### Séminaires de l’année 2021

# Séminaire du LPTMS : Andreas Mayer (Princeton)

## A Langevin approach to the dynamics of the human immune system

### Andreas Mayer (Princeton)

Online seminar --- Zoom Meeting ID: 951 1138 2280 -- Passcode: Ask L. Mazza or D. Petrov -- Note the unusual time! New sequencing techniques put studying populations with a very large number of distinct species within reach. As the number of species increases, so does the number of interactions between species and with the environment. Statistical physics provides a framework to adapt ecological theory to this new setting: key phenomena can be captured by replacing interactions with effective stochastic forces. In my talk, I will present how we have applied this Langevin approach to study the dynamics of adaptive immunity. We propose a theory for the emergence and maintenance of an observed power-law scaling in the sizes of different immune cell populations. Remarkably, linking our theory to snapshots of immune composition at different ages we demonstrate that exposures in infancy leave a lifelong imprint with implications for pathogen defense and autoimmunity.# Séminaire du LPTMS : Valentina Ros (LPTMS)

## High-dimensional random landscapes: statistics of critical points and dynamical instantons

### Valentina Ros (LPTMS)

Online seminar --- ZOOM Meeting ID: 919 8494 7729 -- Password: Ask L. Mazza and D. Petrov -- High-dimensional random functionals emerge ubiquitously when modeling complex systems: for example as energy landscapes in physics, fitness landscapes in biology, or more recently loss landscapes in machine learning. They are typically very non-convex: their optimization with stochastic dynamics is highly non-trivial due to the abundance of local minima that trap the dynamics for very large times. In this talk, I will focus on random functionals with Gaussian statistics and discuss simple activated processes, in which the system jumps between trapping local minima passing through the saddles (or transition states) connecting them. In particular, I will discuss how to use random matrix theory to gain information on the distribution and reciprocal arrangement of the local minima and saddles in configuration space, and how to exploit this information to build dynamical instantons describing the activated jumps.# Séminaire du LPTMS : Matthieu Mangeat (Saarland University)

## Flocking and reorientation transition in the q-state active Potts model

### Matthieu Mangeat (Saarland University)

Online seminar --- Zoom Meeting ID: 947 5714 7410 -- Passcode: Ask L. Mazza or D. Petrov -- We study the q-state active Potts model (APM) on a two-dimensional lattice in which active particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to a collective motion at high densities and low noise. We formulate a coarse-grained hydrodynamic theory with which we compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is found, which is absent in the Vicsek model [1] and the active Ising model [2]. The origin of this reorientation transition is revealed by a stability analysis : for large velocities the transverse diffusion constant approaches zero and then stabilizes longitudinal band motion. Computer simulations corroborate the analytical predictions of the flocking and reorientation transitions and validate the phase diagrams of the APM [3]. [1] T. Vicsek, A. Czirok, E. Ben-Jacob, I. Cohen, and O. Shochet, Phys. Rev. Lett. 75, 1226 (1995). [2] A. P. Solon and J. Tailleur, Phys. Rev. Lett. 111, 078101 (2013) ; Phys. Rev. E 92, 042119 (2015). [3] S. Chatterjee, M. Mangeat, R. Paul, and H. Rieger, EPL 130, 66001 (2020) ; M. Mangeat, S. Chatterjee, R. Paul, and H. Rieger, Phys. Rev. E 102, 042601 (2020).# Séminaire du LPTMS : Pierre Suret (PhLAM - Lille)

## Integrable turbulence and soliton gas: experiments and theoretical approaches

### Pierre Suret (PhLAM - Lille)

Online seminar --- Zoom Meeting ID: 996 1840 3246 -- Passcode: Ask L. Mazza or D. Petrov -- Exactly integrable partial differential equations (PDEs) such as the Korteweg-de-Vries (KdV) or the one-dimensional nonlinear Schrödinger equation (1DNLSE) can be studied in the framework of the Inverse Scattering Transform (IST). Integrable PDEs exhibit an infinite hierarchy of invariants that prevent the development of "standard" Wave Turbulence and energy cascade. Despite the existence of the IST technique, there is no general theory describing of the propagation of random waves in integrable systems such as 1DNLSE. For this reason, Integrable Turbulence, which deals with random fields, has been recently introduced as a completely "new chapter of turbulence theory" by V.E. Zakharov, one of the creators both of the wave turbulence theory and of the IST [1]. Soliton gas (SG) is one example of integrable turbulence. The concept of SG as a large ensemble of solitons randomly distributed on an infinite line and elastically interacting with each other originates from the work of Zakharov [2], who introduced the kinetic equation for a nonequilibrium diluted gas of weakly interacting solitons of the KdV equation. Zakharov’s kinetic equation has been generalized to the case of a dense SG in Ref. [3]. Optical fibers and 1D water tanks are very favorable experimental platforms for the investigation of integrable turbulence and soliton gas (described by the focusing 1DNLSE). In this talk, I will present our recent experimental results obtained both in optical fibers and water tanks [4-6]. In the second part of my talk, by using the famous example of the modulation instability, I will show that SG is a promising model to describe the statistical properties of integrable turbulence. The spontaneous modulation instability (MI) also named “noise-induced MI” arises when a plane wave is perturbed by noise in 1DNLSE. We will show that the long-term evolution of MI can be described by a carefully designed SG [7]. [1] V. E. Zakharov, Stud. Appl. Math. 122, 219 (2009) [2] V. E. Zakharov, Sov. Phys. JETP 33, 538 (1971) [3] G. El, Phys. Lett. A 311, 374 (2003) [4] A. Tikan, S. Bielawski, C. Szwaj, S. Randoux, and P. Suret, Nature Photonics 12, 228 (2018) [5] A. E. Kraych, D. Agafontsev, S. Randoux, and P. Suret, Phys. Rev. Lett. 123, 093902 (2019). [6] P Suret et al. Phys. Rev. Lett. 125, 264101 (2020) [7] A Gelash, D Agafontsev, V Zakharov, G El, S Randoux, P Suret, Phys. Rev. Lett 123, 234102 (2019)# Séminaire du LPTMS : Alberto Biella (LPTMS)

