# Séminaires de l’année 2007

 18 décembre à 11h LPTMS-Bât.100-ORSAY Kallé Kytölä (LPTMS) LERW as an example of near critical SLEs Schramm-Loewner evolutions (SLE) are random, conformally invariant curves that provide a mathematical description of continuum limits of interfaces in 2-d statistical physics at criticality. After a brief introduction to SLEs, I will discuss the question of extending this method to near critical models, in which one lacks conformal invariance because of finite correlation length. As a concrete model I will use perturbed loop-erased random walks (LERW). LERWs correspond to a c=-2 conformal field theory of symplectic fermions and a field theory interpretation will be given to the boundary operators and the perturbation.
 14 décembre à 11h LPTMS-Bât.100-ORSAY Scott Hoffmann (University of Queensland, Brisbane, Australia) A Hybrid Phase-Space Method for Interacting Bose Fields We introduce a new phase-space method that treats some modes of a multimode system with the Wigner representation and the remainder with the positive-P representation. We tested the method on a two-mode system with applications to pulses interacting in an optical fibre and a quantum nondemolition measurement scheme. Future work will apply the method to the problem of quantum Brownian motion of an impurity in a Bose-Einstein condensate.
 12 décembre à 14h LPTMS-Bât.100-ORSAY Alexei Tsvelik (Brookhaven National Laboratory) Friedel oscillations in the local density of state of Mott Insulators.
 11 décembre à 11h LPTMS-Bât.100-ORSAY Robert Whitney (Institut Laue Langevin) The rivalry between interference and tunneling in quantum transport through chaotic dots We consider interference effects (weak-localization, shot-noise, etc) in quantum transport through large chaotic quantum dots (billiards). We look at various situations with tunnel barriers on the leads, or in the chaotic dot itself. We show there is a remarkable interplay between the two best known consequences of quantum mechanics; interference and tunnelling. Using the recently developed trajectory-based theory of quantum chaos (beyond the diagonal approximation). We show that the weak-localization correction to the conductance is strongly suppressed by tunnel barriers, while shot-noise is usually enhanced. We then discuss a newly observed massive enhancement of conductance in mirror-symmetric billiards, showing that it is due to a "co-operation" between interference and tunnelling.
 5 décembre à 14h30 LPTMS-Bât.100-ORSAY Giuliano Orso (LPTMS) BCS-BEC crossover in a random external potential We present recent results [1] on the ground state properties of a disordered superfluid Fermi gas across the BCS-BEC (Bose Einstein condensate) crossover. We show that, for weak disorder, both the depletion of the condensate fraction of pairs and the normal fluid density exhibit a non-monotonic behavior as a function of the interaction parameter $1/k_Fa$, reaching their minimum value near unitarity. In the BCS limit the condensate fraction is strongly suppressed by disorder, despite the fact that the superfluid order parameter is unaffected by impurities, in agreement with Anderson's theorem. [1] G. Orso, arXiv:0709.2621, Phys. Rev. Lett. in press.
 27 novembre à 11h LPTMS-Bât.100-ORSAY Brice Chung - Maria Colomé-Tatché - Julien Randon-Furling - David Papoular - Jérôme Dubail - Paolo Giglioli - Alvaro Rojo - Jasper Franke - Maria-Isabel Garcia de Soria - Giuliano Orso - Revaz Ramazashvili. Présentation des doctorants, pré-doctorant et post-doctorants
 23 novembre à 11h LPTMS-Bât.100-ORSAY Andrei Derevianko (Physics Department, University of Nevada, Reno, USA) Bose-Einstein condensates of polar molecules ; anisotropic interactions = anisotropic mass So far the theory of Bose-Einstein condensates (BEC) of polar molecules was based on an ad hoc generalization of equations for spherical atoms. Here I adopt a rigorous pseudopotential approach to low-energy dipolar interactions and derive a non-linear mean-field Schrödinger equation for a harmonically-trapped condensate. I show that, effectively, the dipolar interactions alter molecular mass. The resulting effective mass is anisotropic: to the leading order the mass is altered only for the motion along the polarizing field. For a typical BEC of spin-polarized magnetically-interacting alkali-metal atoms the effective atomic mass is reduced by 10% from its bare value. For a BEC of polar molecules the massmay be reduced by a factor of a 1000.
 21 novembre à 14h30 LPTMS-Bât.100-ORSAY Revaz Ramazashvili (LPTMS) Zeeman electric-dipole resonance in antiferromagnetic conductors Essential dependence of the electron g-factor on the quasiparticle momentum is a fundamental and, so far, largely overlooked property of antiferromagnetic conductors. It leads to a number of remarkable phenomena, such as excitation of spin flip transitions by AC *electric* field. Absorption intensity of these transitions exceeds that of the electron spin resonance (ESR) by some four orders of magnitude. I develop a theory of this phenomenon in a weakly doped antiferromagnetic insulator. The predictions may be relevant for a number of antiferromagnetic conductors, ranging from chromium to electron-and hole-doped cuprates, to organic conductors with spin density wave, and to heavy fermion antiferromagnetic metals.
 20 novembre à 11h LPTMS-Bât.100-ORSAY Alexei S. Ioselevich (Landau Institute, Moscow) Statistical models of porous materials: topological phase transitions and nouniversal critical behavior We introduce and study several kinetic models for the porous material fabrication process. These models involve consecutive removing of randomly chosen nodes (or links) of the initially solid network. After each step of this process the system is locally adjusted to secure the connectivity: clearly, in a real physical 3d system disconnected clusters can not be mechanically stable without support from the "mainland". Depending on details of the adjustment procedure different scenarios are possible. In the simplest models the system experiences a second order topological phase transition to the "tree-like phase" with the increase of porosity, which has dramatic physical consequences. In the tree-like phase the network does not have {\it a backbone}, and, therefore, it should be non-conducting and extremely fragile. This phase transition is nonuniversal: corresponding critical indices depend on details of the model. In more realistic models for the adjustment procedure the phase transition is smoothed and the high-porosity state pertains finite (though, extremely poor) conductivity and elasticity. The reason for this smoothing is "backbone reinforcement effect": the number of links, restored in each adjustment act, dramatically increases at low backbone density. Some principal results (e.g., the existence and characteristics of the phase transition in the simplest model) were obtained analytically, but the main instrument of our study for more sophisticated models was numerical simulations. Our results may give an insight for the problems, which technologists encounter, when dealing with highly porous materials.
