Séminaires de l’année 2018
Séminaire du LPTMS: Nicolas Cherroret
Novel perspectives from Anderson localization of atomic matter waves
Nicolas Cherroret (Laboratoire KastlerBrossel, Université Pierre et Marie Curie)
In the last decades, the field of atom optics has allowed for accurate experimental investigations of quantum transport with cold atoms. In this context, the physics of Anderson localization (AL) can today be finely studied, using tunable atomic matter waves in well controlled optical random potentials. After briefly introducing the main concepts of atom optics in random optical potentials, I will address the problem of the outofequilibrium evolution of a noninteracting matter wave in a random potential. The discussion will be focused on two different dynamical scenarios where unexpected manifestations of AL show up. First, I will discuss the spatial spreading of a narrow wave packet, a situation where AL triggers a « mesoscopic echo » peak in the density distribution. This phenomenon has been observed experimentally with cold atoms only recently. In the second scenario, I will consider the evolution of a plane matter wave in the random potential. In this case, the interesting dynamics takes place in momentum space, where AL manifests itself as a surprising « coherent forward scattering peak », twin of the wellknown coherent backscattering effect. I will conclude the talk with open questions on the role of atomic interactions.PhysicsBiology interface seminar: Stéphanie Bonneau
Controled oxidation in living systems
Stéphanie Bonneau (Laboratoire Jean Perrin, Paris)
Living systems produce energy by oxidizing carbon : in aerobic organisms, a major step of this oxidation is processed by the respiratory chain in mitochondria. Energy production involves oxidation and subsequent ageing of the cellular materials. The control of their oxidative activity allows cells to remain far enough to the thermodynamic equilibrium and consequently the balance between respiration and ageing is a major regulation parameter of cell's fate. The key role of mitochondria in this phenomenon will be discussed.
Experimentally, the control of the cellular oxidation is performed by using chosen photosensitizers. Due to their macrocycle, such molecules present very special photophysical properties. Their light irradiation generates, through their triplet state, reactive oxygen species. The lifetime of these molecular species is very short and their action is very localized. To specifically target photosensitizers to one or the other cell compartments is thus the basis of their potential to modify and control the physiology of the cells. For example, the photochemical internalization (PCI) of macromolecules into cells is based on the photoinduced alteration of endosomal membranes  before their maturation in lysosomes  allowing the escape of the macromolecules, free to reach its targets within cell. More extensive photoinduced changes, in particular to the mitochondria, lead to cell death by necrosis or apoptosis. This photoinduced cell death is basis of an anticancer therapy socalled PDT.
First, we focussed on the photoinduced modifications of the cellular trafficking. By combining measurements of local cytoplasmic viscosity and active trafficking, we found that photodynamic effect induced a only slight increase in viscosity but a massive decrease in diffusion. These effects are the signature of a return to thermodynamic equilibrium of the system after photoactivation. Secondly, to better apprehend such complex effects, we turned to model systems. In particular, we focused on photooxidation of membranes lipids, that are important oxidative targets. We extensively studied their modifications under photooxidation. Our purpose is to demonstrate that the photoinduced permeabilization of the membranes is correlated with a deep physical stress, which can be relaxed by various pathways, depending on its lipids composition, which is characteristic of the targeted cellular compartment.
Séminaire du LPTMS: Rémy Dubertrand & Pierre Illien
A semiclassical perspective to study quantum interacting particles
Rémy Dubertrand (Université de Liège, Belgique)
Quantum chaos have had a great success to describe various types of oneparticle quantum systems in the semiclassical regime (e.g. large quantum numbers). I will describe how these techniques can be used to describe quantum systems of interacting particles. For example I contributed to look at Bose Hubbard model and justify why the spectral statistics agrees with RMT for a certain regime of the ratio between onsite interaction and hopping energies. I will discuss in a more general framework how fruitful a semiclassical approach can be to study such systems of interacting particles.Effect of crowding and hydrodynamic interactions on the dynamics of fluctuating systems
Pierre Illien (EC2M laboratory, ESPCI Paris)
Describing the interactions of a fluctuating object with its environment is an ubiquitous problem of statistical physics. I will first focus on the dynamics of a driven particle in a host medium which hinders its motion through crowding interactions. Going beyond the usual effective descriptions of the environment of the active tracer, we propose a lattice model which takes explicitly into account the correlations between the dynamics of the tracer and the response of the bath and for which we determine analytically exact and approximate solutions, that reveal intrinsically nonlinear and nonequilibrium properties. I will then present recent results that reveal how the diffusivity of enzymes can be enhanced when they are catalytically active. In order to identify the physical mechanisms at stake in this phenomenon, we perform measurements on the endothermic and relatively slow enzyme aldolase. We propose a new physical paradigm, which reveals that the diffusion coefficient of a model enzyme hydrodynamically coupled to its environment increases significantly when undergoing changes in conformational fluctuations in a substrate concentration dependent manner, and is independent of the overall turnover rate of the underlying enzymatic reaction.Séminaire du LPTMS: Andrei Bernevig *** séminaire exceptionnel ***
Topological Quantum Chemistry
Andrei Bernevig (Department of Physics, Princeton University, USA)
Séminaire du LPTMS: PierreElie Larré & Clément Tauber
Quantum simulating manybody phenomena with propagating light
PierreElie Larré (Université de CergyPontoise)
We consider the propagation of a quantum light field in a cavityless nonlinear dielectric. In this alloptical platform, the space propagation of the field's envelope may be mapped onto the time evolution of a quantum fluid of interacting photons. The resulting manybody quantum system constitutes a particular class of quantum fluids of light and presently attracts a growing interest as a powerflul tool for quantum simulation. I will present recent theoretical and experimental progresses in this rapidly emerging research field, including investigations on superfluidity, elementary excitations, disorder, quantum quenches, prethermalization, thermalization, and BoseEinstein condensation.Bulkedge correspondence for Floquet topological insulators
Clément Tauber (ETH, Zürich)
Floquet topological insulators describe independent electrons on a lattice driven out of equilibrium by a timeperiodic Hamiltonian, beyond the usual adiabatic approximation. In dimension two such systems are characterized by integervalued topological indices associated to the unitary propagator, alternatively in the bulk or at the edge of a sample. In this talk I will give new definitions of the two indices, relying neither on translation invariance nor on averaging, and show that they are equal. In particular weak disorder and defects are intrinsically taken into account. Finally indices can be defined when two driven sample are placed next to one another either in space or in time, and then shown to be equal. The edge index is interpreted as a quantized pumping occurring at the interface with an effective vacuum.PhysicsBiology interface seminar: Shiladitya Banerjee
Adaptive division control in stressed bacterial cells
Shiladitya Banerjee (UCL, UK)
Control of cell size is a fundamental adaptive trait that underlies the coupling between cell growth and division. Cells possess the unique ability to adapt their size and shapes in response to environmental cues, thereby translating extracellular information into decisions to grow or divide. However, the physical mechanisms mediating the regulation of cell size and division timing remain poorly understood. In this talk, I will discuss our recent discovery of an adaptive model of cell size control in bacteria, where the decision to divide is tightly regulated by the spatial patterning of cell wall growth modes. Using a combination of stochastic mechanical modelling and singlecell experiments, I will elucidate the implications of the size control model for cellular fitness adaptation under stress. In particular, our results show that morphological transformations provide fitness and survival advantages to bacteria under sustained antibiotic treatment.
Séminaire du LPTMS: Christopher Joyner
A random walk approach to linear statistics in random tournament ensembles
Christopher Joyner (Queen Mary University of London, UK)
We investigate the linear statistics of random matrices with purely imaginary Bernoulli entries exhibit global correlations in terms of row sums. These are related to ensembles of socalled random regular tournaments. Specifically, we construct a random walk within the space of these matrices and show the induced motion of the first k traces in a Chebyshev basis converges to a suitable OrnsteinUhlenbeck process. Coupling this with Stein’s method allows us to compute the rate of convergence to a Gaussian distribution in the limit of large matrix dimension.Séminaire du LPTMS: Beatriz Seoane Bartolomé & Ulisse Ferrari
Phase transitions in computer simulations : the Tethered Monte Carlo method
Beatriz Seoane Bartolomé (LPTENS, Paris)
In this talk, I will present a powerful Monte Carlo method that I developed during my PhD [1,2] and extended recently [3], designed to efficiently study phase transitions at equilibrium. The principle is very simple, by means of external constraints to the system, we are able to avoid the traditional critical (exponential) slowing down associated to the second (first) order transition, and thus reach much larger system sizes than with traditional methods. Furthermore, the reconstruction of the constrained free energy is much simpler than in other similar methods, such as the famous Umbrella Sampling, allowing us to both fix multiple constraints at the same time, and to extract magnitudes such as the interfacial free energy with an unusual high precision. In particular, I will discuss the Tethered Monte Carlo strategy in the context of a toy model for crystalline porous media [3].Statistical Physicsinspired models of biological network: collective behavior in neuronal ensembles
Ulisse Ferrari (Institut de la Vision, Inserm & UPMC)
PhysicsBiology interface seminar: Pere RocaCusachs
Sensing the matrix: transducing mechanical signals from integrins to the nucleus.
Pere RocaCusachs (Institute for Bioengineering of Catalonia, Universitat de Barcelona, Spain)
Cell proliferation and differentiation, as well as key processes in development, tumorigenesis, and wound healing, are strongly determined by the properties of the extracellular matrix (ECM), including its mechanical rigidity and the density and distribution of its ligands. In this talk, I will explain how we combine molecular biology, biophysical measurements, and theoretical modelling to understand the mechanisms by which cells sense and respond to matrix properties. I will discuss how the properties under force of integrinECM bonds, and of the adaptor protein talin, drive and regulate matrix sensing. I will further discuss how this sensing can be understood through a computational molecular clutch model, which can quantitatively predict the role of integrins, talin, myosin, and ECM receptors, and their effect on cell response. Finally, I will analyze how signals triggered by rigidity at cellECM adhesions are transmitted to the nucleus, leading to the activation of the transcriptional regulator YAP.