## Measurement-induced entanglement phase transitions and the Quantum Zeno Effect

### Alberto Biella (LPTMS)

Online seminar --- ZOOM Meeting ID: 918 3216 1661 -- Password: Ask L. Mazza or D. Petrov -- It is well known that by repeatedly measuring a quantum system it is possible to completely freeze its dynamics into a well defined state, a signature of the Quantum Zeno Effect. In my talk I will discuss how, for a many-body system evolving under competing unitary evolution and variable-strength measurements, the onset of the Zeno effect takes the form of a sharp phase transition [1]. Using the Quantum Ising chain with continuous monitoring of the transverse magnetization as paradigmatic example, we show that for weak measurements the entanglement produced by the unitary dynamics remains protected, while only above a certain threshold the system is sharply brought into an uncorrelated Zeno subspace. Furthermore, we show that this transition is invisible to the average dynamics, but encoded in the rare fluctuations of the stochastic measurement process. Finally we trace an intriguing connection between the onset of the Zeno effect and the subradiance transition of the associated non-Hermitian Hamiltonian. [1] AB, M. Schirò, arXiv:2011.11620# Séminaire du LPTMS : Francesca Pietracaprina (Trinity College Dublin)

## Many-Body Localization in 2D and constrained models

### Francesca Pietracaprina (Trinity College Dublin)

Online seminar --- ZOOM Meeting ID: 955 7840 0964 -- Password: ask L. Mazza and D. Petrov -- Many-body localization is a way to break ergodicity, thermalization and transport in disordered and quasiperiodic interacting quantum systems. The existence of a many-body localization transition in 2D systems is an open question that is being addressed experimentally, theoretically and numerically. In this talk, I will show some recent numerical results for a model whose features make it especially accessible to exact diagonalization. We numerically study the possibility of many-body localization transition in a constrained system: a disordered quantum dimer model on the honeycomb lattice. By using the peculiar constraints of this model and state-of-the-art exact diagonalization and time evolution methods, we probe large two-dimensional systems of up to N=108 sites. F Pietracaprina and F Alet, SciPost Phys. 10, 044 (2021) C Chiaracane, F Pietracaprina, A Purkayastha and J Goold, arXiv:2101.01111 [cond-mat.dis-nn] (2021)# Séminaire du LPTMS : Blagoje Oblak (Sorbonne Université)

## Berry Phases and Drift in the KdV Equation

### Blagoje Oblak (Sorbonne Université)

Online seminar --- ZOOM Meeting ID: 952 7571 4537 -- Password: ask L. Mazza and D. Petrov -- I consider a model of fluid motion closely related to the Korteweg-de Vries equation that governs shallow water waves. Upon reformulating this model as a geodesic in an infinite-dimensional group, the fluid's drift velocity can be recast as an ergodic rotation number. The latter is sensitive to Berry phases, inspired by conformal field theory and gravity, that are produced by adiabatic deformations. Along the way, I show that the topology of coadjoint orbits of wave profiles affects drift in a dramatic manner: orbits that are not homotopic to a point yield quantized rotation numbers. These arguments rely on the general structure of Euler equations, suggesting the existence of other applications of infinite-dimensional geometry to nonlinear waves.# Séminaire du LPTMS : Benoît Vermersch (LPMMC Grenoble)

## Probing mixed-state, symmetry-resolved, entanglement with randomized measurements

### Benoît Vermersch (LPMMC Grenoble)

Online seminar --- ZOOM Meeting ID: 970 5162 7987 -- Password: ask L. Mazza and D. Petrov -- Recently, protocols based on statistical correlations of randomized measurements have been introduced to probe entanglement in synthetic quantum systems. This includes protocols to access Renyi entropies, many-body state fidelities, out-of-time-ordered correlators (OTOCs) and topological invariants, etc. In this seminar, I will first give a tutorial introduction to randomized measurements. Then, I will present our protocol for detecting and quantifying mixed-state entanglement in the framework of the positive-partial-transpose condition (PPT). Finally, I will discuss symmetry-resolved entanglement in synthetic quantum systems, and present a universal mechanism of purification and entanglement creation of many-body quantum states. In all parts of my talk, I will show experimental results obtained in the Innsbruck's trapped ion quantum simulator.# Séminaire du LPTMS : Valerio Sorichetti (LPTMS)

## Nonequilibrium simulation of cytoskeletal proteins: assembly, bundling and gelation

### Valerio Sorichetti (LPTMS)

Online seminar --- ZOOM Meeting ID: 996 5227 7025 -- Password: ask L. Mazza and D. Petrov -- The cytoskeleton of living cells is a dynamical network with an extremely rich behavior: it maintains the shape of the cell, gives it resistance to deformation, allows it to deform and migrate and provides the structure necessary for intracellular transport. In order to gain a better understanding of the biological role of the cytoskeleton, it is fundamental to understand how it assembles, how it remodels itself and which factors determines its structure. The competition between filament bundling and elongation is one of the key factors in determining the structure and mechanical properties of the cytoskeleton. In order to capture this out-of-equilibrium process, we simulate a system of "patchy" monomers which can bind irreversibly to each other to form long filaments. We first consider monomers which interact via a simple isotropic excluded volume interaction in addition to the patchy interaction. We study the assembly kinetics of the filaments, finding that the mean filament length increases linearly with time, and give a theoretical interpretation of this finding. Successively, we allow the monomers to stick reversibly to each other via isotropic short-ranged interactions. We show that this simple model leads to a very rich range of different behaviors, giving rise to filaments, bundles, and complex bundle networks, depending on the thermodynamic parameters considered# Séminaire du LPTMS : Mert Terzi (LPTMS)