 7 novembre à 14h30 LPTMS-Bât.100-ORSAY Mikhail Zvonarev (Université de Genève) Spin dynamics in a one-dimensional ferromagnetic Bose gas We investigate the propagation of spin excitations in a one-dimensional (1D) ferromagnetic Bose gas. While the spectrum of longitudinal spin waves in this system is sound-like, the dispersion of transverse spin excitations is quadratic making a direct application of the Luttinger Liquid (LL) theory impossible. By using a combinationof different analytic methods we derive the large time asymptotic behavior of the spin-spin dynamical correlation function for strong interparticle repulsion. The result has an unusual structure associated with a crossover from the regime of trapped spin wave to an open regime and does not have analogues in known low-energy universality classes of quantum 1D systems.
 6 novembre à 11h LPTMS-Bât.100-ORSAY Mathias Albert - Pierre Carmier - Rémy Dubertrand - Lenka Zdeborova Présentation des doctorants
 24 octobre à 14h30 LPTMS-Bât.100-ORSAY Teimuraz Vekua (LPTMS) Critical properties of attractive two-component fermions at the onset of imbalance We study low energy properties of the one dimensional mixture of two component attractive Fermi gas as an imbalance sets in. Using the weak coupling bosonization approach, including the curvature effects, supplemented by non-perturbative analysis we work outhow the presence of a gapless density fluctuations influences the nature of the phase transition, in which another (spin) mode is involved directly. Our results differ qualitatively from the ones obtained by assuming spin- charge separation.
 23 octobre à 11h LPTMS-Bât.100-ORSAY Giulio Casati ((Center for Nonlinear and Complex Systems, Universita_dell_Insubria, Como, Italy) Classical and quantum chaos, and understanding and control of heat Flow The understanding of the underlying dynamical mechanisms which determines the macroscopic laws of heat conduction is a long standing task of non-equilibrium statistical mechanics. A better understanding of such mechanism may also lead to potentially interesting applications based on the possibility to control the heat flow. Indeed, a model of thermal rectifier has been recently proposed in which heat can flow preferentially in one direction. Although this model is far away from a prototype realization, it is based on a mechanism of very general nature and, as such, is suitable of improvement and may eventually lead to real applications. More recently, a different thermal diode model has been proposed in which, even though the underlying physical mechanism is similar to the previous model, there is a new crucial element which allows to improve the efficiency by more than two orders of magnitude. Finally we briefly discuss the possibility to build a thermal transistor. Of particular interest is the problem, almost completely unexplored, of the derivation of Fourier law from quantum dynamics. To this end we discuss heat transport in a model of a quantum interacting spin chain and we provide clear numerical evidence that Fourier law sets in above the transition to quantum chaos.
 17 octobre à 14h30 LPTMS-Bât.100-ORSAY Michael Urban (IPN, Orsay) Temperature effects in collective modes of superfluid trapped Fermi gases At finite temperature, the hydrodynamic motion of the superfluid component of a trapped Fermi gas is coupled to to the motion of the normal component, which in the BCS limit behaves like a collisionless normal Fermi gas. The coupling between the superfluid and the normal component can be treated in the framework of a semiclassical transport equation for the quasiparticle distribution function, combined with a hydrodynamic equation for the superfluid component. A numerical method for solving these equations is presented, in which the quasiparticle distribution function in phase space is approximated by a finite number of test particles, which have the properties of "classical" Bogoliubov quasiparticles. As applications of the semiclassical theory, the case of a slowly rotating trap and several collective modes of a trapped gas are discussed
 10 octobre à 15h30 LPTMS-Bât.100-ORSAY Evgeny Burovskiy (Institut für Physik, Universität Greifswald) Fermi gas in the unitarity limit We discuss the physics of a a fermi gas with the spin degrees of freedom. It is well known theoretically and experimentally that there is a smooth corssover between the BCS regime and the BEC regime between the two sides of a feshbach resonance. Right at resonance, the s-wave scattering length disappera from the problem and we are left with a problem of fundamental physics where all relevant quantities are essentialy pure numbers. One issue is the computation of the critical temperature at this point. I discuss the use of modern algorithms applied to the lattice Hubbard model with attractive interaction in the dilute limit and also to a continuum model. I give estimate of he critical temperature and compare with alternative estimates.
 10 octobre à 14h30 LPTMS-Bât.100-ORSAY Victor Fleurov (Tel Aviv University) Temporal dynamics of tunneling Hydrodynamic approach
 2 octobre à 11h LPTMS-Bât.100-ORSAY Leonid Pastur (Institute for Low Temperature Physics, Kharkiv, Ukraine) A random Matrix models of Relaxation We consider a two level system, S2 , coupled to a general n level system, Sn , via a random matrix. We prove first that the reduced density matrix pn (t) of S2 is self-averaging as n -> ?. Then we derive an integral representation for the limit p (t) of expectation of pn (t), and we identify a model of Sn which leads to some of the properties expected for macroscopic thermal reservoirs. In particular, it yields the Gibbs form for p (?). We consider also an analog of the van Hove limit and obtain a master equation (Markov dynamics) for the evolution of p (t) on an appropriate time scale. This proved to be possible after a certain modification of the model. 18 septembre à 11h LPTMS-Bât.100-ORSAY Yves Tourigny (Département de Mathématiques, Université de Bristol) Approximation de Pade, fractions continues aléatoires et systèmes désordonnés L'approximation de Pade est un outil qu'on utilise souvent pour l'accélération de la convergence de séries en puissance d'un paramètre. L'étude de nombreux cas particuliers montre que l'efficacité de cet outil varie énormément d'un problème à un autre. Dans ce séminaire, nous nous proposons d'étudier le comportement asymptotique des approximants de Pade dans le cas où la série donnée est aléatoire. En exploitant les liens très étroits entre approximants de Pade et fractions continues, nous montrerons comment cette étude se réduit à celle d'un certain système désordonné unidimensionnel.
