Séminaires de l’année 2018

Séminaire du LPTMS: Nicolas Cherroret

Novel perspectives from Anderson localization of atomic matter waves

Nicolas Cherroret (Laboratoire Kastler-Brossel, 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 out-of-equilibrium evolution of a non-interacting 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 well-known coherent backscattering effect. I will conclude the talk with open questions on the role of atomic interactions.

Physics-Biology 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 photo-physical 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 photo-chemical internalization (PCI) of macromolecules into cells is based on the photo-induced alteration of endosomal membranes - before their maturation in lysosomes - allowing the escape of the macromolecules, free to reach its targets within cell. More extensive photo-induced changes, in particular to the mitochondria, lead to cell death by necrosis or apoptosis. This photo-induced cell death is basis of an anticancer therapy so-called PDT.

First, we focussed on the photo-induced 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 photo-activation. Secondly, to better apprehend such complex effects, we turned to model systems. In particular, we focused on photo-oxidation of membranes lipids, that are important oxidative targets. We extensively studied their modifications under photo-oxidation. Our purpose is to demonstrate that the photo-induced 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 one-particle 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)

The past decade has seen tremendous success in predicting and experimentally discovering distinct classes of topological insulators (TIs) and semimetals.  We review the field and we propose an electronic band theory that highlights the link between topology and local chemical bonding, and combines this with the conventional band theory of electrons. Topological Quantum Chemistry is a description of the universal global properties of all possible band structures and materials, comprised of a graph theoretical description of momentum space and a dual group theoretical description in real space. We classify the possible band structures for all 230 crystal symmetry groups that arise from local atomic orbitals, and show which are topologically nontrivial. We show how our topological band theory sheds new light on known TIs, and demonstrate the power of our method to predict a plethora of new TIs.

Séminaire du LPTMS: Pierre-Elie Larré & Clément Tauber

Quantum simulating many-body phenomena with propagating light

Pierre-Elie Larré (Université de Cergy-Pontoise)

We consider the propagation of a quantum light field in a cavityless nonlinear dielectric. In this all-optical 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 many-body 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 Bose-Einstein condensation.

Bulk-edge correspondence for Floquet topological insulators

Clément Tauber (ETH, Zürich)

Floquet topological insulators describe independent electrons on a lattice driven out of equilibrium by a time-periodic Hamiltonian, beyond the usual adiabatic approximation. In dimension two such systems are  characterized by integer-valued 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.

Physics-Biology 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 single-cell 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 so-called 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 Ornstein-Uhlenbeck 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 (LPT-ENS, 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 inter-facial 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].
[1] J. Stat. Phys. 144, 554 (2011). [2] Phys. Rev. Lett. 108, 165701 (2012). [3] The Journal of Chemical Physics 147 , 084704 (2017).

Statistical Physics-inspired models of biological network: collective behavior in neuronal ensembles

Ulisse Ferrari (Institut de la Vision, Inserm & UPMC)

In both cortices and sensory systems, information is represented and transmitted through the correlated activity of large neuronal networks. Neurons, in fact, do not work independently: each of them drives the activity of the others, thus working as a collective ensemble. Methods borrowed from Statistical Physics and Machine Learning are powerful tools for characterizing the collective behavior of large systems and hence offer promising approaches to understand the activity of neuronal populations. In this talk I will show how the Maximum Entropy principle, applied to cortical in-vivo recording, allows for characterizing and comparing the population behavior during wakefulness and deep sleep. Then, I will use hidden-layer models, point processes and “experimental” linear response theory to account for non-linear stimulus processing in sensory networks, such as the retina. These approaches allow for constructing high performing models of the retinal population response to visual stimuli and thus for characterizing how a network of neurons can encode and transmit visual information.

Physics-Biology interface seminar: Pere Roca-Cusachs

Sensing the matrix: transducing mechanical signals from integrins to the nucleus.

