, Properties of equilibria and glassy phases of the random Lotka-Volterra model with demographic noise, arXiv:2009.10565 (2020).

[2] P. Charbonneau, J. Kurchan, G. Parisi, P. Urbani, Fractal free energy landscapes in structural glasses, Nature Communications 5, 3725 (2014).

[3] A. Altieri, Jamming and Glass Transitions: In Mean-Field Theory and Beyond, Springer Nature (2019).

### May 25: Alessandro Siria

LPENS
Fluid Transport at nano and molecular scale

New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale, where the behaviour of matter strongly departs from common expectations.
This is the field of Nanofluidics : taking inspiration from the solutions found by evolved biological systems, new functionalities will emerge from the nanometre scale, with potential applications in ultrafiltration, desalination and energy conversion.
Nevertheless, advancing our fundamental understanding of fluid transport at the smallest scales requires mass and ion dynamics to be ultimately characterized across channels with dimensions close to the molecular size. A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties.
In this talk we will revisit the state of the art of fluid and ion transport at nano and molecular scale and we will discuss the recent advances obtained thanks to the development of 2D and 1D carbon systems.

### June 1: Laurent Lellouch

Centre de Physique Théorique, CNRS et Aix-Marseille Université
Muon g-2: experiment, standard model and lattice quantum chromodynamics

Twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon’s anomalous magnetic moment, $a_\mu=(g_\mu-2)/2$, with a remarkable precision of 0.54 parts per million. Since then, the standard model prediction for $a_\mu$ has exhibited a discrepancy with experiment of over 3 standard deviations, raising the tantalizing possibility of physical particles or forces as yet undiscovered. On April 7 a new experiment at Fermilab presented its first results, brilliantly confirming Brookhaven’s measurement and bringing the discrepancy with the standard model to a near discovery level of 4.2 sigma. To fully leverage this and future measurements, and possibly claim the presence of new fundamental physics, it is imperative to check the standard model prediction with independent methods, and to reduce its uncertainties. After an introduction and a discussion of the current experimental and theoretical status of $a_\mu$, I will present a precise lattice QCD calculation, by the BMW collaboration, of the contribution to this quantity that most limits the precision of the standard model prediction. The result of this calculation significantly reduces the gap between the standard model and experiment, and suggests that new physics may not be needed to explain the current, experimental, world-average value of $a_\mu$.

Next student seminar :