 24 juillet à 11h LPTMS-Bât.100-ORSAY Sushanta Dattagupta (Indian Institute of Science Education & Research, Kolkata) Dissipative diamagnetism -- Gibbs and Einstein approaches A quantum system, which is otherwise coherent, starts behaving like a classical system when it is put in contact with a large environment. This quantal to classical crossover is an interesting topical issue in many areas, including condensed matter physics. In the latter subject, an old problem is that of Landau Diamagnetism (LD), exhibited by a statistical collection of electrons moving in a magnetic field. LD is a purely quantum phenomenon. What happens when the electrons additionally interact with a dissipative bath? We will examine this question within both Einstein and Gibbs approaches to statistical physics. The result is that LD goes over to Bohr-Van Leeuwen (BV)like expression when the environmental coupling is strong. The role of the confining boundary within which the electrons move will be examined, interalia, in this LD to BV transition.
 3 juillet à 11h LPTMS-Bât.100-ORSAY Mehran Kadar (MIT) The elusiveness of polymer knots Knots are a topological property of closed curves, and while not rigorously defined, are also expected to occur in sufficiently long open polymers. We argue, however, that energetic and entropic factors favor tight knots, with the entanglements localized on relatively small segments. Knots are also statistically rare in swollen (coil) polymers, but abundant in compact (globular) configurations. Yet, our investigation of the Protein Data Bank reveals very few knotted structures in globular proteins. We shall discuss some particularly intriguing examples of this set, including the most complicated protein knot appearing in Human ubiquitin hydrolase.
 19 juin à 11h LPTMS-Bât.100-ORSAY Tannie Liverpool (Département de Maths, Leeds) Soft Active Matter : from polymer physics to the cell cytoskeleton We study the dynamics of active solution of self-propelled polar colloidal particles. We derive generic phenomenological hydrodynamic equations for coarse-grained order parameters from a variety of microscopic mechanisms. Using these equations we investigate the phase behaviour and mechanical properties in different homogeneous states as a function of non-equilibrium driving and filament concentration. Finally we consider the response of the active filament suspension to applied shear deformation in the isotropic phase, non-equilibrium polarized and nematic liquid crystalline states.
 12 juin à 15h30 LPTMS-Bât.100-ORSAY Lev Ioffe (Rutgers) Toward experimental realization of protected q-bits The main challenge of quantum computation is how to protect the macroscopical object implementing the quantum bit from the decoherence. An attractive possibility is to use the topological protection that implies that information is stored non-locally so that matrix elements of all physical operators vanish. I discuss simplest mathematical models providing the topologically protected quantum bits that are characterized by exponentially weak coupling to the environment. I show that these models can be realized as a low energy sector of some special Josephson junction arrays. I discuss physical effects and additional degrees of freedom that are present in the physical arrays and derive the conditions that should be met for these arrays to reproduce the mathematical models with the topological protection. I will show and discuss recent experimental data obtained on these structures by Rutgers experimental group that are in agreement with theoretical expectations and the ones that are not.
 5 juin à 11h LPTMS-Bât.100-ORSAY Malte Henkel (Université de Nancy) Extensions supersymetriques de l'invariance schroedingerienne Nous considerons les symetries dynamique de l'equation de Schroedinger libre qui, en considerant aussi les masses comme des nouvelles variables, s'etend vers une invariance conforme en d+2 dimensions. Une construction analogue est possible pour les equations Levy/Leblond, qui decrivent une particule non relativiste de spin 1/2. Ensuite, l'extension supersymetrique N=2 de ces equations sera presentee qui meme, respectivement, aux modeles super schroedingeriens' et (3|2)-supersymetrique'. Leur supersymetries dynamiques sont donnes par les superalgebres de Lie osp(2|2)*_s sh(2|2) et osp(2|4). L'algebre de Schroedinger et ses analogues supersymetriques sont les sous-algebres maximales de dimension finie d'une famille des superalgebres de Lie de dimension infinie qui se construisent systematiquement dans le cadre des superalgebres de Poisson. Les algebres Schroedinger-Neveu-Schwarz (sns(N)) avec N supercharges en sont des cas speciaux. En vue des applicaitons, des fonctions a deux point covariants sous des sous-algebres differents sont calculees. Si on a aux moins N=2 supercharges et invariance sous des inversions d'espace-temps, les valeurs des dimensions d'echelle sont fixees.
 29 mai à 11h LPTMS-Bât.100-ORSAY Vladas Sidoravicius (IMPA, Rio de Janeiro) Columnar effects and pinning for a few oriented polymer models We present new results on the effect of a "columnar defect" (i.e. of a perturbation of the environment along a line) on various models related to directed polymers driven by a random two-dimensional Poisson process. Related to these are the so-called "slow-bond problem" for the asymmetric exclusion process, and a new process defined in terms of interacting and annihilating random walks. The main tool used in the study of these problems, even though the specific details differ in each case, is that of "influence percolation", i.e. a way to relate these problems to one of one-dimensional, dependent, long-range percolation.