Séminaire du LPTMS: Nicola Bartolo & Mathieu Hemery
Exact results for nonequilibrium phase transitions
Nicola Bartolo (Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot et CNRS)
In outofequilibrium systems, the competition between Hamiltonian evolution and dissipation can result in dissipative phase transitions. We investigate this kind of phenomena in the steady state of a general class of drivendissipative systems, consisting of a nonlinear Kerr resonator in the presence of both coherent (onephoton) and parametric (twophoton) driving and dissipation. We analytically derive the exact steadystate solution via the formalism of the complex Prepresentation. The exact solution applies to any photon density regime, allowing to investigate the thermodynamic limit of large photon densities. In this regime, we point out and characterise the emergence of dissipative phase transitions of both first and second order.φevo: from function to network
Mathieu Hemery (McGill University, Montréal, Canada)
Molecular networks are at the core of most cellular decisions, but there architecture is often intricate and difficult to grasp. Starting from the putative function of a network and using an evolutionnary algorithm (EA) to explore the possible architectures seems however to gives good predictions. After a brief introduction to EA and regulatory network, I will present my own work with phievo: the lacoperon that show its validity and a logarithmic network that highlight its possibilities.Séminaire du LPTMS: Chikashi Arita
Variational calculation of diffusion coefficients in stochastic lattice gases
Chikashi Arita (Universität des Saarlandes, Saarbrücken)
Deriving macroscopic behaviors from microscopic dynamics of particles is a fundamental problem. In stochastic lattice gases one tries to demonstrate this hydrodynamic limit. The evolution of a stochastic lattice gas with symmetric hopping rules is described by a diffusion equation with densitydependent diffusion coefficient. In practice, even when the equilibrium properties of a lattice gas are analytically known, the diffusion coefficient cannot be explicitly computed, except when a lattice gas additionally satisfies the "gradient condition", e.g. the diffusion coefficients of the simple exclusion process and noninteracting random walks are exactly identical to their hopping rates. We develop a procedure to obtain systematic analytical approximations for the diffusion coefficient in nongradient lattice gases with known equilibrium. The method relies on a variational formula found by Varadhan and Spohn. Restriction on test functions to finitedimensional subspaces allows one to perform the minimization and gives upper bounds for the diffusion coefficient. We apply the procedure to the following two models; onedimensional generalized exclusion processes, where each site can accommodate at most two particles (2GEPs) [1], and the KobAndersen (KA) model on the square lattice, which is classified into kineticallyconstrained gas [2]. The prediction of the diffusion coefficient depends on the domain ("shape") of test functions. The smallest shapes give approximations which coincide with the meanfield theory, but the larger shapes, the more precise upper bounds we obtain. For the 2GEPs, our analytical predictions provide upper bounds which are very close to simulation results throughout the entire density range. For the KA model, we also find improved upper bounds when the density is small. By combining the variational method with a perturbation approach, we discuss the asymptotic behavior of the diffusion coefficient in the high density limit. [1] C. Arita, P. L. Krapivsky and K. Mallick, Variational calculation of transport coefficients in diffusive lattice gases, Phys. Rev. E 95, 032121 (2017)
 [2] C. Arita, P. L. Krapivsky and K. Mallick, Bulk diffusion in a kinetically constrained lattice gas, preprint condmat arXiv:1711.10616
Séminaire du LPTMS: Alexandre Lazarescu
On the hydrodynamic behaviour of interacting lattice gases far from equilibrium
Alexandre Lazarescu (Centre de Physique Théorique, École Polytechnique)
Lattice gases are a particularly rich playground to study the large scale emergent behaviour of microscopic models. A few things are known in general for models that are sufficiently close to equilibrium (i.e. with rates close to detailed balance, and where the dynamics is typically diffusive): in particular, the local density of particles behaves autonomously in the macroscopic limit, even at the level of large deviations, and the system can be described through a Langevin equation involving only a few quantities called transport coefficients. As demonstrated in the previous talk, obtaining those coefficients in practice can be quite challenging, but we can usually be confident that they exist. I will be talking about a situation that is quite different at first sight: systems far from equilibrium, where the dynamics is propagative, and where very little is known in general. The question is then whether one can hope to be able to describe those models with a similar hydrodynamic structure, or if that description breaks down (if, for instance, longrange correlations become relevant). I will present recent results showing that, for a broad class of 1D models with hardcore repulsion but also interactions and spacedependent rates, the answer is yes and no: all those models exhibit a dynamical phase transition between a hydrodynamic regime and a highly correlated one, which can be related to the socalled "third order phase transitions". The methods involved are quite general and likely to be applicable to many more families of models. A. Lazarescu, Generic Dynamical Phase Transition in OneDimensional BulkDriven Lattice Gases with Exclusion, J. Phys. A: Math. Theor. 50, 254004 (2017)
Séminaire du LPTMS: Sergej Moroz
Séminaire du LPTMS: Laurent de Forges de Parny
Multicomponent BoseHubbard Model: Nematic Order in Spinor Condensates
Laurent de Forges de Parny (AlbertLudwigs University of Freiburg, Germany)
Since the seminal work of D. Jaksch et al. [1], the motivation of considering bosonic mixtures has emerged from the promising perspectives of observing coexisting quantum phases, spindynamics, and quantum magnetism. Recently, ultracold bosons with effective spin degree of freedom allowed to engineer magnetic quantum phase transitions and non trivial magnetic phases, e.g. the nematic order, which breaks the spinrotation symmetry without magnetic order [2]. I will discuss the magnetic properties of strongly interacting spin1 bosons in optical lattices. Employing a combined strategy based on exact numerical methods (quantum Monte Carlo simulations and exact diagonalization) and analytical calculations, we have derived the phase diagrams and characterized the phase transitions beyond the mean field description [3,4]. Furthermore, we have established the low energy spectrum of the nematic superfluid phase and have confirmed a singlettonematic phase transition inside the Mott insulator phase. References: D. Jaksch, C. Bruder, J. I. Cirac, C. W. Gardiner and P. Zoller, Cold Bosonic Atoms in Optical Lattices, Phys. Rev. Lett. 81, 3108 (1998)
 T. Zibold, V. Corre, C. Frapolli, A. Invernizzi, J. Dalibard and F. Gerbier, Spinnematic order in antiferromagnetic spinor condensates, Phys. Rev. A 93, 023614 (2016).
 L. de Forges de Parny, F. Hébert, V. G. Rousseau, and G. G. Batrouni, Interacting spin1 bosons in a twodimensional optical lattice, Phys. Rev. B 88, 104509 (2013).
 Laurent de Forges de Parny, Hongyu Yang, and Frédéric Mila, Anderson Tower of States and Nematic Order of Spin1 Bosonic Atoms on a 2D Lattice, Phys.Rev.Lett. 113, 200402 (2014)
Séminaire du LPTMS: Andrea de Luca
Solution of a minimal model for manybody quantum chaos
Andrea de Luca (Rudolf Peierls Centre for Theoretical Physics, Oxford University, UK)
I present a minimal model for quantum chaos in a spatially extended manybody system. It consists of a chain of sites with nearestneighbour coupling under Floquet time evolution. Quantum states at each site span a qdimensional Hilbert space and time evolution for a pair of sites is generated by a q^{2}×q^{2} random unitary matrix. The Floquet operator is specified by a quantum circuit, in which each site is coupled to its neighbour on one side during the first half of the evolution period, and to its neighbour on the other side during the second half of the period. I will introduce a diagrammatic formalism useful to average the manybody dynamics over realisations of the random matrices. This approach leads to exact expressions in the largeq limit and sheds light on the universality of random matrices in manybody quantum systems and the ubiquitous entanglement growth in outofequilibrium dynamics.Séminaire du LPTMS: Shuang Wu
Thouless bandwidth formula in the Hofstadter model
Shuang Wu (LPTMS, Université ParisSud)
I will show a method of D. J. Thouless to calculate the Hofstadter spectrum bandwidth in relation to the Catalan constant. And I will present how we generalize Thouless bandwidth formula to its nth moment and obtain a closed expression in terms of polygamma, zeta and Euler numbers. Reference: S. Ouvry and S. Wu, Thouless bandwidth formula in the Hofstadter model, J. Phys. A: Math. Theor. 50, 495204 (2017)
Séminaire du LPTMS: Giulio Bertoli
Finite temperature disordered bosons in two dimensions
Giulio Bertoli (LPTMS, Université ParisSud)
In this talk, I will present a study of the phase transitions in a two dimensional weakly interacting Bose gas in a random potential at finite temperatures. It is possible to identify superfluid, normal fluid and insulator phases. The study of the effect of interaction between particles on localization demonstrates that interacting particles can undergo a manybody localizationdelocalization transition, that is the transition from insulator to fluid state. I will also discuss the influence of disorder on the BKT transition between superfluid and normal fluid, in order to construct the phase diagram. At T=0 one has a tricritical point, where the three phases coexist. It is shown that the truncation of the energy distribution function at the trap barrier, which is a generic phenomenon in evaporative cooling of cold atoms, limits the growth of the localization length, so that the insulator phase is present at any temperature. Reference: G. Bertoli, V.P. Michal, B.L. Altshuler, G.V. Shlyapnikov, Finite temperature disordered bosons in two dimensions, preprint condmat.disnn arXiv:1708.03628
PhysicsBiology interface seminar: Pierre Sens
Mechanosensitive adhesion in cell spreading and crawling
Pierre Sens (Institut Curie, Paris)
Crawling cell motility is powered by actin polymerization and actomyosin contraction. When moving over a flat and rigid substrate, cells usually develop thin and broad protrusions at their front, called lamellipodia, where actin polymerisation generates a protrusive force pushing the front edge of the cell forward. The lamellipodium displays interesting dynamics, including normal and lateral waves, possibly relevant to cell polarisation and the initiation of motion. I will discuss a stochastic model of mechanosensitive cell adhesion, and discuss its relevance for symmetry breaking, cell polarisation, and motility. I will then discuss a generic model of microcrawlers, built as an extension of low Reynolds number microswimmers, that highlights the crucial role of mechanosensitive adhesion for the active crawling of cells and biomimetic objects.