## Collective deformation modes promote fibrous self-assembly in protein-like particles

### Mert Terzi (LPTMS)

Online seminar --- ZOOM Meeting ID: 996 6280 8820 -- Password: ask L. Mazza and D. Petrov -- Self-assembly is a crucial and ubiquitous process for biological systems, in which the building blocks spontaneously organize into larger complexes. If the building blocks fit each other, self-assembly leads to space filling aggregates. However, in the case of misfitting particles, the resulting aggregates may have limiting sizes. When the misfitting particles are deformable, elastic energy builds up during the assembly. The energy cost of elastic deformation competes with a surface tension which drives the particles into assembly. In the regime in which these two energies are comparable, particles can assemble into self-limiting structures. The relationship between characteristics of the individual particles and the resulting aggregates is not well understood. Through numerical simulations and elastic coarse-graining we show that this relationship is dominated by collective aggregate deformation modes in a broad class of soft particles. We identify two characteristics of particles predictive of the overall aggregate structure. When individual particles have soft deformation modes, these modes collectively control the size of self-limiting aggregates and lead to large-scale structures. The second characteristics is incompressibility which favors anisotropic, and hence fibrous aggregates. Finally, we discuss the implications of our results to fiber formation in protein aggregation.# Séminaire du LPTMS : Laura Foini (IPhT - CEA)

## Quenches in initially coupled Luttinger liquids

### Laura Foini (IPhT - CEA)

Online seminar --- ZOOM Meeting ID: 969 0061 5152 -- Password: ask L. Mazza and D. Petrov -- We study the quantum quench in two coupled, and generically different, Luttinger liquids, after that the coupling between the two systems is removed. To solve the problem we exploit the factorization of the initial state in terms of a massive and massless mode which emerges in the low energy limit, and we encode the non-equilibrium dynamics in a proper rescaling of the time. With this the solution of the problem can be derived in terms of the known solutions for single Luttinger liquids and, in particular, can rely on techniques exploiting conformal invariance. The emergent dynamics is characterized by a sequence of regimes and depending on the observable, the contribution from the massive or from the massless mode can be the dominant one, giving rise to exponential or power-law decay in space-time, respectively. While the approach can be relevant in a larger set of problems, I will discuss the application to two tunnel coupled tubes in cold atomic experiments, discussing the complex thermalization properties of the system.# Séminaire du LPTMS : Giulia Pisegna (University of Rome La Sapienza)

## Dynamical Renormalization Group Approach to the Collective Behavior of Swarms

### Giulia Pisegna (University of Rome La Sapienza)

Online seminar --- Zoom Meeting ID: 951 4623 4806 -- Passcode: ask L. Mazza and D. Petrov -- We study the critical behavior of a model with nondissipative couplings aimed at describing the collective behavior of natural swarms, using the dynamical renormalization group under a fixed-network approximation. At one loop, we find a crossover between an unstable fixed point, characterized by a dynamical critical exponent z=d/2, and a stable fixed point with z=2, a result we confirm through numerical simulations. The crossover is regulated by a length scale given by the ratio between the transport coefficient and the effective friction, so that in finite-size biological systems with low dissipation, dynamics is ruled by the unstable fixed point. In three dimensions this mechanism gives z=3/2, a value significantly closer to the experimental window, 1.0≤z≤1.3, than the value z≈2 numerically found in fully dissipative models, either at or off equilibrium. This result indicates that nondissipative dynamical couplings are necessary to develop a theory of natural swarms fully consistent with experiments.# Séminaire du LPTMS : Antoine Fruleux (Ecole Polytechnique)

## Fluctuations in biological tissues

### Antoine Fruleux (École Polytechnique, LadHyX)

Biophysics — Modeling & Data analysis — Morphogenesis Online seminar --- Zoom Meeting ID: 991 9867 1888 -- Passcode: ask L. Mazza and D. Petrov -- [caption id="" align="aligncenter" width="640"] Figure 1: Cell growth is heterogeneous in space and time. Example of a sepal (green organ that protects a flower before it opens) from the model plant Arabidopsis thaliana. The colour scale corresponds to growth rates (high in red, low in blue).[/caption] The two hands of most humans almost superimpose. Similarly, flowers of an individual plant have similar shapes and sizes. This is in striking contrast with growth and deformation of cells during organ morphogenesis, which feature considerable variations in space and in time, raising the question of how organs and organisms reach well-defined size and shape. In order to link cell and organ scales, I built a stochastic hydrodynamic model of growing tissue with fibre-like structural elements that may account for the plant cell wall or animal cytoskeleton or extracellular matrix [1]. The model gave two important predictions. First, fluctuations occurring at cellular scale exhibit long-range correlations. Second, the response of fibres to growth-induced mechanical stress may enhance or buffer cellular variability of growth, making it possible to modulate the robustness of morphogenesis. I will present in more details these results as well as a mathematical tool I defined to analyze signal in tissues: the Cellular Fourier Transform (CFT) [2]. It is suited to those signals which can only be defined at a cellular scale, or which are smoothed out of their sub-cellular variations, and it allows to overcome the constrains met when studying biological tissue, foams, granular materials or other geometrically disordered materials. I will introduce the method and explain how it reveals the physical mechanisms setting spatial heterogeneity in growing tissues.### References

[1] Antoine Fruleux and Arezki Boudaoud. Modulation of tissue growth heterogeneity by responses to mechanical stress. Proceedings of the National Academy of Sciences, 116(6):1940–1945, 2019. [2] Antoine Fruleux and Arezki Boudaoud. Cellular fourier analysis for geometrically disordered materials. Physical Review Research, 3(2):023036, 2021.# Séminaire du LPTMS : Orazio Scarlatella (Collège de France)