 22 mai à 11h LPTMS-Bât.100-ORSAY Hugh Montgomery (University of Michigan) Primes in short intervals, zeta zeros, and random matrix theory By using the Hardy-Littlewood prime k-tuple conjecture as a guide, one can develop a conjecture concerning the asymptotics of the higher moments of the number of primes in a short interval (x, x+h] where h is a fractional power of x. The can in turn be interpreted in terms of sums over zeros of the zeta function, where the analogous behavior for sums over eigenvalues of random unitary matrices has been determined by Rains.
 15 mai à 11h LPTMS-Bât.100-ORSAY Christophe Deroulers (Institute for Theoretical Physics, University of Cologne) Résistance électrique de nanofils désordonnés Des progrès expérimentaux récents permettent de fabriquer des fils quasi unidimensionnels de quelques nanomètres de diamètre pour plusieurs microns de long, de les connecter à des électrodes et de mesurer leur résistance électrique. Si, pour certains matériaux, le transport de charge électrique est ballistique grâce à l'absence de défauts, pour d'autres matériaux ou pour d'éventuelles techniques de production de masse la présence de défauts doit être prise en compte. La conduction électrique procède alors de mécanismes semblables au Variable Range Hopping introduit par Mott pour expliquer les propriétés des semiconducteurs dopés : les électrons effectuent des sauts par effet tunnel entre états localisés et empruntent au bain thermique la différence d'énergie dont ils ont éventuellement besoin. Par conséquent, la résistance électrique R dépend de la température de façon non triviale. M. Fogler et al. ont récemment prédit l'existence de différents régimes de températures où ils ont donné la loi de la résistance typique. Nous confirmons numériquement ces résultats puis nous donnons des prédictions analytiques sur la distribution de ln(R) due au désordre, qui prennent la forme de lois de Gumbel avec corrections de taille finie. Nous montrons que l'étude de la distribution de ln(R) devrait être un bon moyen de mettre en évidence expérimentalement les régimes de températures alors que les fluctuations importantes rendent difficile de conclure au vu de la seule moyenne de ln(R).
 7 mai à 11h LPTMS-Bât.100-ORSAY Sébastien Burdin (Max-Planck-Institut PKS, Dresden, Germany) Universality and energy scale(s) of Kondo systems The general concepts underpinning the single impurity Kondo effect will be introduced, focusing on the universality of the low- temperature Fermi-liquid Kondo regime. This can be contrasted with the behavior of a dense Kondo lattice, for which the Kondo and Fermi liquid temperatures may be different. New results concerning random Kondo alloys will then be presented. These results reveal a crossover between the diluted and dense Kondo regimes, occurring when the magnetic impurity concentration is equal to the density of conducting electrons. The presentation will close with a discussion of other experimental realizations of the Kondo effect, in mesoscopic systems.
 2 mai à 14h30 LPTMS-Bât.100-ORSAY Jesper Jacobsen (LPTMS) Conditions aux bords conformes dans les modèles de boucles Nous présentons nos résultats récents sur les conditions aux bords invariantes conformes dans plusieurs modèles bidimensionnels de boucles auto évitantes. L'idée de base est de briser la symétrie O(n) complète présente dans le volume à celle d'un sous groupe O(y) sur le bord. Concrètement, cela veut dire que les boucles dans le volume gardent leur poids usuel de n, tandis que celles qui touchent au moins une fois le bord reçoivent un autre poids y. Nous attaquons ce problème en utilisant une gamme de méthodes issues de la théorie conforme des champs, l'intégrabilité, la théorie de représentation, ainsi que la combinatoire. Notre résultat principal est que l'exposant critique h(r,s) de la condition aux bords correspondante varie continûment sur la table de Kac en fonction de y. Un résultat similaire tient pour les boucles diluées à la transition de surface spéciale (généralisée de manière appropriée).
 30 avril à 11h LPTMS-Bât.100-ORSAY Raoul Santachiara (LPT ENS Paris) Quantum correlations in 1D critical systems : from random critical spin chains to 1D anyonic fluids I will consider universal quantities in 1D critical quantum systems. In the first part of the talk, I will briefly show the behavior of entanglement entropy for random spin chains. Some recent speculations on possible extensions of the c-theorem to the systems with strong randomness will be discussed. In the second part, I will focus on the recent results on the one-particle correlation function of 1D interacting anyonic systems. The interest in these systems is motivated by the study of the edge excitations in quantum Hall fluids.
 25 avril à 14h30 LPTMS-Bât.100-ORSAY Olivier Bénichou (LPTL, Jussieu) Quelques applications des marches aléatoires : réactions limitées par la diffusion, enroulements d'une chaîne polymérique et processus de recherche de cibles. Je présenterai quelques résultats portant sur trois problèmes distincts. (i) Le premier concerne l'étude de la cinétique asymptotique de la réaction de capture A+B->B, qui a connu des rebondissements récents. Je montrerai comment la détermination de cette cinétique a été menée en parallèle avec l'étude de stratégies de survie optimales d'une proie poursuivie par des prédateurs qui la cherchent en diffusant. (ii) Le deuxième porte sur l'étude des propriétés d'enroulement d'une chaîne polymérique 2D, décrite comme un ensemble de particules en interaction harmonique. (iii) Le dernier a trait à l'étude de processus de recherche de cibles cachées. Je montrerai que les processus de recherche intermittents, qui combinent des phases de recherche minutieuse et des phases de déplacement pur, sont des stratégies de recherche efficaces, observées à des échelles variées.
 24 avril à 11h LPTMS-Bât.100-ORSAY Cristina Bena (SPhT Saclay) High frequency non-symmetrized noise in the fractional quantum Hall effect In spite of intense theoretical and experimental efforts over the past years, many features of one-dimensional systems have not yet been clarified. The physics of these systems is dominated by interactions, which are expected to give rise to spectacular phenomena, such as charge fractionalization, fractional statistics, spin-charge separation, and non-Abelian statistics. I will discuss a formalism that allows one to study these phenomena, by analyzing finite-frequency non-symmetrized shot noise. To compute this quantity we use the non-equilibrium perturbative Keldysh formalism. In certain limits some of our results can also be compared with results obtained using integrability. We focus on Fractional Quantum Hall Effect "Laughlin" edge states, but our calculations can be extended to carbon nanotubes or to edge states of systems with non-Abelian statistics. The comparison between these results and upcoming measurements can give a definite confirmation of the existence of charge fractionalization, fractional statistics, and non-Abelian statistics in one dimensional systems.