Pere Roca-Cusachs (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 integrin-ECM 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 cell-ECM 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 non-equilibrium phase transitions

Nicola Bartolo (Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot et CNRS)

In out-of-equilibrium 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 driven-dissipative systems, consisting of a nonlinear Kerr resonator in the presence of both coherent (one-photon) and parametric (two-photon) driving and dissipation. We analytically derive the exact steady-state solution via the formalism of the complex P-representation. 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.

φ-evo: 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 lac-operon 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 density-dependent 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   non-interacting random walks are exactly identical to their hopping  rates. We develop a procedure to obtain systematic analytical approximations for the diffusion coefficient in non-gradient lattice  gases with known equilibrium. The method relies on a variational  formula found by Varadhan and Spohn. Restriction on test functions to  finite-dimensional sub-spaces allows one to perform the minimization  and gives upper bounds for the diffusion coefficient. We apply the   procedure to the following two models; one-dimensional generalized  exclusion processes, where each site can accommodate at most two   particles (2-GEPs) [1], and the Kob-Andersen (KA) model on the square  lattice, which is classified into kinetically-constrained 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 mean-field theory, but the larger shapes, the   more precise upper bounds we obtain. For the 2-GEPs, 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 cond-mat 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, long-range correlations become relevant). I will present recent results showing that, for a broad class of 1D models with hard-core repulsion but also interactions and space-dependent 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 so-called "third order phase transitions". The methods involved are quite general and likely to be applicable to many more families of models.

Séminaire du LPTMS: Sergej Moroz

Séminaire du LPTMS: Laurent de Forges de Parny

Multicomponent Bose-Hubbard Model: Nematic Order in Spinor Condensates

Laurent de Forges de Parny (Albert-Ludwigs 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, spin-dynamics, 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 spin-rotation symmetry without magnetic order [2]. I will discuss the magnetic properties of strongly interacting spin-1 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 singlet-to-nematic phase transition inside the Mott insulator phase. References:
  1. 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)
  2. T. Zibold, V. Corre, C. Frapolli, A. Invernizzi, J. Dalibard and F. Gerbier, Spin-nematic order in antiferromagnetic spinor condensates, Phys. Rev. A 93, 023614 (2016).
  3. L. de Forges de Parny, F. Hébert, V. G. Rousseau, and G. G. Batrouni, Interacting spin-1 bosons in a two-dimensional optical lattice, Phys. Rev. B 88, 104509 (2013).
  4. Laurent de Forges de Parny, Hongyu Yang, and Frédéric Mila, Anderson Tower of States and Nematic Order of Spin-1 Bosonic Atoms on a 2D Lattice, Phys.Rev.Lett. 113, 200402 (2014)

Séminaire du LPTMS: Andrea de Luca

Solution of a minimal model for many-body 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 many-body system. It consists of a chain of sites with nearest-neighbour coupling under Floquet time evolution. Quantum states at each site span a q-dimensional Hilbert space and time evolution for a pair of sites is generated by a q2×q2 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 many-body dynamics over realisations of the random matrices. This approach leads to exact expressions in the large-q limit and sheds light on the universality of random matrices in many-body quantum systems and the ubiquitous entanglement growth in out-of-equilibrium dynamics.

Séminaire du LPTMS: Shuang Wu

Thouless bandwidth formula in the Hofstadter model

Shuang Wu (LPTMS, Université Paris-Sud)

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 n-th moment and obtain a closed expression in terms of polygamma, zeta and Euler numbers.  

Séminaire du LPTMS: Giulio Bertoli

Finite temperature disordered bosons in two dimensions

Giulio Bertoli (LPTMS, Université Paris-Sud)

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 many-body localization-delocalization 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 cond-mat.dis-nn arXiv:1708.03628

Physics-Biology interface seminar: Pierre Sens

Mechano-sensitive adhesion in cell spreading and crawling

Pierre Sens (Institut Curie, Paris)

Crawling cell motility is powered by actin polymerization and acto-myosin 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 mechano-sensitive cell adhesion, and discuss its relevance for symmetry breaking, cell polarisation, and motility. I will then discuss a generic model of micro-crawlers, built as an extension of low Reynolds number micro-swimmers, that highlights the crucial role of mechano-sensitive 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 salt-free 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 Paris-Sud)