Séminaire du LPTMS: Alexandre P. dos Santos *** séminaire exceptionnel ***
**** ATTENTION: horaire inhabituel !!! ****
Pressure between charged polarizable surfaces
Alexandre Pereira Dos Santos (Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil)
We obtain phase diagrams and binodal curves for the repulsion/attraction between charged low dielectric/metallic surfaces in a saltfree environment. Punctual counterions are confined between the surfaces, however, they are not allowed to approach the surfaces nearer than a characteristic length which can model the ionic hydration. The polarization of the surfaces is considered with a recently introduced method based on periodic Green functions. We show that the density profiles are strongly dependent on the dielectric contrast in special for high electrostatic couplings. However, the pressure curves and consequently the binodal curves and critical points slightly change for different polarizable surfaces and hydration lengths.Séminaire du LPTMS: Alexandre Nicolas
Bottleneck flows of pedestrians and grains
Alexandre Nicolas (LPTMS, Université ParisSud)
Simple models from Statistical Physics can be surprisingly useful to study practical problems that lie outside the traditional realm of Physics. I will illustrate this statement with two distinct (yet related!) examples: granular flows through a constriction and competitive passages of pedestrians through a door. To destroy clogs in constricted granular flows, vibrations are often applied to the hopper or silo, but clogs do not yield instantly. Instead, they exhibit an anomalously broad distribution of lifetimes. This elusive feature was recently captured within a model in which we likened the destruction of clogs to thermally activated escapes from a set of energy traps; the model also reproduces other salient experimental observations [1]. Turning to the second example, competitive pedestrian flows through a narrow door look very disordered, and yet they were found to present robust statistical features, such as anticorrelated time gaps between escapes and exponentially distributed bursts of escapes. On the basis of simple models, we have shown that in fact these features emerge generically in constricted flows [2]. [1] Alexandre Nicolas, Ángel Garcimartín and Iker Zuriguel, "A trap model for clogging and unclogging in granular hopper flows" (2017), preprint arxiv:1711.04455
 [2] Alexandre Nicolas and Ioannis Touloupas, Origin of the correlations between exit times in pedestrian flows through a bottleneck, J. Stat. Mech.: Theor. Exp. 2018(1), 013402.
Séminaire du LPTMS: Izaak Neri
Thermodynamic bounds on the statistics of firstpassage times and extreme values of stochastic processes
Izaak Neri (Max Planck Institute for the Physics of Complex Systems, Dresden, Allemagne)
Stochastic thermodynamics generalizes concepts from thermodynamics, and makes them useful to study mesoscopic systems driven far from thermal equilibrium, such as, optically driven colloidal particles, noisy processes in cell biology or microelectronic devices. In mesoscopic systems thermodynamic observables  such as, entropy production, heat and mesoscopic currents  are fluctuating quantities, and stochastic thermodynamics characterizes universal properties of these fluctuating quantities. Established results are the fluctuation relations and the thermodynamic uncertainty relations, which express universal properties of fluctuations of stochastic currents at a fixed time. In this talk I will present thermodynamic bounds for the statistics of firstpassage times and extreme values of stochastic currents, which are fluctuating properties of trajectories of stochastic currents. Some interesting results are: a bound for the mean firstpassage time of current variables in terms of the dissipation rate, a fluctuation theorem for firstpassage times of entropy production, and a universal bound on the supremum statistics of the heat absorbed by a nonequilibrium system. These results will be illustrated on examples of physical processes, such as, the dynamics a molecular motor and charge transport in microelectronic devices.Séminaire du LPTMS: Samuel Belliard *** séminaire exceptionnel ***
Modified Bethe Ansatz for models without U(1) symmetry
Samuel Belliard (IPhT, CEA, Saclay)
Séminaire du LPTMS: Bertrand LacroixàchezToine *** séminaire exceptionnel ***
Extreme value statistics in a gas of 2D charged particles
Bertrand LacroixàchezToine (LPTMS, Université ParisSud)
We study a system of N charged particles in two dimensions with Coulomb logarithmic repulsion and confined in an external symmetric potential. At the inverse temperature of interest β = 2, the positions of the charges form a 2D determinantal point process. In the case of a quadratic potential, there is a mapping between the positions of these charges and the eigenvalues of complex Ginibre matrices. We focus on the extremal statistics of the positions of the charges and in particular we highlight a new universal regime (with respect to a large class of confining potentials) which had been overlooked before [1]. It allows to solve a puzzle of matching between the typical regime of fluctuations [2] and the large deviation regime [3]. Finally we also considered potentials that deviates from this universality class and computed the extremal statistics in these cases.
[1] B. LacroixAChezToine, A. Grabsch, S. N. Majumdar, G. Schehr, Extremes of 2d Coulomb gas: universal intermediate deviation regime, J. Stat. Mech. P013203, (2018).

[2] B. Rider, J. Phys. A 36(12), 3401 (2003).
 [3] F. D. Cunden, F. Mezzadri, P. Vivo, J. Stat. Phys. 164(5), 10621081 (2016).
Séminaire du LPTMS: Lev Truskinovsky
Rigidity generation by nonthermal fluctuations and muscle contraction
Lev Truskinovsky (ESPCI, Paris)
Active stabilization in systems with zero or negative stiffness is an essential element of a wide variety of technological processes. We discuss a prototypical example of this phenomenon in a biological setting and show how active rigidity, interpreted as a formation of a pseudowell in the effective energy landscape, can be generated in an overdamped stochastic system. We link the transition from negative to positive rigidity with time correlations in the additive noise, and show that subtle differences in the outofequilibrium driving may compromise the emergence of a pseudowell. We apply our results to the description of the power stroke machinery in skeletal muscles which is behind their remarkable ability to take up an applied slack in a ms time scale. Along the way we draw some interesting parallels between muscle physiology and the theory of spin glasses.PhysicsBiology interface seminar: Ulisse Ferrari
Nonlinear stimulus processing by the retina
Ulisse Ferrari (Institut de la Vision, Paris)
Understanding how sensory systems process information is an open challenge. This is mostly because these systems are nonlinear, making it extremely difficult to model the relation between the stimulus and the sensory response. In this talk I will discuss two strategies to tackle this problem and apply them to the retina.
First, we use exvivo multielectrode array experiments to record the retinal activity and directly model the ganglion cell response to complex stimuli, such as videos of moving objects. Here I will show that standard, nearlylinear, models are not enough and highly nonlinear models are required. Then I will present the result of a closedloop experiment where we adapted the stimulus online to investigate how the response changes when the visual stimulation is perturbed.
With this approach we could estimate the optimal performance of a neural decoder and show that the nonlinear sensitivity of the retina is consistent with an efficient encoding of stimulus information.
References:
) U. Ferrari, C. Gardella, T. Mora, O. Marre. eNeuro, vol. 4, 6. 2017.
) S. Deny, U. Ferrari, P. Yger, R. Caplette, S. Picaud, G. Tkacik, O. Marre, Nature Commun. 8 (1) 2017
Séminaire du LPTMS: Mario Collura *** séminaire exceptionnel ***
Full counting statistics out of equilibrium: melting of antiferromagnetic order
Mario Collura (Oxford University, UK)
One of the basic principles of quantum mechanics is the statistical nature of measurements of observables. The result of measurements is indeed described by a probability distribution, and measuring the same observable in identical systems will give different outcomes in accordance with this distribution. The full probability distribution carries very detailed information about the system and, on top of expectation value, encodes all fluctuations of the system. I will focus on the nonequilibrium dynamics of a fully polarised antiferromagnetic state under the unitary evolution induced by the XXZ Hamiltonian. It turns out that, depending on the value of the interactions, the full probability distribution of the subsystem staggered magnetisation may retain informations about the original antiferromagnetic order, thus acquiring a shape much different from a simple Gaussian distribution.Séminaire du LPTMS: Simon Pigeon *** séminaire exceptionnel ***
Vibrational assisted conduction in a molecular wire
Simon Pigeon (LKB, UPMC, Paris)
This work establishes how vibrational coupled hopping affects the electronic properties of a molecular wire. These crucial results pave the way to a better understanding and more complete description of electronic properties of these promising devices.
Ref:
 S. Pigeon, L. Fusco, G. De Chiara & M. Paternostro, Vibrational assisted conduction in a molecular wire, Quantum Science and Technology 2, 025006 (2017) ; arXiv:1612.01809
Quantum Physics Journal Club: Giovanni Martone
Supersolids: a short overview
Giovanni Italo Martone (LPTMS)
Séminaire du LPTMS: Michele Filippone *** séminaire exceptionnel ***
Controlled Parity Switch of Persistent Currents and Topological chargepumping effects induced by bulk magnetic fluxes
Michele Filippone (Université de Genève, Suisse)
We investigate persistent currents for a fixed number of fermions in periodic quantum ladders threaded by AharonovBohm and transverse magnetic fluxes Φ and χ. We show that the coupling between ladder legs provides a way to effectively change the groundstate fermionnumber parity, by varying χ. We demonstrate that varying χ by 2π (one flux quantum) leads to an apparent fermionnumber parity switch. We find that persistent currents exhibit a robust 4π periodicity as a function of χ, despite the fact that χ→χ+2π leads to modifications of order 1/N of the energy spectrum, where N is the number of sites in each ladder leg. We connect the parity switching effect to the quantum Hall regime in twodimensional systems. We show that the parity switching effect is related to the parity of the number of filled Landau levels and that it inherits strong robustness against disorder in the HarperHofstadter quantum Hall regime. Indeed, we show that the 4π periodicity is a mesoscopic manifestation of a novel type of fermionic pumping in topological systems, complementary to Thouless' pump. Focusing on the lowenergy edge physics in the general framework of ChernSimons theory, we discuss this alternative type of pumping in the context of integer and fractional quantum Hall systems. Our construction provides an intuitive setting to understand known effects and explore new ones. In particular, we show that adding superconductivity to the picture allows us to recover the 4π Josephson effect of Majorana fermions and its generalizations to parafermions. The parityswitching and the 4π periodicity effects are robust with respect to temperature and disorder and we outline potential physical realizations using Corbino disk geometries in solid state systems, quantum ladders with cold atomic gases and, for bosonic analogs of the effects, photonic lattices. Ref: Michele Filippone, CharlesEdouard Bardyn, Thierry Giamarchi, Controlled parity switch of persistent currents in quantum ladders, preprint condmat.meshall arXiv:1710.02152
Séminaire du LPTMS: Alexandru Petrescu *** séminaire exceptionnel ***
Fluxonbased quantum simulation in circuit QED
Alexandru Petrescu (Department of electrical engineering, Princeton University, USA)
 [1] N. A. Masluk, I. M. Pop, A. Kamal, Z. K. Minev, and M. H. Devoret, Phys. Rev. Lett. 109, 137002 (2012).