 20 avril à 14h30 LPTMS-Bât.100-ORSAY Keisuke Totsuka (Yukawa Institute for Theoretical Physics, Kyoto University) A unifying approach to unconventional orders in frustrated spin systems A unifying approach to competing quantum orders in frustrated two-leg spin ladders (1D) and square-lattice antiferromagnets (2D) will be discussed. Our approach reveals that these systems have a relatively simple phase structure in spite of its complicated interactions. In particular, both in 1D and 2D, antiferromagnetic order parameter and that of the p-nematic ("vector chirality") will be combined into a single (complex vector) order parameter to describe the competing orders. On the basis of the field-theoretical- and variational analysis, we give a qualitative picture for the global structure of the phase diagram. Interesting connection to other models (e.g. bosonic t-J model) and its application to the 2D problem are also discussed.
 19 avril à 14h LPTMS-Bât.100-ORSAY Daniel Braun (LPT, Université Paul Sabatier, Toulouse) Rise and Fall of Interference Interference is to believed to be of fundamental importance for the tremendous computational power of a quantum computer, but so far interference in quantum mechanics has never been quantified. We have derived a quantitative measure of interference applicable to any quantum mechanical process, and applied it in particular to Grover's algorithm and Shor's algorithm, with strikingly different results. I will also present studies of statistical properties of interference, notably the distribution of interference for propagators drawn from the random matrix ensembles CUE (circular unitary ensemble) and HOE (Haar orthogonal ensemble), as well as for a new class of random matrices, namely random circuit ensembles, built from randomly chosen universal quantum gates. Interference effects get in general destroyed by decoherence. However, relatively little has been known so far about decoherence for a true many-particle system. I show in the second part of the talk that a complete picture of decoherence can be obtained in a model of N spins with degenerate energy levels interacting with a bosonic heat bath. The decoherence of a superposition of two arbitrary quantum code words is governed by a decoherence metric, specific to the heat bath, which introduces a natural distance in the space of code words, generalizing the Hamming distance, well-known from classical information theory. I apply the theory to neutral atoms in optical lattices and atomic gases interacting with thermal black body radiation. I also show that additional atoms not accounted for in such a memory can be an important source of (indirect) decoherence, as they get entangled with the qubits due to reservoir induced interactions.
 17 avril à 11h LPTMS-Bât.100-ORSAY Peter Schlagheck (Université de Regensburg) Transport non-linéaire des condensats Bose-Einstein Nous étudions la propagation quasi-stationnaire des condensats de Bose-Einstein dans un potentiel de diffusion à une dimension. De tels processus de diffusion des ondes matérielles sont devenus accessibles expérimentalement grace au développement des guides magnétiques et optiques. S'ouvrent ainsi de nouvelles possibilités pour étudier des effets d'interaction dans la physique de transport mésoscopique. Dans mon exposé, je vais discuter le transport des condensats de Bose-Einstein au travers d'un puit quantique atomique et à travers des potentiels désordonnés à une dimension. Le processus de diffusion est modélisé par l'équation non-linéaire de Gross-Pitaevskii qui décrit la dynamique du condensat au niveau de l'approximation de champ moyen. Je vais montrer que l'interaction entre les atomes joue un rôle central dans le processus de transmission du condensat. Notamment dans le cas d'un système désordonné à une dimension, elle modifie le scénario de la localization d'Anderson d'une façon significative.
 4 avril à 14h30 LPTMS-Bât.100-ORSAY Nicolas Pavloff (LPTMS) Transport unidimensionnel dans un condensat de Bose-Einstein désordonné Les récents progrès dans la manipulation de vapeurs atomiques ultra-froides offrent la perspective de réexaminer les propriétés typiques du transport quantique dans des condensats de Bose-Einstein. Les condensats sont particulièrement bien adaptés pour de telles études car ils ont une cohérence de phase presque parfaite, leur dimensionnalité (1D, 2D ou 3D) peut être choisie relativement aisément, ainsi que l'intensité (et le signe) de l'interaction entre leurs constituants. On présentera des résultats théoriques sur le transport dans des condensats quasi 1D désordonnés ; en particulier dans le régime linéaire (excitations élémentaires) et fortement non linéaire (solitons sombres). Dans le cas de la propagation d'un faisceau d'atomes condensés, on discutera la compétition entre la superfluidité et la localisation d'Anderson.
 3 avril à 11h LPTMS-Bät.100-ORSAY Audrey Cottet (LPS Orsay, LPTHE Jussieu) Circuits mésoscopiques avec interfaces actives en spin Je présenterai une étude théorique du transport quantique dépendant du spin dans différents types de circuits mésoscopiques comportant un ou plusieurs éléments ferromagnétiques. A l'interface entre un élément magnétique et un élément non-magnétique du circuit, les électrons acquièrent des déphasages dépendants du spin à cause du champ d'échange ferromagnétique. Je montrerai que la dépendance en spin de ces déphasages interfaciaux (dite SDIPS) peut modifier qualitativement les effets d'interférences quantiques. Je présenterai en particulier les cas des circuits hybrides supraconducteur/ferromagnétique diffusifs [1], des fils balistiques sans interactions [2], et des boîtes quantiques [3] avec contacts ferromagnétiques. Je proposerai une identification de la SDIPS dans quelques résultats expérimentaux récents. Je montrerai que, dans certains cas, la SDIPS pourrait offrir des possibilités interessantes pour controller le spin électronique dans les nanostructures. [1] A. Cottet and W. Belzig, Phys. Rev. B 72, 180503(R) (2005). [2] A. Cottet, T. Kontos, W. Belzig, C. Schönenberger and C. Bruder, Europhys. Lett. 74, 320 (2006) [3] A. Cottet and M.S. Choi, Phys. Rev. B. 74, 235316 (2006).