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 first-passage 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 first-passage 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 first-passage time of current variables in terms of the dissipation rate, a fluctuation theorem for first-passage 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)

I will present a modified version of the algebraic Bethe ansatz (MABA) that allows to characterize the eigenvalues and the eigenstates of spins chains without U(1) symmetry.  In the case of the XXX Heisenberg spins chain on the circle with a twisted boundary condition, the Bethe vectors and associated eigenvalues will be constructed and the correlation function problem will be discussed. This method also works for the XXX and XXZ Heisenberg spins chains on the segment.


Séminaire du LPTMS: Bertrand Lacroix-à-chez-Toine *** séminaire exceptionnel ***

Extreme value statistics in a gas of 2D charged particles

Bertrand Lacroix-à-chez-Toine (LPTMS, Université Paris-Sud)

  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. Lacroix-A-Chez-Toine, 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), 1062-1081 (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 pseudo-well in the effective energy  landscape, can be generated in an over-damped 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 out-of-equilibrium  driving may compromise the emergence of a pseudo-well. 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.

Physics-Biology interface seminar: Ulisse Ferrari

Non-linear 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 non-linear, 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 ex-vivo multi-electrode 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, nearly-linear, models are not enough and highly non-linear models are required. Then I will present the result of a closed-loop experiment where we adapted the stimulus on-line 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 non-linear sensitivity of the retina is consistent with an efficient encoding of stimulus information.

-) 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 non-equilibrium 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)

I will present a detailed study of the conduction properties of a molecular wire where hopping processes between electronic sites are coupled to a vibrational mode of the molecule. This description is inspired by the idea that physically the vibrational mode does not need to change the energetic structure of the electronic part but can just perturb the exchange taking place on this subsystem. It is shown that the presence of the vibrational system can give rise to strong enhancement of the wire conductivity. Moreover through the control of the vibrational properties (temperature and position) one can accurately control the electronic flux crossing the device. An increase of the temperature enhances the conduction, while the control of the equilibrium position of the oscillator can switch on and off the conduction.

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.

  • 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 charge-pumping 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 Aharonov-Bohm and transverse magnetic fluxes Φ and χ. We show that the coupling between ladder legs provides a way to effectively change the ground-state fermion-number parity, by varying χ. We demonstrate that varying χ by (one flux quantum) leads to an apparent fermion-number parity switch. We find that persistent currents exhibit a robust 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 two-dimensional 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 Harper-Hofstadter quantum Hall regime. Indeed, we show that the periodicity is a mesoscopic manifestation of a novel type of fermionic pumping in topological systems, complementary to Thouless' pump.  Focusing on the low-energy edge physics in the general framework of Chern-Simons 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 parity-switching and the 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, Charles-Edouard Bardyn, Thierry Giamarchi, Controlled parity switch of persistent currents in quantum ladders, preprint cond-mat.mes-hall arXiv:1710.02152


Séminaire du LPTMS: Alexandru Petrescu *** séminaire exceptionnel ***

Fluxon-based quantum simulation in circuit QED

Alexandru Petrescu (Department of electrical engineering, Princeton University, USA)