 [2] A. Petrescu, H. E. Türeci, A. V. Ustinov, and I. M. Pop, ArXiv eprints (2017), arXiv:1712.08630 [condmat.meshall].
 [3] E. Orignac and T. Giamarchi, Phys. Rev. B 64, 144515 (2001).
 [4] A. Petrescu and K. Le Hur, Phys. Rev. Lett. 111, 150601 (2013).
 [5] M. Piraud, F. HeidrichMeisner, I. P. McCulloch, S. Greschner, T. Vekua, and U. Schollwöck, Phys. Rev. B 91, 140406 (2015).
 [6] A. Petrescu, M. Piraud, G. Roux, I. P. McCulloch, and K. Le Hur, Phys. Rev. B 96, 014524(2017).
 [7] R. Jackiw and C. Rebbi, Phys. Rev. D 13, 3398 (1976).
 [8] W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. Lett. 42, 1698 (1979).
Séminaire du LPTMS: Raffaela Cabriolu
Creep response of a soft glass
Raffaela Cabriolu (Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway)
In this work we discuss finite size effects in the fluidization process of dense amorphous mate rials subjected to an external load. By means of molecular dynamics simulations we study the mechanical response of a densly packed 3D particle system to a sudden applied shear stress. In order to disentangle possible boundary effects from finite size effects, we use an unusual setup by implementing a geometryconstraint protocol with periodic boundary conditions in all directions. We show that this protocol is well controlled and that the long time fluidization process is to a great extend independent of the details of the protocol parameters. This procedure allows for a robust study of finite size effects regarding the creep exponents and the fluidization process. The slow dynamics show a powerlaw creep with exponents that do not depend on the system size whereas the fluidisation time shows strong finite size effects, that we can rationalize within a finite size scaling relation.Séminaire du LPTMS: Mehdi Bouzid
Athermal analogue of sheared colloidal suspensions
Mehdi Bouzid (LPTMS, Université ParisSud)
Sandpiles, window glass, tomato ketchup, are three materials that would not necessarily strike the larger public for their similarities. However, they take part of one of the most lingering enigma of condensed matter physics as they are examples of fluids undergoing dynamical arrest and becoming solid in a way essentially different from a thermodynamic phase transition. Such complex fluids, developing a yieldstress and becoming very hard solids (metallic or oxide glasses) or soft glassy materials (colloidal pastes, granular packing, polymer melts ...), are of central importance in statistical physics, material science or chemical engineering. In this talk I will highlight an analogy between the rheology of Brownian and nonBrownian suspensions, we show that these systems can be described by a HerschelBulkley law as soon as the shear rate and shear stress are respectively normalized by an energy scale and a microscopic time of reorganization, which are both functions of the normal confinement stress. The pressurecontrolled approach, originally developed for granular flows, reveals a striking physical analogy between the colloidal glass transition and the granular jamming transition.PhysicsBiology interface seminar: Emmanuel de Langre
Plant vibration, from wind flutter to phenotyping
Emmanuel de Langre (École polytechnique, Palaiseau, France)
Plants are often very flexible objects. This results in motion under stimuli such as wind or currents, but also hosts such as insects. Motion are known to influence plant development by thigmomorphogenesis. I will review methodologies and results from the past ten years, aimed at quantifying and understanding the vibration of plants, or parts of plants, from Arabidopsis Thaliana to large trees. I will focus on experimental techniques indoor and outdoor, on simple models of motions, and the role of the plant architecture. The recent application to high throughput plant phenotyping by vibrations will also be presented.
Séminaire du LPTMS: Jacopo de Nardis
Edge singularities and quasilongrange order in nonequilibrium steady states
Jacopo de Nardis (LPTENS, Paris)
I will present a theoretical study of exotic states of matter realized via the bipartite quantum quench protocol, where two manybody systems at different density or temperatures are put in contact. I will mostly focus on the 1d interacting Bose gas and I will show how the nonequilibrium steady state emerging from the junction displays strong quantum correlations that are more typical of gapless ground states than thermal states in one spacial dimension. I will then show how by means of new form factors techniques we can provide a full characterization of the strong edge singularities which in principle could also be studied via universal field theory methods similar to nonlinear Luttinger liquids. Ref: Jacopo De Nardis, Miłosz Panfil, Edge singularities and quasilongrange order in nonequilibrium steady states, preprint condmat.quantgas arXiv:1801.08079.
Séminaire du LPTMS: Leonardo Mazza
Majorana zero modes in onedimensional Ytterbium quantum gases
Leonardo Mazza (Département de Physique, ENS, Paris)
After their introduction in the context of the relativistic Dirac equation, Majorana fermions have recently experienced a renewed interest for their relevance in the description of topological superconducting models, where they appear as Majorana zero modes. In this talk I will investigate the possibility of realizing Majorana zero modes in coldatom experiments, and in particular I will argue that onedimensional Ytterbium gases offer a natural experimental playground for their experimental implementation. Reference: [1] F. Iemini, L. Mazza, L. Fallani, P. Zoller, R. Fazio and M. Dalmonte, Majorana Quasiparticles Protected by ℤ_{2} Angular Momentum Conservation, Phys.Rev.Lett. 118 200404 (2017)
Séminaire du LPTMS: Corrado Rainone *** séminaire exceptionnel ***
Mechanical Failure in Amorphous Solids: Scale Free Spinodal Criticality
Corrado Rainone (Dpt of Chemical and biological physics, Weizmann Institute of Science, Israël)
The mechanical failure of amorphous media is a ubiquitous phenomenon from material engineering to geology. It has been noticed for a long time that the phenomenon is "scalefree", indicating some type of criticality. In spite of attempts to invoke "SelfOrganized Criticality", the physical origin of this criticality, and also its universal nature, being quite insensitive to the nature of microscopic interactions, remained elusive. Recently we proposed that the precise nature of this critical behavior is manifested by a spinodal point of a thermodynamic phase transition. Moreover, at the spinodal point there exists a divergent correlation length which is associated with the systemspanning instabilities (known also as shear bands) which are typical to the mechanical yield. Demonstrating this requires the introduction of an "order parameter" that is suitable for distinguishing between disordered amorphous systems, and an associated correlation function, suitable for picking up the growing correlation length. The theory, the order parameter, and the correlation functions used are universal in nature and can be applied to any amorphous solid that undergoes mechanical yield. Critical exponents for the correlation length and the system size dependence are estimated. We conclude with some perspectives and modelling ideas on the subject. Réf: Itamar Procaccia, Corrado Rainone, and Murari Singh, Mechanical failure in amorphous solids: Scalefree spinodal criticality, Phys. Rev. E 96, 032907 (2017)
Séminaire du LPTMS: Grégoire Ithier *** séminaire exceptionnel ***
Typicality and unconventional equilibrium states of an embedded quantum system.
Grégoire Ithier (Royal Holloway, London, UK)
 [1] C. Gross and I. Bloch, Quantum simulations with ultracold atoms in optical lattices, Science 357, 6355, pp. 9951001 (2017)
 [2] M. Schreiber et al, Observation of manybody localization of interacting fermions in a quasirandom optical lattice, Science 349, 6250, pp. 842845 (2015)
 [3] G. Ithier and F. BenaychGeorges, Dynamical typicality of embedded quantum systems, Phys. Rev. A 96, 012108 (2017)
 [4] G. Ithier, S. Ascroft, and F. BenaychGeorges, Typical equilibrium state of an embedded quantum system, Phys. Rev. E 96, 060102(R) (2017)
Séminaire du LPTMS: Alfredo Ozorio de Almeida
!!! Attention : jour inhabituel !!!
Translations and reflections on the torus: Identities for discrete Wigner functions and transforms
Alfredo Miguel Ozorio de Almeida (Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brazil)
Séminaire du LPTMS: José Lebreuilly
Stabilizing zero temperature quantum phases and incompressible states of light via nonMarkovian reservoir engineering
José Lebreuilly (Laboratoire Pierre Aigrain, ENS, Paris)
We study the possibility of stabilizing strongly correlated quantum fluids of light in drivendissipative devices through novel nonMarkovian reservoir engineering techniques. This approach allows to compensate losses and refill selectively the photonic population so to sustain a desired steadystate. It relies in particular on the use of a frequencydependent incoherent pump which can be implemented, e.g., via embedded twolevel systems maintained at a strong inversion of population. As specific applications of these methods, we discuss the generation of a photonic Mott Insulator (MI). As a first step, we present the case of a narrow band emission spectrum and show how this allows for the stabilization of MI states under the condition that the photonic states are relatively flat in energy. As soon as the photonic bandbwidth becomes comparable to the emission linewidth, important nonequilibrium signatures and entropy generation appear, and a novel dissipative phase transition from a Mott Insulating state toward a superfluid (SF) phase is unveiled. As a second step, we present a more advanced configuration based on reservoirs with a broadband frequency distribution, and we highlight the potential of this configuration for the quantum simulation of equilibrium quantum phases at zero temperature with tunable chemical potential. As a proof of principle we establish the applicability of our scheme to the BoseHubbard model by confirming the presence of a perfect agreement with the groundstate predictions both in the MI and SF regions, and more generally in all parts of the parameter space.PhysicsBiology interface seminar: Quan Li
Nanodiamond based quantum sensors for biological applications
Quan Li (Chinese University of Hong Kong, China)
Special location: Laboratoire Aimé Cotton, Orsay
Nanodiamond (ND) with Nitrogenvacancy (NV) centers serves as promising biosensor due to its excellent biocompatibility, high photostability, and the long spin coherence time at room temperature. However, the complicated biological environment, e.g. in a single cell, imposes stringent requirements on the sensor probes to be internalized. In this talk, I will discuss the requirements on nanodiamond as intracellular sensor, and the possible strategies that will enable various biosensing measurements. I will start with the understanding of nanodiamondcell interfaces, from anchoring of ND on the plasma membrane to their internalization, and eventually to their intracellular trafficking. Other than the conventional threedimensional trajectories tracking of the ND, it is also possible to track their orientations (rotation), providing additional information of the intracellular environment. One problem with NV based biosensing is that the NV center is less sensitive to certain parameters such as temperature and pressure, and not at all response to many other important biochemical parameters such as pH and nonmagnetic biomolecules. I will also discuss possible schemes of constructing nanodiamond based hybrid sensors, which lead to significantly enhanced sensitivity and/or potentially enable the measurement of various biochemical parameters using NV based quantum sensing.