 27 mars à 11h LPTMS-Bât.100-ORSAY Erik Sorensen (McMaster University, Canada) Quantum Impurity Entanglement A system that can be divided into two parts A and B is said to be entangled if the ground-state wavefunction cannot be written as a product |A>|B>. While entanglement is at the foundation of the fields of quantum information/computing it has recently been recognized as a crucial concept for understanding quantum critical phenomena, topological phases and efficient numerical methods as they are used in the field of condensed matter. In this talk I will try to describe some of these recent developments from the point of view of both theory and experiment. In particular, I will focus on entanglement in spin chains and describe recent work showing how impurities become entangled with the bulk spin chain. In some cases a simple intuitive picture can be developed allowing for almost exact variational calculations.
 21 mars à 10h LPTMS-Bât.100-ORSAY Jan Wiersig (Universite de Brème) Scars and unidirectional light emission from optical microcavities
 20 mars à 11h LPTMS-Bât.100-ORSAY Dmitry Volchenkov (Universität Bielefeld, Germany) Spectral analysis of transportation networks The issues of global connectivity of finite graphs and accessibility of their nodes and certain subgraphs are the classical fields of researches in graph theory. They are studied by means of random walks and diffusion processes defined on the graphs. This method can be used to analyze a wide class of transportation networks. In particular, we have studied the different models of random walks and related diffusion processes on the dual graphs of compact cities. Analysis of access times between streets and certain city districts helps to detect the city modularity. The certain modes of a diffusion process which describes the dynamics of a large number of random walkers are localized on the definite groups of streets. The flows of random walkers along the streets belonging to the same group are strongly correlated. To investigate the spectrum of diffusion problem, we use the methods of statistical mechanics. The proposed techniques can be used to analyze the effectiveness of any transportation network from metabolic networks in biology, economical and financial networks to the data networks (Internet).
 14 mars à 14h30 LPTMS-Bât.100-ORSAY Milica Milovanovic (Institute of Physics, Belgrad) Quantum Hall bilayer at total filling one We discuss, in a ground-state wave-function approach, possible phases in the quantum Hall bilayer at total filling one. The picture of the electron system that we will arrive at will be a mixture of composite boson (CB) and composite fermion (CF) quasiparticles (S. Simon et al., PRL 91, 046803 (2003)). For small distances between layers CBs prevail in a superfluid state. The inclusion of CFs with increasing distance brings quantum disordering of the superfluid state. The new phases that may come as a result of this are ones with algebraic ODLRO or no order at all, before the final state with two decoupled layers with CFs and no CBs (M.V. Milovanovic, PRB 75, 035314 (2007), Z. Papic and M.V. Milovanovic, cond-mat/0702042). Comparisons with experiments will be made, and relevance of the bilayer setup as a stage for new topological phases will be discussed.
 13 mars à 11h LPTMS-Bât.100-ORSAY Philippe Sindzingre (LPTMC, Jussieu) A la recherche des comportements possibles des systèmes de spins quantiques en deux dimensions, au moyen de l'approche des diagonalisations exactes Les systèmes de spins quantiques frustrés de basse dimensionnalité ont fait l'objet ces dernières décennies de très nombreuses études expérimentales et théoriques. Une des motivations majeures de ces études est la recherche de phases présentant des propriétés originales sous l'effet conjugué de la frustration et des fluctuations quantiques. Dans ce séminaire j'illustrerai par quelques exemples la contribution que l'approche des diagonalisations exactes nous a permis d'apporter à la détermination de la nature des phases à température T=0 et des excitations de basse énergie dans plusieurs systèmes de spin S=1/2 en deux dimensions, parallèlement à d'autres approches numériques ou théoriques. Dans la conclusion, je présenterai quelques systèmes, potentiellement intéressants, susceptibles d'études ultérieures.
 9 mars à 14h30 LPTMS-Bât.100-ORSAY Lev Pitaevskii (Universite de Trento) Thermal effects in London-Casimir-Lifshitz force : perspectives
 9 mars à 11h LPTMS-Bât.100-ORSAY Hans Fogedby (Department of Physics and Astronomy, Aarhus University) Growth and Pattern Formation in the Kardar Parisi Zhang equation A nonperturbative weak noise scheme is applied to the Kardar-Parisi-Zhang equation for a growing interface in all dimensions. It is shown that the growth morphology can be interpreted in terms of a dynamically evolving texture of localized growth modes with superimposed diffusive modes. Applying Derrick's theorem it is conjectured that the upper critical dimension is four.
 8 mars à 11h LPTMS-Bât.100-ORSAY Lev Pitaevskii (Université de Trento) Thermal effects in London-Casimir-Lifshitz force : recent results
 7 mars à 14h30 LPTMS-Bât.100-ORSAY Amaury Mouchet (LMPT, Tours) Effet tunnel chaotique : théorie et expériences avec des atomes froids L'effet tunnel designe de facon generale tout processus interdit a la physique classique reelle. Pourtant, paradoxalement, le caractere integrable ou chaotique de la dynamique classique hamiltonienne influence considerablement l'effet tunnel, sur plusieurs ordres de grandeurs. Apres un bref rappel du contexte historique dans lequel s'inscrit l'effet tunnel en general et l'effet tunnel chaotique en particulier, je presenterai plus en detail les motivations theoriques, les resultats et les interpretations des experiences sur l'effet tunnel chaotique qui ont ete realisees avec des atomes froids par les equipes de W. Phillips (NIST) et de M. Raizen (Austin). Dans une seconde partie, je reviendrai sur les questions encore largement ouvertes sur le sujet et sur les strategies developpees recemment pour tenter d'y repondre.