Long-lived fluxon excitations can be trapped inside a superinductor ring, which can be realized with a long array of Josephson junctions, one of which offers the input/output path for the magnetic flux [1]. The superinductor ring can be separated into smaller loops by a periodic sequence of Josephson junctions in the quantum regime, thereby allowing fluxons to tunnel between neighboring loops [2]. This model is dual to that of two-leg ladder bosons, which have a rich phase diagram depending on flux and density [3–6]. By tuning the Josephson coupling, and implicitly the tunneling probability amplitude of fluxons, a wide class of 1D tight-binding lattice models may be implemented and populated with a stable number of fluxons. In this context, fluxons are lattice bosons with repulsive interactions. We illustrate this quantum simulation platform by discussing the Su-Schrieffer-Heeger model in the 1-fluxon subspace, which hosts a symmetry-protected topological phase with fractionally charged bound states at the edges [7,8]. This pair of localized edge states could be used to implement a superconducting qubit increasingly decoupled from decoherence mechanisms.
  1. [1]  N. A. Masluk, I. M. Pop, A. Kamal, Z. K. Minev, and M. H. Devoret, Phys. Rev. Lett. 109, 137002 (2012).
  2. [2]  A. Petrescu, H. E. Türeci, A. V. Ustinov, and I. M. Pop, ArXiv e-prints (2017), arXiv:1712.08630 [cond-mat.mes-hall].
  3. [3]  E. Orignac and T. Giamarchi, Phys. Rev. B 64, 144515 (2001).
  4. [4]  A. Petrescu and K. Le Hur, Phys. Rev. Lett. 111, 150601 (2013).
  5. [5]  M. Piraud, F. Heidrich-Meisner, I. P. McCulloch, S. Greschner, T. Vekua, and U. Schollwöck, Phys. Rev. B 91, 140406 (2015).
  1. [6]  A. Petrescu, M. Piraud, G. Roux, I. P. McCulloch, and K. Le Hur, Phys. Rev. B 96, 014524(2017).
  2. [7]  R. Jackiw and C. Rebbi, Phys. Rev. D 13, 3398 (1976).
  3. [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 geometry-constraint 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 power-law 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 Paris-Sud)

Sand-piles, 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 yield-stress 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  non-Brownian  suspensions, we show that these systems can be described by a Herschel-Bulkley 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 pressure-controlled approach, originally developed for granular flows, reveals a striking physical analogy between the colloidal glass transition and the granular jamming transition.

Physics-Biology 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 quasi-long-range order in non-equilibrium steady states

Jacopo de Nardis (LPT-ENS, Paris)

I will present a theoretical study of exotic states of matter realized via the bi-partite quantum quench protocol, where two many-body 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 non-equilibrium 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 non-linear Luttinger liquids. Ref:

Séminaire du LPTMS: Leonardo Mazza

Majorana zero modes in one-dimensional 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 cold-atom experiments, and in particular I will argue that one-dimensional 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 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 "scale-free", indicating some type of criticality. In spite of attempts to invoke "Self-Organized 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 system-spanning 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: Scale-free 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)

In recent years, the progress in quantum engineering has provided new tools for simulating the dynamics of truly isolated quantum systems. These systems, made of trapped ions or cold atoms[1], can be prepared in a global pure state and their level of isolation is such that they evolve unitarily according to the Schrödinger equation. Surprisingly, despite being at all times in a pure quantum state, they display signatures of a local equilibration which can be in strong disagreement with the predictions of statistical physics [2]. These experimental facts are clearly questioning what kind of statistical description is relevant for isolated many body quantum systems.
In this talk, I will present our recent results on a theoretical model dedicated to this problem. This model considers a quantum system coupled to a large quantum environment, and introduces  some randomness at the level of the interaction Hamiltonian. We then demonstrate that the system has a typical dynamics for several classes of random interactions and most importantly for arbitrary system, environment, and global initial state [3]. In other words, the microscopic structure of interaction Hamiltonians does not matter and reduced density matrices have a self-averaging property. 
These results have two important consequences: first they can explain the absence of sensitivity to microscopic details of processes like e.g. thermalization. Second they provide the rigorous ground for an averaging procedure over random interactions which can be used for analytical non perturbative calculations performed with full generality i.e. for arbitrary system, environment, and initial state.
We apply this technique to calculate analytically the stationary state at long times of the system and find a new thermodynamical ensemble more general than the microcanonical one [4]. 

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)

A finite Hilbert space can be associated to a periodic phase space, that is, a torus. A finite subgroup of operators corresponding to reflections and translations on the torus form respectively the basis for the discrete Weyl representation, including the Wigner function, and for its Fourier conjugate, the chord representation. They are invariant  under Clifford transformations and obey analogous product rules to the continuous representations, so allowing for the calculation of expectations and correlations for observables. We here import new identities from the continuum for products of pure state Wigner functions and chord functions, involving, for instance the inverse phase space participation ratio and correlations of a state with its translation. Connections between products of Wigner functions and mixed (transition) Wigner functions also arise. Finally, generalizations of translations and reflections to a doubled phase space connect the Weyl representation of the evolution operator to the propagator of Wigner functions.