Séminaire du LPTMS: Jacopo Rocchi
Selfsustained clusters in spin glass models
Jacopo Rocchi (LPTMS, Université ParisSud)
While macroscopic properties of spin glasses have been thoroughly investigated, their manifestation in the corresponding microscopic configurations is much less understood. To identify the emerging microscopic structures with macroscopic phases at different temperatures, we introduce the concept of selfsustained clusters (SSC). SSC are regions of the space where incluster induced fields dominate over the field induced by outcluster spins. We study their properties in the Ising pspin model with p=3 using replicas. The intuition gained using fully connected models is then used in the study of models defined on random graphs. A messagepassing algorithm is developed to determine the probability of individual spins to belong to SSC. Results for specific instances, which compare the predicted SSC associations with the dynamical properties of the spins, are obtained from numerical simulations. This insight gives rise to a way to predict individual spin dynamics from a single snapshot of spin configurations.PhysicsBiology interface seminar: Gervaise Mosser
Collagen and gelatin from sol to gel states for the synthesis of biomaterials
Gervaise Mosser (Laboratoire de Chimie de la Matière Condensée, Université Pierre et Marie Curie)
Collagen type I, the most abundant protein of connective tissues (bones, dermis, tendons, etc), is a macromolecular mesogen that can form lyotropic liquidcrystal phases. With this approach, our team works on elaborating several biomimetic biomaterials. However, the use of collagen can be hindered due to its price and the possibility to easily denature into gelatine and noticeably during sterilization processes. In this context, we wanted to determine whether collagen could be partially replaced by gelatine without modification of the overall hierarchical structure of the biomaterial.
Séminaire du LPTMS: Marcel Filoche
Localization landscape and localization potential in disordered or complex structures
Marcel Filoche (Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique)
Standing waves in disordered or complex systems can be subject to a strange and intriguing phenomenon which has puzzled the physics and mathematical communities for more than 60 years, namely wave localization. This phenomenon consists of a concentration (or a focusing) of the wave energy in a very restricted subregion of the entire domain. It has been evidenced experimentally in mechanics, acoustics and quantum physics. Determining the conditions for the onset of localization, depending on the disorder amplitude, the energy, or the wave type, is the aim of many theoretical studies. We will present a theory that unifies different types of localization within a single mathematical framework [1]. To that end, we will introduce the notion of "localization landscape", solution to an associated Dirichlet problem. Going further, this will enable us to define an "effective localization potential", providing a new insight into the confinement of the waves in disordered media. This potential allows us to predict the localization region, the energies of the localized modes, the density of states, and the long range decay of the wave functions. We will present experimental and numerical examples of this theory in mechanics, in semiconductor physics, and in molecular systems, as well as theoretical perspectives with cold atom systems. [1] M. Filoche and S. Mayboroda, Universal mechanism for Anderson and weak localization, PNAS 109, 14761–14766 (2012).Séminaire du LPTMS : Shamashis Sengupta
Gatetunable superconductivity in oxide heterostructures
Shamashis Sengupta (CSNSM, Université ParisSud)
The realization of twodimensional electronic gases (2DEGs) in oxidebased heterostructures (e.g. LaAlO3/SrTiO3) has led to important discoveries about superconductivity in low dimensions. There have been reports of the observation of pairing interactions without superconductivity (Cheng et al., Nature 521, 196 (2015)) and densityofstates features resembling the pseudogap in cuprates (Richter et al., Nature 502, 528 (2013)). Consequently, this 2DEG has emerged as a model system to study the physics of Cooper pair formation in two dimensions and to gain useful insights about complex problems, e.g., the phase diagram of high temperature superconductors. In this talk, we will discuss about a new method developed in our group for realizing such superconducting systems in oxide heterostructures, and the results of experiments to characterize their properties. Due to the low carrier density, it is possible to change it using a gate voltage following the principle of a fieldeffect transistor. The superconducting critical parameters (temperature and field) are tunable as a function of the gate voltage, leading to a 'superconducting dome' in the phase diagram. The possibility of continuously varying the carrier density allows us to study different equilibrium and nonequilibrium features characterizing the electronic phases. Results of some recent experiments will be presented.PhysicsBiology interface seminar: David Bensimon
Quantitative analysis of the somitogenetic wavefront
David Bensimon (LPSENS, Paris, France)
Somitogenesis is the process by which the anterioposterior axis is segmented in all vertebrates thus defining the coordinate system that will serve for positioning of the appendices and organs. This process of segmentation is due to the interaction between a posterior moving wavefront of morphogens and a posterior located clock generating somites (segments) at regular times and places. The existence and characterization of the clock has been amply demonstrated. In this talk I will focus on the molecular network behind the wavefront. I will discuss the wavefront response to various perturbations and compare our observations with a model of this network.
Quantum Physics Journal Club: Raoul Santachiara
Quantum mechanics in multiconnected space and the origin of new statistics in low dimensional system
Raoul Santachiara (LPTMS, Université ParisSud)
We recall how to define the problem of N indinstinguishible quantum particles and argue that the topology of the configuration space plays a crucial role. This observation, that has been put on solid grounds by Leinaas and Mirheim in the 1977, has provided the theoretical framework for the existence of anyonic statistics in two dimensions. Moreover, it inspired the connection between the Conformal field theory and topological phases in two dimensions: via this connection, the occurence of nonAbelian anyons in the fractional quantum Hall effect has been suggested.Séminaire du LPTMS: Serguey Andreev
Effective interactions in a quantum BoseBose mixture
Serguey Andreev, ITMO University, St. Petersburg, Russia
Application of the methods of Quantum Electrodynamics (QED) to a system of bosons at absolute zero temperature put forward by Spartak Beliaev in 1958 has been one of the most powerful analytical methods in studies of BoseEinstein condensates. The Beliaev theory provides a prescription of replacement of the actual microscopic interaction by an effective potential which can be used for perturbative expansion of the manybody Hamiltonian. Originally designed for onecomponent systems, the method has recently been applied to binary Bose mixtures in the context of supersolidity and stabilization of collapsing BoseEinstein condensates by quantum fluctuations. The present work is aimed at investigation of legitimacy of extrapolation of the Beliaev prescription to twocomponent systems. We show that quantum scatterings of different components, which until now have been assumed independent, can interfere due to the AndreevBashkin entrainment effect. The effect manifests itself in renormalization of the elementary excitations of the system. This result has escaped the earlier considerations based on the Fourier expansion of smallamplitude oscillations of the order parameter. We explain how one can account for the effect by using a properly generalized Bogoliubov approach. In 3D the effect appears in the second order of the perturbation theory, which makes possible using the concept of effective potential in this case. The entrainment arises due to "dressing" of magnons with Bogoliubov phonon modes, by analogy with the physics of Bose polaron. We exploit this fruitful analogy to speculate on possible formation of a magnon crystal in the stronglyinteracting regime.
Séminaire du LPTMS: Eoin Quinn *** séminaire exceptionnel ***
Splitting of electrons and violation of the Luttinger sum rule
Eoin Quinn (University of Amsterdam, The Netherlands)
We present a framework for organising the correlations of interacting electrons, which allows us to describe a regime of strongly correlated behaviour. We highlight two ways to characterise the electronic degree of freedom, either by the canonical fermion algebra or by the graded Lie algebra su(22). The first underlies the Fermi liquid description of correlated matter, and we identify a novel regime governed by the latter. We derive a systematic expansion of the electronic correlations, and compute the electronic spectral function at the leading order. This reveals a splitting in two of the electronic band, a violation of the Luttinger sum rule, and a Mott metalinsulator transition. Réf: Eoin Quinn, Splitting of electrons and violation of the Luttinger sum rule, Phys. Rev. B 97, 115134 (2018)
Séminaire du LPTMS: Senthil Todadri
*** ATTENTION: horaire inhabituel ***
Dualities in condensed matter physics
Senthil Todadri (Massachusetts Institute of Technology, Cambridge, USA)
There has been much recent progress in unearthing and understanding dualities of theories in 2 space dimensions. I will describe some of this progress and their relevance to frontier problems in condensed matter physics. I will specifically focus on a class of new dualities that generalize the familiar chargevortex duality of bosons to theories that include fermions.Quantum Physics Journal Club: Maurizio Fagotti
LiebRobinson Bounds
Maurizio Fagotti (LPTMS, Université ParisSud)
Séminaire du LPTMS: Aurélien Decelle
Spectral learning of Restricted Boltzmann Machines
Aurélien Decelle (Laboratoire de Recherche en Informatique, Université Paris Sud)
In this presentation I will expose our recent results on the Restricted Boltzman Machine (RBM). The RBM is a generative model very similar to the Ising model, it is composed of both visible and hidden binary variables, and traditionally used in the context of machine learning. In this context, the goal is to infer the parameters of the RBM such that it reproduces correctly a dataset's distribution. Although they have been widely used in computer science, the phase diagram of this model is not known precisely in the context of learning. In particular, it is not known how the parameters influence the learning, and what exactly is learned within the parameters of the model. After an introduction to some aspects of Machine learning, I will expose our work, showing how the SVD of the data governs the first phase of the learning and how this decomposition helps to understand the dynamics and the equilibrium properties of the model. Réf: Aurélien Decelle, Giancarlo Fissore and Cyril Furtlehner, Spectral dynamics of learning in restricted Boltzmann machines, EuroPhys. Lett. 119, 60001 (2017)
 Aurélien Decelle, Giancarlo Fissore and Cyril Furtlehner, Thermodynamics of Restricted Boltzmann Machines and related learning dynamics, preprint condmat arXiv:1803.01960 (2018).