## Dynamical Mean-Field Theory for Markovian Open Quantum Many-Body Systems

### Orazio Scarlatella (Collège de France)

Online seminar --- Zoom Meeting ID: 962 2949 3343 -- Passcode: ask L. Mazza and D. Petrov -- Markovian quantum many body systems describe a number of experimental platforms relevant for quantum simulations. Their theoretical understanding is hampered by the exponential scaling of their Hilbert space and by their intrinsic nonequilibrium nature, limiting the applicability of many traditional approaches. In this talk I will present an extension of the nonequilibrium Dynamical Mean Field Theory (DMFT) to bosonic Markovian open quantum systems. Within DMFT, a Lindblad master equation describing a lattice of dissipative particles is mapped onto an impurity problem describing a single site of the lattice coupled to a self-consistent environment, which for bosons accounts for fluctuations beyond Gutzwiller mean-field theory due to the finite lattice connectivity. I will present a non-perturbative approach to solve this impurity problem, which is tailored for Markovian open quantum systems. As a first application of this DMFT approach, I will discuss the steady-state of a driven-dissipative Bose-Hubbard model with two-body losses and incoherent pump. I will show that this model features a normal phase at small hopping and a phase transition towards a non-equilibrium superfluid. Remarkably, this transition occurs as a finite-frequency instability, leading to an oscillating in time order parameter. Then, I will show that DMFT captures hopping-induced dissipative processes, completely missed in Gutzwiller mean-field theory, which crucially determine the properties of the normal phase, including the suppression of local gain and the emergence of a stationary quantum-Zeno regime. I will also argue that these processes compete with coherent hopping processes to determine the phase transition towards the superfluid phase, leading to a large extension of the normal phase due to finite-connectivity.# Séminaire du LPTMS : Marko Medenjak (LPENS)

## Bounds on time translation symmetry breaking

### Marko Medenjak (LPENS)

Online seminar --- Zoom Meeting ID: 951 9908 9593 -- Passcode: ask L. Mazza and D. Petrov -- Abstract: Isolated systems consisting of many interacting particles are generally assumed to relax to a stationary state, whose macroscopic properties are described by the laws of thermodynamics and statistical physics. In this seminar we will explore whether quantum systems can avoid relaxation and if stationarity itself can be unstable, meaning that even the slightest disturbance of the thermal state leads to perpetually changing physical properties. Analogous behaviour was experimentally observed in driven quantum systems (Floquet time crystals), in which the response of the system does not follow the period of the driving. We will show that time translation symmetry breaking is possible even in perfectly isolated quantum systems without any driving, and provide rigorous bounds on it in terms of dynamical symmetries. Finally, we will see how time translation symmetry breaking occurs in the Heisenberg XXZ spin chain, and show that its properties are a no-where continuous (fractal) function of the system parameters. [1] M. Medenjak, B. Buča, and D. Jaksch, Phys. Rev. B 102, 041117(R) (2020). [2] M. Medenjak, T. Prosen, and L. Zadnik, SciPost Phys. 9, 3 (2020).# Séminaire du LPTMS : Sanjay Ramassamy (IPHT)

## Barak-Erdös graphs and the infinite-bin model

### Sanjay Ramassamy (IPHT)

Online seminar --- Zoom Meeting ID: 916 6984 4150 -- Passcode: ask L. Mazza and D. Petrov !! NEW PASSWORD !! -- Barak-Erdös graphs are the directed acyclic version of Erdös-Rényi random graphs : the vertex set is {1,...,n} and for each i# Soutenance de thèse: Nadia Milazzo

*Optimal measurement strategies for quantum state and quantum channel estimation*

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### Nadia Milazzo

Abstract:The rapid advance of quantum information technology requires precise control and manipulation of quantum systems; in particular, it is essential to certify that quantum processors truly work quantum mechanically, in order to validate experiments and their results. The problem of certification of quantum states and devices is a demanding one, thus many attempts have been put forward to find ways of efficiently testing their basic functionalities, such as their entanglement properties. The aim of the present thesis is to find optimal strategies for the estimation and characterization of quantum states and channels, with a special focus on entanglement correlations. We consider in particular the certification of entanglement across a given partition of a multi-qubit system, when only partial information about the corresponding quantum state is available. We also discuss the problem of separability of quantum channels in terms of the Choi matrix representation, and we shall mention some preliminary results about quantum functional testing based on Bayesian adaptive strategies. As a separate issue, we also highlight the relevance of quantum information in the context of analogue gravity.

Jury: Prof. Otfried Gühne: Department Physik, Universität Siegen (rapporteur) Prof. Jens Siewert: Dpto. de Química-Física Facultad de Ciencia y Tecnología, Universidad del País Vasco, UPV/EHU (rapporteur) Professeure d'université Rosa Tualle Brouri: Institut d'Optique Graduate School - Université Paris Saclay Prof. Daniel Braun: Institut für Theoretische Physik, Universität Tübingen Prof. Igor Lesanovsky: Institut für Theoretische Physik, Universität Tübingen Dr. Olivier Giraud, CNRS - Université Paris Saclay Location: Institut für Theoretische Physik, Universität Tübingen, Room H33**Zoom link:** https://zoom.us/j/9347693638

# Soutenance de thèse : Alexandre Pricoupenko

*Beyond-mean-field effects in ultracold gases*

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### Alexandre Pricoupenko

Ultracold gases are well controllable quantum systems that are described by a set of few parameters. The idea to look at systems with partially attractive and repulsive forces, fine-tuned to an approximate overall cancellation of the mean-field term, provide an interesting platform for studying various beyond-mean-field phenomena, remarkable recent examples being quantum droplets and dipolar supersolids. In this thesis, we take a step towards understanding the phase diagram of the 1D Bose-Bose mixture with attractive interspecies and repulsive intraspecies contact interactions. We address the one-dimensional three-body problem with two- and three-body interactions that we solve analytically. Then, we develop the perturbation theory for systems with a weak two-body potential interaction, where the attractive and repulsive parts compensate each other. We calculate in every dimension the leading nonpairwise contribution, which represents an effective three-body interaction. We apply this result particularly to tilted dipoles in quasi-low-dimensional geometries. Interestingly, we show the consistency of this few-body perturbative approach with the Bogoliubov one.