Séminaire du LPTMS: José Lebreuilly

Stabilizing zero temperature quantum phases and incompressible states of light via non-Markovian reservoir engineering

José Lebreuilly (Laboratoire Pierre Aigrain, ENS, Paris)

We study the possibility of stabilizing strongly correlated quantum fluids of light in driven-dissipative devices through novel non-Markovian reservoir engineering techniques. This approach allows to compensate losses and refill selectively the photonic population so to sustain a desired steady-state. It relies in particular on the use of a frequency-dependent incoherent pump which can be implemented, e.g., via embedded two-level 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 non-equilibrium 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 Bose-Hubbard model by confirming the presence of a perfect agreement with the ground-state predictions both in the MI and SF regions, and more generally in all parts of the parameter space.

Physics-Biology 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 Nitrogen-vacancy (NV) centers serves as promising bio-sensor due to its excellent bio-compatibility, high photo-stability, 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 intra-cellular sensor, and the possible strategies that will enable various bio-sensing measurements. I will start with the understanding of nanodiamond-cell interfaces, from anchoring of ND on the plasma membrane to their internalization, and eventually to their intracellular trafficking. Other than the conventional three-dimensional 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 bio-sensing 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 non-magnetic 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

Self-sustained clusters in spin glass models

Jacopo Rocchi (LPTMS, Université Paris-Sud)

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 self-sustained clusters (SSC). SSC are regions of the space where in-cluster induced fields dominate over the field induced by out-cluster spins. We study their properties in the Ising p-spin 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 message-passing 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.

Physics-Biology 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 liquid-crystal 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 sub-region 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 localizationPNAS 109, 14761–14766 (2012).

Séminaire du LPTMS : Shamashis Sengupta

Gate-tunable superconductivity in oxide heterostructures

Shamashis Sengupta (CSNSM, Université Paris-Sud)

  The realization of two-dimensional electronic gases (2DEGs) in oxide-based 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 density-of-states 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 field-effect 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 non-equilibrium features characterizing the electronic phases. Results of some recent experiments will be presented.

Physics-Biology interface seminar: David Bensimon

Quantitative analysis of the somitogenetic wavefront

David Bensimon (LPS-ENS, Paris, France)


Somitogenesis is the process by which the anterio-posterior 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 multi-connected space and the origin of new statistics in low dimensional system

Raoul Santachiara (LPTMS, Université Paris-Sud)

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 non-Abelian anyons in the fractional quantum Hall effect has been suggested.

Séminaire du LPTMS: Serguey Andreev

Effective interactions in a quantum Bose-Bose 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 Bose-Einstein 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 many-body Hamiltonian. Originally designed for one-component systems, the method has recently been applied to binary Bose mixtures in the context of supersolidity and stabilization of collapsing Bose-Einstein condensates by quantum fluctuations. The present work is aimed at investigation of legitimacy of extrapolation of the Beliaev prescription to two-component systems. We show that quantum scatterings of different components, which until now have been assumed independent, can interfere due to the Andreev-Bashkin 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 small-amplitude 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 strongly-interacting 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(2|2). 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 metal-insulator transition. Réf:

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 charge-vortex duality of bosons to theories that include fermions.

Quantum Physics Journal Club: Maurizio Fagotti

Lieb-Robinson Bounds

Maurizio Fagotti (LPTMS, Université Paris-Sud)

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 cond-mat arXiv:1803.01960 (2018).

