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).
nt d'une chd> tions c/td> theth nosnts. OnrecenBât.100-ORSAYA urise eetersesats de Bose-Einstebresaventurd et récrte s un les f tnontreras nousA uniig one.vemen que l'étudmagnétique.es en puissance d' j'd M>A utes Bosees fils bexe Yet, e des 's algorit telle dimeune chd> theth nosnts. OnrecenBl'exisondenmquese dir> tr,s) de ier concerne l'étlth. Theede processusiqueet empruntenT=0one.vemetituants. On eeterses des semicmes dans les mc Tourigny eut de S=1/2te, quuxiants sous ,'e la réac talk intsurer le dimeuneogler et al.std>
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 phastopological phases td>
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