Jury : Frédéric Chevy, LKB ENS, Paris, examinateur Mario Gattobigio, INPHYNI, Nice, rapporteur Denis Lacroix, IJCLab, Orsay, examinateur Dmitry Petrov, LPTMS, Orsay, directeur de thèse Luis Santos, Leibniz Universität Hannover, rapporteur Leticia Tarruel, ICFO, Barcelona, examinatrice# Séminaire du LPTMS : Isabelle Bouchoule (LCF)

## Effect of losses in correlated quantum gases

### Isabelle Bouchoule (Laboratoire Charles Fabry)

Hybrid seminar: onsite + zoom (ID:932 9180 1483, PW:cuTj9X). I propose, during this seminar, to make a review of different results we obtained in the last years, both theoretical and experimental, concerning the effect of losses in quantum gases. Losses are always present in cold atoms experiments. However in presence of correlations between atoms, evaluating their effect is a difficult theoretical problem which is mainly unexplored. This is illustrated by our recent work on the effect of losses on a gas with contact interactions: if one makes the usual assumption that the environment involved in the loss process has a vanishing correlation time, which is the assumption that leads to the famous Lindblad equation used so far to describe losses in the context of cold atoms, then we show that the energy increase rate in the gas diverges in dimensions larger than 1. The description of losses requires either taking into account the finite correlation time of the reservoir, or taking into account the finite interaction range between atoms. In dimension 1, such a divergence is absent and for short enough correlation time of the reservoir, losses are well described by the Lindblad equation, even in the presence of contact interactions. We have investigated the effect of losses in the Lieb-Liniger model of 1D bosons with contact interaction. We assume slow losses so that the gas has time to relax at any time. In contrast to chaotic systems which relax towards a thermal state parameterized by 2 quantities, the particle density and the energy density, the local properties of the Lieb-Liniger model after relaxation are parameterized by a whole function, the rapidity distribution. We have computed the evolution of the rapidity distribution in presence losses, thus characterizing entirely the effect of slow losses. We observe that losses bring the system to a non-thermal state, and we show that a manifestation of the non-thermal nature of the state is the breakdown of the popular Tan's relation. In the particular case when the gas lies into the quasi-condensate regime, we computed the effect of losses within the Bogoliubov description of the gas. Experimentally, our measurements, which concern long wave-length collective modes, are in agreement with those theoretical predictions.# Journée des laboratoires FFJ

Annonce J-FFJ 16 sept 2021# Séminaire du LPTMS : Pierfrancesco Urbani (IPHT)

## Marginal stability in soft anharmonic mean field spin glasses

### Pierfrancesco Urbani (Institut de Physique Théorique)

Onsite blackboard talk + zoom (zoom ID:995 6963 5229, pw: M6Z5ib). I will consider simple mean field spin glasses of real degrees of freedom subjected to anharmonic quartic potential and study the glass phase. I will show how this phase realizes marginal stability through soft, pseudogapped, non-linear excitations. I will also discuss how these findings provide a new scenario for the longstanding problem of the spin glass transition in a field and why this is important for the physics of low temperature amorphous solids.# Soutenance de thèse : Hugo Le Roy

*Elasticity of self assembling bio-materials*

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### Hugo Le Roy

Hybrid PhD defense: onsite + zoom (https://cnrs.zoom.us/j/95416861091?pwd=NlhHOW9BWXM2aDN4UmJSSktpYTZmQT09 Meeting ID: 954 1686 1091 - For pwd, please contact martin.lenz@universite-paris-saclay.fr)Self organization is crucial for the wealth of living systems, both at the molecular and cellular level. To correctly achieve their role a tight control over the shape and structure of protein for instance, is required. Any mistake can lead to various disease like Alzheimer, where proteins aggregate into fibrous structures. On a larger scale, cells need to probe the mechanical response of their environment -the extra cellular matrix- and to adapt their own rigidity to collectively orientate. In this thesis, we are looking a two different models. In the first part, we study protein aggregation beyond the microscopical details. By considering elastic assembling particles, we are able to derive generic law and understand the very persistent formation of fibers. In the second part, we model the dynamical response of a fashionable class of hydrogels for their bio-compatibility. Material engineers are now able to synthesize materials with more and more subtle behavior, although understanding certain emergent properties -such as non-exponential relaxation- can be a major challenge. We design a simple model for the dynamical response of hydrogels connected by large multivalent crosslinkers. We are able to account for experimental results,and rationalize their origin.

Jury : Thibaut Divoux, Laboratoire de Physique ENS Lyon, examinateur Greg Grason, Univ. of Massachusetts Amherst, rapporteur Martin Lenz, LPTMS, directeur de thèse Emmanuel Trizac, LPTMS, examinateur Emanuela Zaccarelli, Univ. La Sapienza, rapportrice Zorana Zeravcic, Laboratoire Gulliver ESPCI, examinatrice# Séminaire du LPTMS: Mikhail Zvonarev

## Spin-charge separation for hardcore fermions in one dimension

### Mikhail Zvonarev (LPTMS)

Onsite + zoom seminar Meeting ID: 985 7618 9384 Passcode: AAs1jh Link: https://cnrs.zoom.us/j/98576189384?pwd=djN5OVozR3pwNGFQWjY1VDlEN2I4QT09The hardcore constraint permits no more than one fermion per lattice site. Such a system is, eventually, strongly interacting, and has no small parameter expansion. I will discuss how spin and charge dynamics factorize at the level of correlation functions. The problem has a long history, and I will show a trick, eventually reducing the complete solution to a few elementary algebraic manipulations.