Séminaire du LPTMS: Urna Basu

Active Brownian Motion in Two Dimensions

Urna Basu (LPTMS, Université Paris-Sud)

We study the dynamics of a single active Brownian particle in a two-dimensional harmonic trap. The active particle has an intrinsic time scale set by the rotational diffusion. The harmonic trap also induces a relaxational time-scale. 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 long-times, 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:

Physics-Biology 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 cell-cell 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


Aurélien Grabsch




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 quasi-1D 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

Out-of-time-order correlators in quantum mechanics

Séminaire du LPTMS: Thorsten Emig

A Minimal Physiological Model for Human Running Performance

Thorsten Emig (LPTMS, Université Paris-Sud)

  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 self-consistency 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

Continuous-time dynamic cavity for equilibrium and non-equilibrium processes

Erik Aurell (KTH-Royal Institute of Technology, Sweden)

Dynamics on locally tree-like 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 continuous-time 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 K-satisfiability 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 Paris-Sud - LPS) Nathan Goldman (Université libre de Bruxelles) Walter Hofstetter (Goethe-Universitat 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 Paris-Sud - LPTMS) directeur de thèse

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 time-dependent perturbations – Floquet engineering. Continuing these ideas, we study the interacting bosonic version of the Kane-Mele 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. Ultra-cold atoms or quantum circuits implementation of both Haldane and Kane-Mele 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 Su-Schrieffer-Heeger model and we test new experimental probes of its topology associated with the Zak phase. We also work on the out-of-equilibrium 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.

13:30-14:00  Walter Hofstetter -- Johann Wolfgang Goethe-Universität, Frankfurt / Main, Germany http://www.goethe-university-frankfurt.de/66535594/AG-Hofstetter 14:00-14:30 Titus Neupert -- University of Zürich, Switzerland http://www.physik.uzh.ch/en/groups/neupert/team/neupert.html 14:30-15:00 Nathan Goldman -- Université libre de Bruxelles, Belgium hhtps://www.nathan-goldman-physics.com 15:00-15:30 Pause 15:30-16:00 Guido Pupillo -- ISIS, Université de Strasbourg https://isis.unistra.fr/laboratoire-de-physique-quantique-guido-pupillo/ 16:00-16:30 Nicolas Regnault -- LPA, ENS http://www.lpa.ens.fr/spip.php?page=annuaire&Chercheurs=1&lang=fr 16:30-17:00 Mark Oliver Goerbig -- LPS, Université Paris-Sud https://www.equipes.lps.u-psud.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 Paris-Sud)

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 coarse-grained 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 many-body localization. Reference:
  • Bradraj Pandey, Kirill Plekhanov and Guillaume Roux, Quantum purification spectroscopy, preprint quant-ph 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)

When a wave such as light propagates through a disordered system, it is scattered many times in various directions before escaping. At first sight, this process is well described by diffusion. However, diffusion neglects interferences, making us believe that the information content of a wave is progressively lost through spreading. This picture is incorrect. In fact, multiple scattering is a linear process that redistributes information among many degrees of freedom which can nowadays be resolved and manipulated. In this talk, we will give an overview of different strategies to achieve original light transport properties in open disordered systems that deviate both from the diffusive picture and the Gaussian field model. First, we will characterize the statistical properties of the transmission matrix to demonstrate large energy transfer or focusing through nominally opaque media [1, 2]. Then, we will discuss how to achieve similar performance in transmission by means of the reflection matrix only [3, 4]. Finally, we will characterize the distribution of scattering times, in order to generate excitations with particularly short or long dwell times. References:
  • [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, Correlation-enhanced 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. Paniagua-Diaz, N. Fayard, A. Goetschy, R. Pierrat, R. Carminati and J. Bertolotti, Non-Gaussian Correlations between Reflected and Transmitted Intensity Patterns Emerging from Opaque Disordered Media, Phys. Rev. X 8, 021041 (2018)

Soutenance de thèse: Ines Rodriguez-Arias

Soutenance de thèse:

Collective behaviors in interacting spin systems


Inès Rodriguez-Arias

  • Cécile Monthus, IPhT, CEA-Saclay, 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 Paris-Sud, directeur de thèse
Résumé: Dynamic nuclear polarization (DNP) is one of the most promising techniques towards a new generation of Magnetic Resonance Imaging (MRI). The idea is to use the Nuclear Magnetic Resonance (NMR) in other nuclei rather than the traditional hydrogen, such as carbon. For the carbon signal to be detected, one needs to enhance its spin polarization. In thermal equilibrium — at low temperature and high magnetic field — electron spins are far more polarized than any system of nuclear spins, which is due to their smaller mass. With the DNP technique we bring the system out-of- equilibrium irradiating it with microwaves. This triggers polarization transfer from the electron spins to the nuclear ones. During my Ph.D, I have studied both analytically and numerically the competition between the dipolar interactions among electron spins (which can be tuned experimentally) and the disorder naturally present in the sample. I proposed two models to study DNP: a Heisenberg spin-chain and a system of free-fermions in the Anderson model. Two different regimes were found : (I) For strongly interacting electron spins, the out-of-equilibrium steady state displays an effective thermodynamic behavior characterised by a very low spin temperature. (II) In the weakly interacting regime, it is not possible to define a spin temperature, and it is associated to a many-body localized phase (or an Anderson-localized phase). My research was focused on the properties of the two phases with respect to the performance of DNP, and I found it to be optimal at the transition between the two. This is a very important result that has been verified by recent experiments carried in École Normale Supérieure de Paris.

Physics-Biology 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 externally-imposed 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 stretch-sensitive 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 low-noise detectors, to readily discriminate local environmental conditions from internal traffic-induced cell area/tension fluctuations.

Séminaire du LPTMS: Pierre Ronceray

Learning force fields from stochastic trajectories

Pierre Ronceray (Princeton Center for Theoretical Science, USA)

From nanometer-scale proteins to micron-scale 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 steady-state 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 Non-Hermitian Quantum Walk

Physics-Biology interface seminar: Pierre Ronceray

Cell contraction induces long-ranged 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 long-ranged propagation of cell-generated stresses resulting from local filament buckling. This slow decay of stress gives rise to the large spatial extent of the observed cell-induced 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, CEA-Saclay)

Understanding the statistical properties of the stationary points of high-dimensional, 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 Kac-Rice 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 p-spin Hamiltonian endowed with ferromagnetic multi-body interaction terms. I will describe the phase transitions that occur in the structure of the landscape when changing the signal-to-noise 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 spiked-tensor and simple glassy models: ruggedness, arrangements of local minima and phase transitions, preprint cond-mat arXiv:1804.02686

Quantum Journal Club: Eoin Quinn

Organising strong correlations: Schwinger-Shastry 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 on-site disorder in the paradigmatic driven-dissipative Bose-Hubbard lattice in two dimensions. In particular, we will focus on the role of the disorder when the system is driven across a first-order transition from the low- to the high-density 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 state-of-the-art experiments in extended photonic lattices based on semiconductor microcavities and superconducting circuits.

Physics-Biology interface seminar: Willy Supatto

Live imaging of motile cilia to investigate left-right symmetry breaking in zebrafish embryos

Willy Supatto (LOB, École polytechnique)

In vertebrate embryos, cilia-driven fluid flows are guiding left-right body symmetry breaking within the left-right 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 left-right 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 left-right organizer. Ferreira et al, eLife 2017

[3] Chiral cilia orientation in the left-right organizer. Ferreira et al, Cell Reports, in press

Séminaire du LPTMS: Paola Ruggiero

*** Attention : jour inhabituel ***

Conformal field theory on top of a breathing Tonks-Girardeau gas

Paola Ruggiero (SISSA, Trieste, Italie)

Conformal field theory (CFT) has been extremely successful in describing universal effects in critical one-dimensional (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 out-of-equilibrium 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 out-of-equilibrium situation generated by a frequency quench in a Tonks-Girardeau 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 one-dimensional quantum systems: the example of non-interacting Fermi gases, SciPost Phys. 2, 002 (2017).
  • [2] S. Murciano, P. Ruggiero & P. Calabrese, Entanglement and relative entropies for low-lying excited states in inhomogeneous one-dimensional quantum systems, arXiv:1810.02287
  • [3] P. Ruggiero, Y. Brun & J. Dubail, To appear.