Séminaire du LPTMS: Urna Basu
Active Brownian Motion in Two Dimensions
Urna Basu (LPTMS, Université ParisSud)
We study the dynamics of a single active Brownian particle in a twodimensional harmonic trap. The active particle has an intrinsic time scale set by the rotational diffusion. The harmonic trap also induces a relaxational timescale. We show that the competition between these two time scales leads to a nontrivial time evolution for the active Brownian particle. At short times a strongly anisotropic motion emerges leading to anomalous persistence properties. At longtimes, the stationary position distribution in the trap exhibits two different behaviours: a Gaussian peak at the origin in the strongly passive limit and a delocalised ring away from the origin in the opposite strongly active limit. The predicted stationary behaviours in these limits are in agreement with recent experimental observations. Réf: Urna Basu, Satya N. Majumdar, Alberto Rosso and Grégory Schehr, Active Brownian Motion in Two Dimensions, preprint condmat.statmech arXiv:1804.09027
PhysicsBiology interface seminar: Knut Drescher
Bacterial collective behaviours
Knut Drescher (Max Planck Institute for Terrestrial Microbiology, Marburg, Germany
In nature, bacteria often engage in a range of collective behaviors. In this presentation, I will demonstrate how two bacterial behaviors, swarming and biofilm formation, are related by physical interactions, chemical signaling, and dynamical transitions. I will show how these collective behaviors arise from cellcell interactions, and the physiological state of individual cells. Furthermore, I will introduce new experimental methods for investigating bacterial collective behaviors.
Soutenance de thèse: Aurélien Grabsch
Soutenance de thèse :
Random matrix theory in statistical physics: quantum scattering and disordered systems
by
Aurélien Grabsch
Jury:
 Alexander Altland (University of Cologne, Germany)
 JeanPhilippe Bouchaud (CFM, Paris)
 David S. Dean (LOMA, Université de Bordeaux)
 Yan V. Fyodorov (King's College London, UK)
 Satya N. Majumdar (LPTMS, Université ParisSud), directeur de thèse
 Cécile Monthus (IPhT, CEASaclay)
 Christophe Texier (LPTMS, Université ParisSud), directeur de thèse
Abstract:
Random matrix theory has applications in various fields: mathematics, physics, finance, ... In physics, the concept of random matrices has been used to study the electonic transport in mesoscopic structures, disordered systems, quantum entanglement, interface models in statistical physics, cold atoms, ... In this thesis, we study coherent AC transport in a quantum dot, properties of fluctuating 1D interfaces on a substrate and topological properties of multichannel quantum wires. The first part gives a general introduction to random matrices and to the main method used in this thesis: the Coulomb gas. This technique allows to study the distribution of observables which take the form of linear statistics of the eigenvalues. These linear statistics represent many relevant physical observables, in dif ferent contexts. This method is then applied to study concrete examples in coherent transport and fluctuating interfaces in statistical physics. The second part focuses on a model of disordered wires: the multichannel Dirac equation with a random mass. We present an extension of the powerful methods used for one dimensional systems to this quasi1D situation, and establish a link with a random matrix model. From this result, we extract the density of states and the localisation properties of the system. Finally, we show that this system exhibits a series of topological phase transitions (change of a quantum number of topological nature, without changing the symmetries), driven by the disorder.Quantum Physics Journal Club: Bradraj Pandey
Outoftimeorder correlators in quantum mechanics
Séminaire du LPTMS: Thorsten Emig
A Minimal Physiological Model for Human Running Performance
Thorsten Emig (LPTMS, Université ParisSud)
Measurements of physiological variables during exercise and performance evaluations and predictions are important for a fundamental understanding of physiological processes, training and assessment of athletes, and beyond sport in the study of complex physiological response related to aging, muscular structure and cardiovascular health. Models for human running performances of various complexities and underlying principles have been proposed, often employing a combination of data for world record performances and concepts that are not always based on simple principles of human physiology. We present a novel, minimal model for human running performance that follows from a selfconsistency relation for the time dependent power output during racing events. The model has a total of four parameters that are not fixed a priori and characterize individual physiological profiles for a runner. The analytic approach presented here is the first to derive the observed logarithmic scaling between world (and other) record running speeds and times from basic principles. Various female and male record performances (world, national) and also personal best performances of individual runners for distances from 800m and to the Marathon are excellently described by our model, with mean absolute errors of (often much) less than 1%. Physiological parameters of our model, as obtained from records and individual runners, are consistent with existing laboratory measurements. The computed maximal power output that can be sustained for a given time describes well existing experimental data for the time to exhaustion dependence of supramaximal oxygen consumption in the anaerobic regime. Our model is used to define and estimate endurance for both the aerobically and anaerobically dominated performances. As an application of our model, we derive personalized training speeds for prescribed duration and intensity. Our findings could be a basis for plethora of further studies including assessment of performance dependence on age, altitude, muscular structure, specialization of athlete, racing strategies, and optimal dosing of recreational exercise.Séminaire du LPTMS: Erik Aurell
Continuoustime dynamic cavity for equilibrium and nonequilibrium processes
Erik Aurell (KTHRoyal Institute of Technology, Sweden)
Dynamics on locally treelike graphs can be described by marginals which satisfy equations known as dynamic cavity. These equations are for probabilities of whole histories of single variables, and therefore need further approximations or closure. I will present a closure for continuoustime processes, and show how it behaves for some standard models in disordered systems which are either in equilibrium, or relaxing towards equilbrium. I will also discuss local search algorithms on Ksatisfiability of the walksat type, processes which do not satisfy detailed balance. This is joint work over the last few years with Gino Del Ferraro, Eduardo Dominguez, David Machado and Roberto Mulet.Soutenance de thèse: Kirill Plekhanov
Soutenance de thèse :Topological Floquet states, artificial gauge fields in strongly correlated quantum fluids
Kirill Plekhanov
Jury : Mark Oliver Goerbig (Université ParisSud  LPS) Nathan Goldman (Université libre de Bruxelles) Walter Hofstetter (GoetheUniversitat Frankfurt) Karyn Le Hur (Ecole Polytechnique  CPHT) directeur de thèse Titus Neupert (University of Zurich) Guido Pupillo (Université de Strasbourg) Nicolas Regnault (ENS  LPA) Guillaume Roux (Université ParisSud  LPTMS) directeur de thèse Abstract:In this thesis we study the topological aspects of condensed matter physics, that received a revolutionary development in the last decades. Topological states of matter are protected against perturbations and disorder, making them very promising in the context of quantum information. The interplay between topology and interactions in such systems is however far from being well understood, while the experimental realization is challenging. Thus, in this work we investigate such strongly correlated states of matter and explore new protocols to probe experimentally their properties. In order to do this, we use various both analytical and numerical techniques.
First, we analyze the properties of an interacting bosonic version of the celebrated Haldane model – the model for the quantum anomalous Hall effect. We propose its quantum circuit implementation based on the application of periodic timedependent perturbations – Floquet engineering. Continuing these ideas, we study the interacting bosonic version of the KaneMele model – the first model of a topological insulator. This model has a very rich phase diagram with an emergence of an effective frustrated magnetic model and a variety of symmetry broken spin states in the strongly interacting regime. Ultracold atoms or quantum circuits implementation of both Haldane and KaneMele bosonic models would allow for experimental probes of the exotic states we observed.
Second, in order to deepen the perspectives of quantum circuit simulations of topological phases we analyze the strong coupling limit of the SuSchriefferHeeger model and we test new experimental probes of its topology associated with the Zak phase. We also work on the outofequilibrium protocols to study bulk spectral properties of quantum systems and quantum phase transitions using a purification scheme which could be implemented both numerically and experimentally.
Miniworkshop 13:3014:00 Walter Hofstetter  Johann Wolfgang GoetheUniversität, Frankfurt / Main, Germany http://www.goetheuniversityfrankfurt.de/66535594/AGHofstetter 14:0014:30 Titus Neupert  University of Zürich, Switzerland http://www.physik.uzh.ch/en/groups/neupert/team/neupert.html 14:3015:00 Nathan Goldman  Université libre de Bruxelles, Belgium hhtps://www.nathangoldmanphysics.com 15:0015:30 Pause 15:3016:00 Guido Pupillo  ISIS, Université de Strasbourg https://isis.unistra.fr/laboratoiredephysiquequantiqueguidopupillo/ 16:0016:30 Nicolas Regnault  LPA, ENS http://www.lpa.ens.fr/spip.php?page=annuaire&Chercheurs=1&lang=fr 16:3017:00 Mark Oliver Goerbig  LPS, Université ParisSud https://www.equipes.lps.upsud.fr/GOERBIG/Séminaire du LPTMS: Mihail Poplavskyi
Pfaffian Point Processes and corresponding gap probabilities
Mihail Poplavskyi (King’s College London, Department of Mathematics)
Stochastic point processes are natural models of complex particle systems. In this talk we discuss point processes with a special structure of correlation functions, called Pfaffian Point Processes (PPP). We then give several examples of PPP such as the Glauber dynamics of Ising spin model, ensembles of random matrices with orthogonal symmetry, random Kac series, etc. In the second part of the talk we present recent results on gap probabilities for PPP and their applications to persistence probabilities for a class of stochastic models.Séminaire du LPTMS: Guillaume Roux
Quantum purification spectroscopy
Guillaume Roux (LPTMS, Université ParisSud)
We discuss a protocol based on quenching a purified quantum system that allows to capture bulk spectral features. It uses an infinite temperature initial state and an interferometric strategy to access the Loschmidt amplitude, from which the spectral features are retrieved via Fourier transform, providing coarsegrained approximation at finite times. It involves techniques available in current experimental setups for quantum simulation, at least for small systems. We illustrate possible applications in testing the eigenstate thermalization hypothesis and the physics of manybody localization. Reference: Bradraj Pandey, Kirill Plekhanov and Guillaume Roux, Quantum purification spectroscopy, preprint quantph arXiv:1802.04638.
Séminaire du LPTMS: Arthur Goetschy
Optimizing energy transfer and dwell times in disordered systems: a statistical approach
Arthur Goetschy (Institut Langevin, ESPCI)
 [1] A. Goetschy and A. D. Stone, Filtering Random Matrices: The Effect of Incomplete Channel Control in Multiple Scattering, Phys. Rev. Lett. 111, 063901 (2013)
 [2] C. W. Hsu, S. F. Liew, A. Goetschy, H. Cao, and A. D. Stone, Correlationenhanced control of wave focusing in disordered media, Nature Phys. 13, 497 (2017)
 [3] N. Fayard, A. Goetschy, R. Pierrat and R. Carminati, Mutual Information between Reflected and Transmitted Speckle Images, Phys. Rev. Lett. 120, 073901 (2018)
 [4] I. Starshynov, A. M. PaniaguaDiaz, N. Fayard, A. Goetschy, R. Pierrat, R. Carminati and J. Bertolotti, NonGaussian Correlations between Reflected and Transmitted Intensity Patterns Emerging from Opaque Disordered Media, Phys. Rev. X 8, 021041 (2018)
Soutenance de thèse: Ines RodriguezArias
Soutenance de thèse:Collective behaviors in interacting spin systems
par
Inès RodriguezArias
Jury: Cécile Monthus, IPhT, CEASaclay, présidente
 Juan Garrahan, University of Notthingham, rapporteur
 Nicolas Laflorencie, LPT Université Paul Sabatier, rapporteur
 Cristiano Ciuti, MPQ Université Paris Diderot, examinateur
 Geoffrey Bodenhausen, Département de Chimie, ENS, examinateur
 Andrea De Luca, University of Oxford, invité.