# Fête de la Science 2021

# Physics-Biology interface seminar: Cécile Leduc

## Structure and assembly of single intermediate filaments

### Cécile Leduc (Institut Jacques Monod)

Intermediate filaments (IF) are involved in key cellular functions including polarization, migration, and protection against large deformations. These functions are related to their remarkable ability to extend without breaking, a capacity that should be determined by the molecular organization of subunits within filaments. However, this structure-mechanics relationship remains poorly understood at the molecular level. Here, using super-resolution microscopy (SRM), we show that vimentin filaments exhibit a ~49 nm axial repeat both in cells and in vitro. As unit-length-filaments (ULFs) precursors were measured at ~59 nm, this demonstrates a partial overlap of ULFs during filament assembly. Using an SRM-compatible stretching device, we also provide evidence that the extensibility of vimentin is due to the unfolding of its subunits and not to their sliding, thus establishing a direct link between the structural organization and its mechanical properties. Overall, our results pave the way for future studies of IF assembly, mechanical and structural properties in cells.

# Séminaire du LPTMS : Camille Aron (LPENS)

## Non-analytic non-equilibrium field theory

### Camille Aron (Laboratoire de Physique de l'Ecole Normale Supérieure)

Onsite seminar + zoom (ID: 981 7596 6473, Passcode: fCjZ6d) The Landau-Ginzburg theory is a cornerstone of modern physics that unifies the various equilibrium phase transitions of matter in a common framework. The free energy, functional of the order parameter, is built on simple principles: locality, symmetry, stability, and analyticity. It is still unclear when and how such a unified principle-based approach can be extended to non-equilibrium phase transitions. In this talk, I will present the recent theoretical efforts to address the non-equilibrium phase transition which occurs during the resistive switching (RS) of a variety of correlated oxides: their resistivity suddenly drops by several orders of magnitude when subject to a finite voltage bias. The particular case of RS will lead me to propose to abandon the principle of analyticity of the Landau potential away from thermal equilibrium. This more general question will be addressed in the simpler context of non-equilibrium versions of the Ising model. I will show how the usual φ^4 potential can be deformed by non-analytic operators of intrinsic non-equilibrium nature and use the renormalization group to discuss their low-energy relevance.# Séminaire du LPTMS : Sara Murciano (SISSA)

## Symmetry-resolved entanglement and negativity in systems with U(1) symmetry

### Sara Murciano (International School for Advanced Studies)

Onsite seminar + zoom (ID: 948 1788 3604, Passcode: B3McPy). Symmetries are a pillar of modern physics and an evergreen research topic is the characterisation of how the presence of a symmetry influences the properties of a physical system. I will present an analysis of the entanglement entropies and negativities related to different symmetry sectors of systems with an internal U(1) symmetry. The entanglement entropy admits a decomposition according to its total preserved charge thanks to the block diagonal form of the density matrix. Despite this structure becomes nontrivial after the operation of partial transposition on the density matrix, it has been shown that negativity admits a resolution in terms of the charge imbalance between two subsystems. I will focus on the resolution of entanglement and negativity for free Dirac fields in two spacetime dimensions at finite temperature and size. To this end, I use a geometrical construction in terms of an Aharonov-Bohm-like flux inserted in the Riemann surface defining the entanglement. The main interesting finding is that both entanglement entropy and negativity are equally distributed among the different symmetry sectors at leading order.# Physics-Biology interface seminar: Renaud Poincloux

## Elasticity of dense actin networks produces nanonewton protrusive forces at macrophage podosomes

### Renaud Poincloux (IPBS, Toulouse)

Actin filaments assemble into force-generating systems that play pivotal roles in diverse cellular functions, including cell motility, adhesion, contractility and division. Thermodynamics and in vitro experiments showed that the polymerization of single actin filaments generates forces in the 1-10 pN range. How networks of crosslinked actin filaments, individually generating piconewton forces, are able to produce forces reaching tens of nanonewtons remains unclear. We used in situ cryo-electron tomography to unveil how the nanoscale architecture of macrophage podosomes enables basal membrane protrusion. We show that the sum of the actin polymerization forces at the membrane is not sufficient to explain the protrusive forces generated by podosomes. Quantitative analysis of podosome organization demonstrates that the core is composed of a dense network of bent actin filaments storing elastic energy. Theoretical modelling of the network as a spring-loaded elastic material reveals that it exerts forces of up to tens of nanonewtons, similar to those evaluated experimentally. Thus, taking into account not only the interface with the membrane but also the bulk of the network is crucial to understand force generation by actin machineries. Our integrative approach sheds light on the elastic behavior of dense actin networks and opens new avenues to understand force production inside cells.

# Séminaire du LPTMS : Juliane Klamser (ESPCI)

## Monte Carlo descriptions of active matter and their continuous-time limits

### Juliane Klamser (École supérieure de physique et de chimie industrielles)

Onsite seminar + zoom (Meeting ID: 984 8271 3951, Passcode: dX3f5t, link: https://cnrs.zoom.us/j/98482713951?pwd=L1AyQUtzeEFEMWRic3pBV2NrWUFRUT09) Coauthors : Olivier Dauchot, Julien Tailleur# Physics-Biology interface seminar: Yves Gaudin

## Interactions between viral factories and cellular innate immunity, a story of liquid biocondensates

### Yves Gaudin (I2BC)

Replication of Mononegavirales (MNV) occurs in viral factories which form inclusions in the host-cell cytoplasm. For rabies virus (RABV), those inclusions are called Negri Bodies (NBs). NBs have characteristics similar to those of liquid organelles: they are spherical, they fuse to form larger structures, and they disappear upon hypotonic shock. Their liquid phase was confirmed by FRAP experiments. Live-cell imaging indicates that viral nucleocapsids are ejected from NBs and transported along microtubules to form either new virions or secondary viral factories.