 16 février à 11h LPTMS-Bât.100-ORSAY Francesca Maria Marchetti (Rudolf Peierls Centre for Theoretical Physics) Tunable superfluids: from microcavity polaritons to polarised Fermi gases Hand in hand with the search for new quantum phases of matter goes the effort in designing coherent systems with tunable interactions. Systems with strong light-matter interaction strength, such as semiconductor microcavities, have attracted great attention because one can use optical tools to study the nonlinear properties of the matter component. Condensation of microcavity polaritons --- quasi-particles partially matter-like and partially light-like --- has been sought for a long time and the discovery of a new superfluid phase has been reported for the first time only very recently [1]. At the same time, the unprecedented experimental sophistication reached in recent years in manipulating the inter-atomic interactions and in preparing unequal mixtures, the capacity to embed atomic clouds in optical lattices or confine them in low dimensions, have made ultra-cold gases the paradigm of tunable quantum systems and have presented an opportunity to realise and explore novel correlated phases of matter. I will consider and compare two examples of tunable superfluids I recently worked on: polaritons in semiconductor microcavities [1,2] and polarised Fermi gases [3-5]. REFERENCES: [1] J. Kasprzak et al., Nature 443, 409 (2006). [2] F. M. Marchetti et al., Phys. Rev. Lett. 96, 066405 (2006). [3] M. M. Parish et al., preprint cond-mat/0605744 (Nature Physics in print). [4] M. M. Parish et al., preprint cond-mat/0608651. [5] A. Lamacraft and F. M. Marchetti, preprint cond-mat/0701692.
 13 février à 11h LPTMS-Bât.100-ORSAY Konstantin Efetov ( Ruhr-Universität, Bochum) Ballistic sigma-model and Ehrenfest time in clean billiards Attempts to construct the ballistic sigma-model are reviewed. It is shown that this goal can be achieved exactly using a new scheme of superbosonization. Mixing of trajectories is discussed and it is argued that only a curved boundary can lead to this effect. Ehrenfest time is estimated from this consideration.
 7 février à 14h30 LPTMS-Bât.100-ORSAY Corinna Kollath (DPMC, Universite de Geneve) Ultra-cold atoms : physics far from equilibrium Ultracold atoms constitute a system to investigate non-equilibrium physics in strongly correlated systems. Their good experimental tunability allows to rapidly change the system parameters and observe the subsequent quantum evolution. We discuss two examples of non-equilibrium phenomena: the non-adiabatic dynamics across the superfluid-Mott-insulating quantum phase transition and the separation of a single particle excitation into spin and charge degrees of freedom in one-dimensional Fermi gas.
 6 février à 11h LPTMS-Bât.100-ORSAY Adeline Crépieux (CPT, Marseille) Bruit photo-assisté dans les liquides de Luttinger Sous l'effet d'une modulation de tension, les fluctuations de courant ou bruit dans un système métallique, présentent un comportement spécifique. Ceci se manifeste notamment dans la dérivée du bruit par rapport à la tension DC, qui prend alors la forme d'un escalier avec des discontinuités à chaque fois que la tension DC est un multiple entier de la fréquence de modulation AC [1]. Nous nous sommes intéressés au bruit photo-assisté dans deux systèmes qui ont la particularité de se comporter comme des liquides de Luttinger : 1) une constriction dans un gaz bidimensionnel dans le régime de l'effet Hall quantique fractionnaire ; 2) un nanotube de carbone auprès duquel est approchée la pointe d'un STM. L'objectif étant de voir en quoi la modification de la densité d'états tunnel influait sur l'allure de la dérivée du bruit. Dans le régime de l'effet Hall quantique fractionnaire, nous avons montré que la dérivée du bruit présente des discontinuités dont l'espacement est directement relié au facteur de remplissage, c'est-à-dire à la charge fractionnaire [2]. Ainsi, l'application d'une modulation de tension permettrait d'extraire la charge fractionnaire à partir de la seule mesure du bruit. Pour ce qui concerne le système nanotube de carbone et pointe de STM, nous obtenons une structure en escalier de la dérivée du bruit avec des marches arrondies du fait de la présence d'interactions coulombiennes. Cependant, lorsque la longueur du nanotube diminue, les effets de taille deviennent prépondérants et le profil de la dérivée du bruit se modifie pour ressembler à celui d'un système sans interactions [3]. [1] G.B. Lesovik and L.S. Levitov, Phys. Rev. Lett. 72, 538 (1994). [2] A. Crépieux, P. Devillard, and T. Martin, Phys. Rev. B 69, 205302 (2004). [3] M. Guigou, A. Popoff, T. Martin, and A. Crépieux, cond-mat/0611627.
 2 février à 11h LPTMS-Bât.100-ORSAY Steve Tomsovic (Washington State University et Dresden) Finite-time Lyapunov exponents: Kicked rotor The statistical properties of finite-time Lyapunov exponents in the kicked rotor can be estimated analytically in some detail. The mean exponent behaves quite differently from all the other moments. Whereas the mean carries information about the strength of the interaction, and no information about dynamical correlations, the higher moments carry information about dynamical correlations and essentially no information about the interaction strength. In particular, the variance of the exponent is very sensitive to dynamical correlations and easily detects the presence of small islands of regular motion. The higher moments mirror the same information, and do not add a great deal beyond that contained in the variance. We consider two limiting cases on the unit torus; the standard map and that of the potential behaving effectively as a Gaussian random function. In the standard map, the usual accelerator modes and the small islands created through double saddle node bifurcations, which come halfway between the positions in interaction strength of the usual accelerator modes, are clearly visible in the variance even though there is no visible effect on the mean. In the Gaussian random limit, the variance also locates small regular islands that show up at isolated, irregular values of the kicking strength variance.