 Alberto Rosso, LPTMS, Université ParisSud, directeur de thèse
PhysicsBiology interface seminar: Alexis Lomakin
How do cells measure their boundaries to tailor physiological responses?
Alexis Lomakin (King's College London, UK)
Much like modern day engineered devices, cells in the human body are able to make precise measurements: intestinal epithelial cells monitor local cell densities to prevent hyperplasia, neutrophils sample their microenvironment to compute the fastest migratory route toward infection sites, and epidermal stem cells use extracellular matrix occupancy to make cell fate decisions. What these examples illustrate is the sensitivity of complex cell behaviors to spatial and mechanical constraints, known in quantitative sciences as boundary conditions. Although the importance of boundary conditions in cell and tissue physiology is increasingly recognized, it remains unclear how cells sample their boundaries to tailor specific behaviors to boundary conditions. Here, using biophysical tools to manipulate cell boundaries in a highly controlled, quantitative manner, we found that cells estimate externallyimposed confinement using their largest and stiffest intracellular component, the nucleus. Cell confinement below a certain threshold deforms the nucleus and expands its envelope area. Unbuffered against area expansion due to slow turnover of constituents, the nuclear envelope becomes stretched. This in turn engages signaling via nuclear membrane stretchsensitive proteins to the actomyosin cortex, activating contractility. The latter provides a motive force for the cell to squeeze through tight pores and constrictions in the extracellular matrix. Interestingly, no increase in cell contractility is observed when cells move through environmental confines that do not significantly deform the nucleus. Thus, the nucleus acts as an internal ruler for environmental confinement size, allowing cells to utilize energetically costly contractility on demand, only when surrounding space becomes restrictive. The advantage of the proposed mechanism is that in contrast to the plasma membrane, nuclear membranes do not participate in constitutive membrane trafficking; their surface area thus fluctuates less. This intrinsic quiescence should privilege them to function as lownoise detectors, to readily discriminate local environmental conditions from internal trafficinduced cell area/tension fluctuations.
Séminaire du LPTMS: Pierre Ronceray
Learning force fields from stochastic trajectories
Pierre Ronceray (Princeton Center for Theoretical Science, USA)
From nanometerscale proteins to micronscale colloidal particles, particles in biological and soft matter systems undergo Brownian dynamics: their deterministic motion due to external forces and interactions competes with the random diffusion due to thermal noise. In the absence of forces, all trajectories look alike: the key information characterizing the system's dynamics thus lies in its force field. However, reconstructing the force field by inspecting microscopy observations of the system's trajectory is a hard problem, for two reasons. First, there needs to be enough information about the force available in the trajectory: the effect of the force field becomes apparent only after a long enough observation time. Second, one needs a practical method to extract that information and reconstruct the force field, which is challenging for force fields with a spatial structure, in particular in the presence of measurement noise. Here we address these two problems for steadystate Brownian trajectories. We first give a quantitative meaning to the information contained in a trajectory, and show how it limits force inference. We then propose a practical procedure to optimally use this information to reconstruct the force field by decomposing it into moments. Using simple model stochastic processes, we demonstrate that our method permits a quantitative evaluation of phase space forces and currents, circulation, and entropy production with a minimal amount of data.Quantum Journal Club: Leonardo Mazza
Topological Transition in a NonHermitian Quantum Walk
PhysicsBiology interface seminar: Pierre Ronceray
Cell contraction induces longranged stress stiffening in the extracellular matrix
Pierre Ronceray (Princeton University, USA)
Animal cells in tissues are supported by biopolymer matrices, which typically exhibit highly nonlinear mechanical properties. While the linear elasticity of the matrix can significantly impact cell mechanics and functionality, it remains largely unknown how cells, in turn, affect the nonlinear mechanics of their surrounding matrix. Here we show that living contractile cells are able to generate a massive stiffness gradient in three distinct 3D extracellular matrix model systems: collagen, fibrin, and Matrigel. We decipher this remarkable behavior by introducing Nonlinear Stress Inference Microscopy (NSIM), a novel technique to infer stress fields in a 3D matrix from nonlinear microrheology measurement with optical tweezers. Using NSIM and simulations, we reveal a longranged propagation of cellgenerated stresses resulting from local filament buckling. This slow decay of stress gives rise to the large spatial extent of the observed cellinduced matrix stiffness gradient, which could form a mechanism for mechanical communication between cells.
Séminaire du LPTMS: Valentina Ros
Arrangement of local minima and phase transitions in the energy landscape of simple glassy models
Valentina Ros (IPhT, CEASaclay)
Understanding the statistical properties of the stationary points of highdimensional, random energy landscapes is a central problem in the physics of glassy systems, as well as in interdisciplinary applications to computer science, ecology and biology. In this talk, I will discuss a framework to perform the computation of the quenched complexity of stationary points, making use of a replicated version of the KacRice formula. I will discuss its application to simple models (the spiked tensor model and its generalizations) which capture the competition between a deterministic signal and stochastic noise, and correspond to a spherical pspin Hamiltonian endowed with ferromagnetic multibody interaction terms. I will describe the phase transitions that occur in the structure of the landscape when changing the signaltonoise ratio, and highlight the implications for the evolution of local dynamics within the landscape. Reference: Valentina Ros, Gerard Ben Arous, Giulio Biroli and Chiara Cammarota, Complex energy landscapes in spikedtensor and simple glassy models: ruggedness, arrangements of local minima and phase transitions, preprint condmat arXiv:1804.02686
Quantum Journal Club: Eoin Quinn
Organising strong correlations: SchwingerShastry formalism
Séminaire du LPTMS: Alberto Biella
Efficient stochastic unraveling of disordered open quantum systems
Alberto Biella (Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot)
The interplay of interaction, dissipation and driving in open quantum systems can trigger transitions between nonequilibrium phases. Such behaviour can emerge in extended lattices, in more then one spatial dimension, when homogenous systems are considered. However, in any realistic experimental realization, disorder cannot be neglected. In this work we develop a method to efficiently unravel the density matrix of a generic disordered open quantum system exploiting stochastic trajectories. We use it to study the effect of onsite disorder in the paradigmatic drivendissipative BoseHubbard lattice in two dimensions. In particular, we will focus on the role of the disorder when the system is driven across a firstorder transition from the low to the highdensity phase. We found that the disorder induces the formation of density domains which progressively smears the sharp transition leading to a crossover behaviour in the thermodynamic limit. We characterize this mechanism in terms of photon density and spatial correlation functions and we discuss how inhomogeneities affects the bistable dynamics of the system at the transition. Our results are relevant for stateoftheart experiments in extended photonic lattices based on semiconductor microcavities and superconducting circuits.PhysicsBiology interface seminar: Willy Supatto
Live imaging of motile cilia to investigate leftright symmetry breaking in zebrafish embryos
Willy Supatto (LOB, École polytechnique)
In vertebrate embryos, ciliadriven fluid flows are guiding leftright body symmetry breaking within the leftright organizer (LRO). To investigate the generation and sensing of flows, it is required to quantify cilia biophysical features in 3D and in vivo [1]. In the zebrafish embryo, the LRO is called the Kupffer’s vesicle (KV) and is a spheroid shape cavity, which is covered with motile cilia distributed at its surface and oriented in all directions of space. This transient structure varies in size and shape during development and from one embryo to the other. As a consequence, the experimental investigation of cilia properties is challenging. It requires quantifying cilia features in vivo and in 3D and combining the data from different embryos to compare one embryo to the other and perform statistical analyses.To reach this goal, we devised an experimental workflow combining live 3D imaging using multiphoton microscopy, image processing, and data registration to quantify cilia biophysical features, such as cilia density, motility, 3D orientation, or length. We integrated such experimental features obtained in vivo into a fluid dynamics model and a multiscale physical study of flow generation and detection. This strategy enabled us to demonstrate how cilia orientation pattern generates the asymmetric flow within the KV [2]. In addition, we could investigate the physical limits of flow detection to clarify which mechanisms could be reliably used for body axis symmetry breaking [2]. Finally, we discovered the distribution of cilia orientation is asymmetric within the KV [3]. Importantly, these results suggested that the asymmetric force detection could result from the cilium being sensitive to its own motion. Together, this work sheds light on the complexity of leftright symmetry breaking and chirality genesis in developing tissues.
[1] From cilia hydrodynamics to zebrafish embryonic development. Supatto & Vermot, Current Topics in Developmental Biology 2011
[2] Physical limits of flow sensing in the leftright organizer. Ferreira et al, eLife 2017
[3] Chiral cilia orientation in the leftright organizer. Ferreira et al, Cell Reports, in press
Séminaire du LPTMS: Paola Ruggiero
*** Attention : jour inhabituel ***
Conformal field theory on top of a breathing TonksGirardeau gas
Paola Ruggiero (SISSA, Trieste, Italie)
Conformal field theory (CFT) has been extremely successful in describing universal effects in critical onedimensional (1D) systems, in situations in which the bulk is uniform. However, in many experimental contexts, such as quantum gases in trapping potentials and in several outofequilibrium situations, systems are strongly inhomogeneous. Recently it was shown that the CFT methods can be extended to deal with such 1D situations [1,2]: the system’s inhomogeneity gets reabsorbed in the parameters of the theory, such as the metric, resulting in a CFT in curved space. Here in particular we make use of CFT in curved spacetime to deal with the outofequilibrium situation generated by a frequency quench in a TonksGirardeau gas in a harmonic trap [3]. We show compatibility with known exact result and use this new method to compute new quantities, not explicitly known by means of other methods, such as the dynamical fermionic propagator and the one particle density matrix at different times. Refs: [1] J. Dubail, JM. Stéphan, J. Viti & P. Calabrese, Conformal field theory for inhomogeneous onedimensional quantum systems: the example of noninteracting Fermi gases, SciPost Phys. 2, 002 (2017).