We developed several minimal systems (both cellular and acellular) allowing the formation of biomolecular condensates recapitulating NBs properties. Those minimal systems established RABV phosphoprotein (P) as the main regulator of the liquid liquid phase separation (LLPS) and identified structural elements of RABV nucleoprotein and P that are key in this process. Formation of liquid viral factories by LLPS has been extended to other MNV. This is a paradigm change in the field of MNV replication that invites us to revisit the interplay between viral factories and innate cellular immunity.

As an example, we previously demonstrated that stress granules, which are also liquid biomolecular condensates containing microbes-associated-molecular-patterns recognition receptors acting as sensors of RNA virus replication, come into close contact with NBs, exchange material with them, but do not fully mix their content. We have now extended those observations to other components of the cellular pathway leading to interferon production demonstrating a key role of viral and cellular biomolecular condensates in innate immunity.

# Séminaire du LPTMS : Cristiano Ciuti (MPQ)

**Cavity-mediated effects in disordered quantum Hall systems**

### Cristiano Ciuti (laboratoire Matériaux et Phénomènes Quantiques)

Onsite seminar + zoom (ID: 961 6880 9559, PW: h2LNDE). The manipulation of matter by giant vacuum fields in electromagnetic resonators is an emergent topic in physics and chemistry [1]. In this seminar, we will see how the cavity vacuum fluctuations affect the physics of disordered quantum Hall systems [2]. In particular, we will show how, in the presence of electronic disorder, the cavity can mediate long-range electron hopping via the exchange of a virtual photon, involving both edge and bulk states. Such an effect produces a breakdown of the topological protection of the integer quantum Hall effect as demonstrated in recent transport experiments [3]. [1] F. J. Garcia-Vidal, C. Ciuti, T. W. Ebbesen,*Manipulating matter by strong coupling to vacuum fields,*Science 373,178 (2021). [2] C. Ciuti,

*Cavity-mediated electron hopping in disordered quantum Hall systems,*Phys. Rev. B 104, 155307 (2021). [3] F. Appugliese, J. Enkner, G. L. Paravicini-Bagliani, M. Beck, C. Reichl, W. Wegscheider, G. Scalari, C. Ciuti, J. Faist,

*Breakdown of the topological protection by cavity vacuum fields in the integer quantum Hall effect,*preprint arXiv:2107.14145 (2021)

# Soutenance de thèse : Lucas Varela Alvarez

*Systèmes coulombiens en une et deux dimensions : résultats exacts*

par

### Lucas Varela Alvarez

Soft matter often features charged units which interact through Coulomb forces, the treatment of which is difficult due to their long-range nature. This thesis gives results for three many-body systems where every long-range interaction is included, without approximations. First, we study exact results for a one-dimensional electroneutral salt-free suspension made of two fixed colloids and N neutralizing mobile counterions, with dielectric jumps at the colloids’ position. This includes the partition function, density profile and pressure. We find that for any N counterions system there may be like-charge attraction, unlike when the dielectric is homogeneous in which case N must be even. Secondly, we consider the previous system out of equilibrium within a homogeneous dielectric space, as a model for the dynamics of two electrical double- layers. Using a combination of exact calculations where possible and Brownian dynamics simulations, we compute the relaxation time towards equilibrium (τ). The parity of N leads to distinctly different dynamics: for N even, thermal effects are detrimental to relaxation, increasing τ, while they accelerate relaxation for N odd. In the two 1D systems we show that in the limit N→∞ at fixed colloid’s charge, the mean-field prediction is recovered. Finally, we analyse exactly the short-distance effective potential between two “guest” charges immersed in a two-dimensional two-component charge-asymmetric plasma composed of positively (q1 = +1) and negatively (q2 = −1/2) charged point particles. The result is valid in the collapse-free regime, where the Coulombic coupling (dimensionless inverse temperature) β < 4. At high Coulombic coupling β > 2, this model features like-charge attraction.

Jury : Julien Barré, Institut Denis Poisson, rapporteur Alonso Botero, Universidad de Los Andes, examinateur Alexei Chepelianskii, LPS, examinateur David Dean, LOMA, rapporteur Bérengère Dubrulle, SPEC, examinatrice Yan Levin, Universidade Federal do Rio Grande do Sul, examinateur Gabriel Téllez, Universidad de Los Andes, directeur de thèse Emmanuel Trizac, LPTMS, directeur de thèse# Seminaire du LPTMS : Mélanie Lebental (LUMIN)