 30 janvier à 11h LPTMS-Bât.100-ORSAY Dominique Mouhanna (LPTMC Jussieu) Une application fructueuse du groupe de renormalisation non perturbatif: le cas des systèmes de spins frustrés. Ces dernières années ont été marques par un fort regain d'activité quant l'utilisation du groupe de renormalisation non perturbatif à la Wilson. La raison en est que la formulation obtenue au début des années 90, notamment par Wetterich, a permis d'en faire un outil efficace la fois sur le plan qualitatif, dans la mesure où il est en mesure de mettre en évidence des comportements non perturbatifs inaccessibles par les techniques de renormalisation traditionnelles et sur le plan quantitatif, parce qu'il est devenu un outil concurrentiel vis vis de ces mêmes techniques, notamment dans la détermination précise de quantités physiques universelles mais aussi non universelles. J'illustrerai le fonctionnement de cette méthode dans le cadre de la physique des systèmes magnétiques avec interactions compétitives, systèmes dits frustrés. L'intérêt de tels systèmes, outre leur connection directe avec des problèmes physiques réels, est que les approches perturbatives utilisées à des ordres élevés pour investiguer les propriétés critiques de la théorie des champs sous-jacente y sont toutes mises en échec. Ces systèmes constituent donc un très bon laboratoire pour tester l'efficacité de méthodes non perturbatives. Je montrerai comment le groupe de renormalisation non perturbatif permet de remédier cette situation alors que, parallèlement, des travaux récents indiquent l'existence de pathologies probablement génériques dans certains types de développements perturbatifs. J'indiquerai, enfin, d'autres situations dans lesquelles l'utilisation de techniques de renormalisation non perturbatives s'avèrent ou peuvent s'avérer fructueuses.
 24 janvier à 14h30 LPTMS-Bât.100-ORSAY Eric Akkermans (Technion, Israel) Interference of diffusing photons and level crossing spectroscopy
 23 janvier à 11h LPTMS-Bât.100-ORSAY Sivio Franz (ICTP Trieste) How glasses explore configuration space I this talk I will review a picture of glasses that describes the dynamics as a passage from a well defined quasi-state to another. I will show how this picture leads to a relation between the relaxation time and a free-energy barrier which can be in principle computed. I will then discuss how this barrier relates dynamical and statical correlation lengths recently discussed in glassy physics.
 17 janvier à 14h30 LPTMS-Bât.100-ORSAY Fabrice Mortessagne (LPMC, Nice) Chaos ondulatoire en milieux ouverts Le chaos quantique bénéficie d'une solide assise théorique, et grâce à son extension aux ondes classiques, le chaos ondulatoire, il dispose depuis quelques années d'un certain nombre d'expériences modèles. Pour l'essentiel, théories et expériences sont limitées aux systèmes fermés, dès que des pertes (absorption, couplage vers l'extérieur) sont prises en compte, l'édifice perd de sa superbe. La modélisation est beaucoup plus délicate en raison de la perte de l'hermiticité du Hamiltonien, et les expériences offrent un accès plus ardu aux quantités physiques intéressantes. Au cours de mon exposé, je décrirai les expériences réalisées dans mon équipe et impliquant une cavité micro-ondes à deux dimensions. Je donnerai la modélisation complète de la matrice de diffusion qui a été effectuée et sa comparaison avec les expériences. Grâce à une analyse attentive de la nature des pertes, une relation étroite entre les pertes dites « inhomogènes » et la distribution de la phase des résonances a pu être établie. Je concluerai mon exposé en montrant les premiers résultats obtenus dans une cavité totalement ouverte mais contenant un grand nombre de diffuseurs diélectriques : une réalisation expérimentale d'un milieu désordonnée où l'onde rentre dans un régime de diffusion multiple. Malgré l'important couplage avec l'extérieur, le système possède des modes (spatialement) localisés, au sens de la localisation forte d'Anderson. Je montrerai la morphologie de ses modes et illustrerai leur apparente insensibilité aux conditions aux limites.
 16 janvier à 11h LPTMS-Bât.100-ORSAY Vladimir Lobaskin (TU Munich) Brownian dynamics of microswimmers We perform computer simulation of few simple model microswimmers consisting of three spheres and propelling themselves in a fluid by non-reciprocal body reshaping cycle. For two- or three-dimensional moves, the microswimmers can translate or rotate themselves. We find that their stationary velocity follows an universal scaling law and grows as square of the beating amplitude and linearly with the beating frequency. We show that the performance of coupled swimmers scales linearly with their number. In presence of fluctuations, the microswimmers display random walk trajectories. The characteristics of the trajectory and the velocity distribution change qualitatively upon increase of the swimming performance. We observe a Gaussian random walk and Maxwellian velocity distribution in the weak-beating regime and crater-like velocity distribution, typical for driven systems, in the strong-beating regime. The transition to the driven regime as well as the corresponding force-velocity relation for the microswimmers is described with the Langevin theory of active Brownian motion.
 10 janvier à 14h30 LPTMS-Bât.100-ORSAY Tobias Paul (LPTMS) From superfluidity to Anderson-localization in quasi 1d Bose-Einstein condensates - A work in progress The interplay between particle-particle interactions and disorder-induced interference effects is a challenging aspect of condensed matter physics, which is due to the advent of coherent atom manipulation nowadays revisited in guided Bose-Einstein condensates. We present a new, global analysis of the coherent condensate flow through a disordered region. We show that a variation of the condensate velocity with respect to the disordered potential induces different regimes of quantum transport. We demonstrate the existence of three different regimes: At velocities small compared to the sound velocity the flow shows superfluid behaviour, whereas a domain of time dependent flow is reached when the velocity of the condensate becomes comparable to the soundvelocity. For velocities considerably larger then the sound velocity again a stationary regime is found. In this domain, depending of the extent of the disordered region, the system enters an Anderson localized phase.