 [2] S. Murciano, P. Ruggiero & P. Calabrese, Entanglement and relative entropies for lowlying excited states in inhomogeneous onedimensional quantum systems, arXiv:1810.02287
 [3] P. Ruggiero, Y. Brun & J. Dubail, To appear.
Séminaire du LPTMS: Herbert Spohn
Nonlinear fluctuating hydrodynamics for onedimensional fluids
Professor Herbert Spohn (Zentrum Mathematik, München)
Séminaire du LPTMS: Alexios Polychronakos *** séminaire exceptionnel ***
!!!! ATTENTION : LIEU INHABITUEL (salle des conseils de l'IPN) !!!!
100 Years of Feynman and 30 without him: reminiscences from his last year
Alexios P. Polychronakos (The City College and Graduate Center of the CUNY, New York)
2018 marks the 100th anniversary of the birth and 30 years since the passing of Richard Feynman, a brilliantly creative physicist and a legendary personality in science and society at large. During the last year of his life at Caltech Feynman became fascinated by integrable models, his involvement and enthusiasm inspiring and motivating both experts and novices in the field. I will attempt to give a glimpse into Feynman's thinking and personality through the lens of personal memories and mementos from that last year.Séminaire du LPTMS: Emmanuel Trizac
When random walkers help solving intriguing integrals
Emmanuel Trizac (LPTMS, Université ParisSud)
We will discuss the properties of a family of integrals involving the cardinal sine fuction, first studied by Borwein & Borwein. The aim is to provide a physicist's perspective onto a curious change of behaviour occurring within this family, noticed when benchmarking computer algebra packages, and initially attributed to a bug. A number of nontrivial generalizations will be obtained.PhysicsBiology interface seminar: Jan Brugues
How to set the proper size and shape of metaphase spindles
Jan Brugues (MPI Dresden, Germany)
Regulation of size and growth is a fundamental problem in biology. A prominent example is the formation of the mitotic spindle, where protein concentration gradients around chromosomes are thought to regulate spindle growth by controlling microtubule nucleation. Previous evidence suggests that microtubules nucleate throughout the spindle structure. However, the mechanisms underlying microtubule nucleation and its spatial regulation are still unclear. In the first part of the talk I will present an assay based on laser ablation to directly probe microtubule nucleation events in Xenopus laevis egg extracts. Combining this method with theory and quantitative microscopy, we show that the size of a spindle is controlled by autocatalytic growth of microtubules, driven by microtubulestimulated microtubule nucleation. The autocatalytic activity of this nucleation system is spatially regulated by the limiting amounts of active microtubule nucleators, which decrease with distance from the chromosomes. This mechanism provides an upper limit to spindle size even when resources are not limiting. Once the necessary amounts of microtubules are created, the activities of motors lead to the proper shape and architecture of spindles. In the second part of the talk I will discuss the origin of motormediated stress in spindles.Soutenance de thèse: Shuang Wu
Soutenance de thèse:Algebraic area distribution of two dimensional random walks and the Hofstadter model
par
Shuang Wu
Jury: Rapporteur: Sergei Matveenko (Landau Institute for Theoretical Physics, Moscow, Russia)
 Rapporteur: Alexios Polychronakos (The City College of New York, USA)
 Examinateur: Angel Alastuey (Laboratoire de Physique, ENS Lyon)
 Examinateur: Vincent Pasquier (IPhT, CEA Saclay)
 Examinatrice: Didina Serban (IPhT, CEA Saclay)
 Invité: Olivier Giraud (LPTMS, Université ParisSud)
 Directeur de thèse: Stéphane Ouvry (LPTMS, Université ParisSud)
Quantum Journal Club: Christophe Texier
Correlations of occupation numbers in the canonical ensemble
Christophe Texier (LPTMS, Université ParisSud)
The connection between the statistical physics of noninteraction indistinguishable particles in quantum mechanics and the theory of symmetric functions will be reviewed. Then, I will study the $p$point correlation function $overline{n_1cdots n_p}$ of occupation numbers in the canonical ensemble ; in the grand canonical ensemble, they are trivially obtained from the independence of individual quantum states, however the constraint on the number of particles makes the problem non trivial in the canonical ensemble. I will show several representations of these correlation functions. I will illustrate the main formulae by revisiting the problem of BoseEinstein condensation in a 1D harmonic trap in the canonical ensemble, for which we have obtained several analytical results. In particular, in the temperature regime dominated by quantum correlations, the distribution of the ground state occupancy is shown to be a truncated Gumbel law. Ref: Olivier Giraud, Aurélien Grabsch & Christophe Texier, Correlations of occupation numbers in the canonical ensemble and application to BEC in a 1D harmonic trap, Phys. Rev. A 97, 053615 (2018).Séminaire du LPTMS: Alexandre Krajenbrink
Linear statistics and pushed Coulombgas at the soft edge of random matrices : four paths to large deviations
Alexandre Krajenbrink (LPTENS, Paris)
In this talk, I will consider the classical problem of linear statistics in random matrix theory. This amounts to study the distribution of the sum of a certain function of the matrix eigenvalues. Varying this function, this problem can describe fluctuations of conductance, shot noise, Renyi entropy, center of mass of interfaces, particle number… This problem has been extensively studied for the bulk of the eigenvalues (macroscopic linear statistics) where interesting phase transitions have been unveiled but not so much at the edge of the spectrum (microscopic linear statistics) on which I will focus. In particular, I will introduce four methods to solve this problem, show their equivalence and I will discuss the physical applications of these results (large deviations of the solution of the KardarParisiZhang equation, existence of phase transitions with continuously varying exponent and possible experimental realization of this setup with nonintersecting Brownian interfaces). Alexandre Krajenbrink & Pierre Le Doussal, Linear statistics and pushed Coulomb gas at the edge of beta random matrices: four paths to large deviations, preprint arXiv:1811.00509
Séminaire du LPTMS: Lucile Julien
La révision du système international d'unités
Lucile Julien (Laboratoire KastlerBrossel, UPMC, Paris)
Le SI, système international d’unités, né en 1960, est l’héritier du système métrique et du système MKSA. Il est fondé sur 7 unités de base, dont les définitions peuvent changer lorsque les besoins des utilisateurs le rendent nécessaire. Ainsi, le mètre a été redéfini en 1983 en fixant la valeur numérique de la vitesse de la lumière dans le vide. La 26ème Conférence Générale des Poids et Mesures, qui s'est réunie du 13 au 16 novembre, a décidé de la même façon de redéfinir le kilogramme, l’ampère, la mole et le kelvin en fixant les valeurs de quatre constantes de la physique. Après une présentation historique du SI, je présenterai les motivations de sa révision actuelle, les travaux qui l’ont rendue possible et la façon dont elle est réalisée.!!!! ATTENTION : LIEU INHABITUEL (Auditorium Irène Joliot Curie) !!!!
**** l'exposé sera en français ****
PhysicsBiology interface seminar: AnaJesus GarciaSaez
Single molecule analysis of mitochondrial permeabilization in apoptosis
AnaJesus GarciaSaez (MPI Tübingen, Germany)
Bax and Bak are key regulators of apoptosis and mediate the permeabilization of the outer mitocondrial membrane that leads to cytochrome and Smac release. Although it is widely accepted that Bax and Bak function and molecular mechanism largely overlap, there is limited evidence how Bak works. In previous studies, we have used single molecule microscopy to characterize the oligomerization of Bax in the membrane and its organization at the nanoscale in the mitochondria of apoptotic cells. We now extended these approaches to Bak and identified key structural differences between the two proteins that may have functional implications.
Séminaire du LPTMS: Christophe Mora
Parafermions and symmetryenriched Majorana fermions in onedimensional fermionic models
Christophe Mora (Laboratoire Pierre Aigrain, ENS, Paris)
Stabilizing and manipulating exotic emergent quasiparticles is one of the main goal of modern condensed matter physics. The quest for observing Majorana fermions and their nonAbelian braiding statistics in superconducting nanostructures is currently attracting a lot of attention, with fascinating prospects in faulttolerant quantum computation. Parafermions are the simplest generalization of Majorana fermions: they show nonAbelian fractional statistics and are typically associated with topological phases. We will discuss the possibility of harboring these exotic excitations in genuinely onedimensional electronic platforms. We focus on a specific model of fermions in one dimension with a generalized Z_{N} multiplet pairing extending the standard and socalled Kitaev chain model. Using a combination of analytical techniques, we find an interesting topological phase intertwined with spontaneous symmetry breaking. Each symmetrybreaking sector is shown to possess a pair of boundary Majorana fermions encoding a topological character. A careful study of the quantum anomaly through pumping in the system finally reveals that parafermions exist in one dimension but only as nonlocal operators. References: Fernando Iemini, Christophe Mora & Leonardo Mazza, Topological phases of parafermions: a model with exactlysolvable ground states, Phys. Rev. Lett. 118, 170402 (2017)
 Leonardo Mazza, Fernando Iemini, Marcello Dalmonte & Christophe Mora, Poor man's parafermions in a lattice model with even multiplet pairing, preprint condmat arXiv:1801.08548.
Séminaire du LPTMS: Christophe Texier
Counting the equilibria of a directed polymer in a random medium and Anderson localisation
Christophe Texier (LPTMS, Université ParisSud)
I will discuss a new connection between two different problems: the counting of equilibria of a directed polymer in a random medium (DPRM) and the problem of Anderson localisation for the 1D Schrödinger equation. Using the KacRice formula, it is possible to express the mean number of equilibria of a DPRM in terms of functional determinants. In the onedimensional situation, these functional determinants can be calculated thanks to the GelfandYaglom method, showing that the mean number of equilibria of the DPRM growth exponentially with the length of the polymer, with a rate controlled by the generalized Lyapunov exponent (GLE) of the localisation problem (cumulant generating function of the log of the wave function). The GLE is solution of a spectral problem studied by combining numerical approaches and WKBlike approximation. Furthermore, the formalism can be extended in order to obtain the number of equilibria at fixed energy, providing the (annealed) distribution of the energy density of the line over the equilibria. Reference: Yan V. Fyodorov, Pierre Le Doussal, Alberto Rosso and Christophe Texier, Exponential number of equilibria and depinning threshold for a directed polymer in a random potential, Annals of Physics 397, 164 (2018)