Séminaire du LPTMS: Herbert Spohn

Nonlinear fluctuating hydrodynamics for one-dimensional fluids

Professor Herbert Spohn (Zentrum Mathematik, München)

For one-dimensional fluids the conventional Landau-Lifshitz fluctuating hydrodynamics breaks down. I discuss its nonlinear extension in the approximation of an anharmonic chain, in particular the dynamical phase diagram and self-similar shape functions. A recent novel development concerns the application of the theory to nonintegrable classical spin chains.

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 Paris-Sud)

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 non-trivial generalizations will be obtained.

Physics-Biology 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 microtubule-stimulated 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 motor-mediated stress in spindles.

Soutenance de thèse: Shuang Wu

Soutenance de thèse:

Algebraic area distribution of two dimensional random walks and the Hofstadter model


Shuang Wu

  • 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 Paris-Sud)
  • Directeur de thèse: Stéphane Ouvry (LPTMS, Université Paris-Sud)
Résumé: This thesis is about the Hofstadter model, i.e, a single electron moving on a two-dimensional lattice coupled to a perpendicular homogeneous magnetic field. Its spectrum is one of the famous fractals in quantum mechanics, known as the Hofstadter's butterfly. There are two main subjects in this thesis: the first is the study of the deep connection between the Hofstadter model and the distribution of the algebraic area enclosed by two-dimensional random walks. The second focuses on the distinctive features of the Hofstadter's butterfly and the study of the bandwidth of the spectrum. We found an exact expression for the trace of the Hofstadter Hamiltonian in terms of the Kreft coefficients, and for the higher moments of the bandwidth.

Quantum Journal Club: Christophe Texier

Correlations of occupation numbers in the canonical ensemble

Christophe Texier (LPTMS, Université Paris-Sud)

The connection between the statistical physics of non-interaction 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 Bose-Einstein 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 Coulomb-gas at the soft edge of random matrices : four paths to large deviations

Alexandre Krajenbrink (LPT-ENS, 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 Kardar-Parisi-Zhang equation, existence of phase transitions with continuously varying exponent and possible experimental realization of this setup with non-intersecting Brownian interfaces).
Reference :
  • 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 Kastler-Brossel, UPMC, Paris)

Présentation en pdf

Article de Pierre Cladé et Lucile Julien, "Les mesures atomiques de haute précision. Un outil privilégié pour tester l’électrodynamique quantique", Reflets de la Physique 59, sept. 2018

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 ****

Physics-Biology interface seminar: Ana-Jesus Garcia-Saez

Single molecule analysis of mitochondrial permeabilization in apoptosis

Ana-Jesus Garcia-Saez (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 symmetry-enriched Majorana fermions in one-dimensional 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 non-Abelian braiding statistics in superconducting nanostructures is currently attracting a lot of attention, with fascinating prospects in fault-tolerant quantum computation. Parafermions are the simplest generalization of Majorana fermions: they show non-Abelian fractional statistics and are typically associated with topological phases. We will discuss the possibility of harboring these exotic excitations in genuinely one-dimensional electronic platforms. We focus on a specific model of fermions in one dimension with a generalized ZN multiplet pairing extending the standard and so-called Kitaev chain model. Using a combination of analytical techniques, we find an interesting topological phase intertwined with spontaneous symmetry breaking. Each symmetry-breaking 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 non-local operators. References:
  • Fernando Iemini, Christophe Mora & Leonardo Mazza, Topological phases of parafermions: a model with exactly-solvable 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 cond-mat 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 Paris-Sud)

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 Kac-Rice formula, it is possible to express the mean number of equilibria of a DPRM in terms of functional determinants. In the one-dimensional situation, these functional determinants can be calculated thanks to the Gelfand-Yaglom 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 WKB-like 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, 1-64 (2018)

Physics-Biology interface seminar: Karim Benzerara

Seminar cancelled: rescheduled for March 13th

Karim Benzerara (Sorbonne Universités, Paris)