## Quantum chaos and geodesics: towards non-euclidean photonics

### Mélanie Lebental (LUMIN)

Onsite seminar + zoom (Meeting ID: 984 4152 4540, Passcode: p5WA0A). Quantum chaos is a research field dedicated to semiclassical physics [1,2], i.e. the relationship between a quantum system and its classical counterpart. The predictions are investigated in any wave system, namely quantum, acoustics, microwaves, optics,… Microlasers are a good platform to implement the predictions of quantum chaos. Reciprocally, semi-classical physics provides efficient theoretical tools to optimize photonic devices, in particular the "trace formula" which describes the spectrum as a sum over periodic classical trajectories. I will review some of these results [3] and discuss on-going works on bifurcation theory and the corresponding microlaser experiments. Recently it became possible to fabricate three-dimensionnal (3D) microlasers with high optical quality by direct laser writing [4], in particular curved surface-like microlasers, leading to the emerging domain of non-Euclidean photonics. Actually, the shortest path between two points within a curved surface is not the straight line anymore, but is called a "geodesic". The corresponding trace formula should then be a sum over periodic geodesics. We checked this hypothesis with Möbius strip microlasers and provided experimental and numerical evidences that the laser modes were indeed located along periodic geodesics [5]. [1] M. Brack and R. K. Bhaduri, Semiclassical physics, Addison-Wesley Publishing Company (1997). [2] H.-J. Stöckmann, Quantum chaos, an introduction, Cambridge University Press (1999). [3] A. Pascal, A. Pascal, S. Bittner, B. Dietz, A. Trabattoni, C. Ulysse, M. Romanelli, M. Brunel, J. Zyss, and M. Lebental, Waves and rays in plano-concave laser cavities, Part II: a semiclassical approach, European Journal of Physics, vol. 38, 034011 (2017). [4] M. A. Guidry, Y. Song, C. Lafargue, R. Sobczyk, D. Decanini, S. Bittner, B. Dietz, L. Huang, J. Zyss, A. Grigis, and M. Lebental, Three-dimensional micro-billiard lasers: The square pyramid, Europhysics Letters, vol. 126, 64004 (2019). [5] Yalei Song, Y. Monceaux, S. Bittner, Kimhong Chao, H. M. Reynoso de la Cruz, C. Lafargue, D. Decanini, B. Dietz, J. Zyss, A. Grigis, X. Checoury, and M. Lebental, Möbius Strip Microlasers: A Testbed for Non-Euclidean Photonics Phys. Rev. Lett. vol. 127, 203901 (2021).# Physics-Biology interface seminar: Diane-Laure Pagès

## Collective amoeboid migration of cancer cell clusters by polarised jiggling

### Diane-Laure Pagès (Institut Gustave Roussy)

**Winner of the PhysBio2021 best short talk award**

Migration is a key step in many biological processes, including the metastatic progression of cancers which accounts for most patient’s deaths. As far as we know, cell locomotion occurs through three distinct mechanisms. In a few words, single cells can migrate via two modes, mesenchymal (adhesive, traction-based) or amoeboid (non-adhesive, propulsion-based). Cell cohorts are generally led by protrusive leaders, towing the collective through adhesion to the substrate.

We have been able to demonstrate the existence of an undescribed mode of collective migration. We study tumour cell clusters’ migration, transformed and non-transformed, in non-adherent microfabricated channels. This collective migration is independent of focal-adhesions and traction but is dependent on integrin-mediated friction to the substrate. Moreover, cell clusters display an actomyosin cortex that is polarised to the rear of clusters, proportionally to migration speed. Inhibiting ROCK and myosin activity decreases migration, while optogenetic activation of RhoA dictates directionality, demonstrating that this migration relies on actomyosin contractility [2]. However, such migration is not driven by a sustained cell or myosin flow. Instead, we observed fluctuating cell and myosin displacements that are correlated with clusters’ speed. We then demonstrate analytically that, together with friction with the substrate and myosin polarisation, this behaviour leads to migration. Our results suggest that cell clusters can use a unique mode of collective migration, based on “polarised jiggling”, that may explain the metastatic potential of these tumour intermediates. We call this new mode of migration “collective amoeboid migration”, by analogy with single cell amoeboid migration.

1. Zajac, O. et al. Tumour spheres with inverted polarity drive the formation of peritoneal metastases in patients with hypermethylated colorectal carcinomas. Nat. Cell Biol. 1 (2018). doi:10.1038/s41556-017-0027-6

2. Pagès, D.-L. et al. Cell clusters adopt a collective amoeboid mode of migration in confined non-adhesive environments. bioRxiv 2020.05.28.106203 (2020) doi:10.1101/2020.05.28.106203.

# Séminaire du LPTMS : Andrea De Luca (LPTM)

## Universal out-of-equilibrium dynamics of 1D noisy critical quantum systems

### Andrea de Luca (Laboratoire de Physique Théorique et Modélisation)

** ZOOM SEMINAR** (ID: 965 3800 3966, Pw: Y8TBFQ).*l*. The corresponding stationary distribution has a 3/2 right tail for all

*l*, and converges to a one-sided Levy stable for large

*l*.

# Séminaire du LPTMS : Denis Ullmo (LPTMS)

## A Mean Field Game description of pedestrian dynamic

### Denis Ullmo (Laboratoire de Physique Théorique et Modèles Statistiques)

Onsite seminar + zoom (ID: 962 4404 9550, PW: 9thN9E). In this talk, I will consider the dynamics of crowds at the "operational" level, which corresponds to the relatively short time and length scale associated for instance with a single obstacle. Comparing various model predictions with experimental data, I will show that, contrarily to what is usually assumed in such context, it is necessary to take into account the fact that pedestrian have the capacity to "anticipate" to reproduce even the qualitative properties of the experimental data. Models based on a analogy with granular materials therefore fails drastically, and even modern models of crowds dynamics including short term (ie up to the next collision) anticipation are unable to reproduce the essential feature of the experiments. Furthermore, I will show that a very simple model based on Mean Field Game, that can be analyzed through a very elegant connection with the non-linear Schrödinger equation, is able (actually by construction) to take into account the effects of anticipation of the pedestrians, and reproduce nicely the important features of the experiment.# Physics-Biology interface seminar: Raphaël Voituriez

## Quantifying memory effects in random search processes

### Raphaël Voituriez (Laboratoire Jean Perrin)

A general question that arises in random walk theory is the quantification of space exploration by a random walker. A key observable is provided by the first-passage time, which quantifies the kinetics of general target search problems, and as such has a broad range of applications from diffusion limited reactions at the molecular scale, to immune cells patrolling tissues to find antigens, or larger scale organisms looking for ressources.

I will present asymptotic results which enable the determination of the first-passage time statistics to a target site for a wide range of random processes, and show how these results generalize to non Markovian processes, which are needed to model non Brownian, complex searchers with memory skills. I will discuss how these results can be used to assess the optimality of general random search processes. An explicit example of cellular system where long range memory effects emerge will be given.