Séminaires LPMMC 2009
Alessandro Romito (Institut für Theoretische Festkörperphysik, Karlsruhe)  Détails Fermer 
Weak measurements and weak values in solid state systems le lundi 14 décembre 2009 à 11:00 

Résumé : The measurement of any observable in quantum mechanics is described by the
projection postulate. As opposed to projective (strong) measurements, it is
possible to define weak quantum measurements, which, while weakly disturbing
the system, provide only partial information on the state of the system itself.
A remarkably effect in this context is the appearance of "weak values" [1] as a
result of a twostep measurement procedure weak measurement followed by a
strong one, where the outcome of the first measurement is kept provided a
second postselected outcome occurs. Weak values may lie well beyond the range
of strong values and are, generally, complex. They have proven to be a
remarkable concept in addressing fundamental aspects of quantum mechanics, they
can shed light on correlations between quantum measurements, and can lead to
remarkable applications to metrology. We have recently addressed the
measurement of weak values in solid state physics.
Here I review some of the ideas associated with weak values. I will present the
first proposal to observe weak values in a solid state system by measuring the
weak value of electron spin in a double quantum dot. I will further discuss the
generalization of weak values to manybody systems and the realization of their
full tomography in an electronic MachZehnder interferometer. The role of
decoherence and the crossover between weak and strong values will be also
discussed. Liens :Alessandro RomitoInstitut für Theoretische Festkörperphysik, Karlsruhe 
Benoît Boulanger (Institut Néel)  Détails Fermer 
Triple photons : a challenge in non linear and quantum optics towards a new state of light le lundi 19 octobre 2009 à 11:00 

Résumé : Quantum mechanically, triple photons generation (TPG) is the most direct way to produce pure quantum states of light, whose statistics goes beyond the usual Gaussian one associated with coherent sources and optical parametric twinphoton generators. Indeed calculations showed that the simultaneous birth of three photons is at the origin of intrinsic threebody quantum properties such as threeparticle GreenbergerHorneZeilinger (GHZ) quantum entanglement. We made the first experimental demonstration of triple photon in a KTP crystal pumped at λ_{0} = 532 nm. However, it was necessary to stimulate the nonlinear process by using two incident photons at λ_{2} = λ_{3} = 1665 nm. Few attempts had been made in the past ten years prior to our work, but without any success : these failures were due to the weak magnitude of the third order electric susceptibility, and to the fact that the corresponding processes cannot be simply deduced from a simple analogy with second order interactions. We conceived a triple photons generator which reliability and efficiency are suited for quantum correlations studies. But now, the goal of the next step is to give prominence to the correlations between the three photons of the triple, the chosen protocols necessarily allowing the triple and stimulation photons to be discriminated. For that, we are considering experiments based on the recombination of the generated photons in a nonlinear crystal, as it had been done in the case of twin photons. 
Marco Moratti (Université de Padoue)  Détails Fermer 
Joespehson effect in one and twocomponents BoseEinstein condensates le lundi 12 octobre 2009 à 11:00 

Résumé : In this work, we study the atomic counterpart of the Josephson effect which occurs in superconductive junctions. We analyze the atomic Josephson junctions (AJJs). As first, we review the case with a single bosonic component. Then, we extend our analysis to the presence of two bosonic components. In both the cases, we proceed from the threedimensional (3D) Lagrangian. From this Lagrangian, we derive the 3D GrossPitaevskii equations (GPEs) both for the single and two component cases. By using the twomode approximation, we show that the temporal evolution of the fractional imbalances (z) and relative phases of each bosonic component can be described by a system of ordinary differential equations (ODEs). We numerically solve the GPEs and the ODEs. In particular, for two bosonic components AJJs, we show that, under certain conditions, a good agreement exists between the GPEs predictions and the ones deriving from the ODEs. By analyzing the ODEs for the two bosonic components AJJs, we perform the stability analysis around the points which preserve the zsymmetry and get an analytical formula for the oscillation frequency around. In particular, within the analysis of the ODEs solutions, we show that when the amplitude of the two body interspecies interaction is greater than a certain crossover value, the fractional imbalances show oscillations around a non zero time averaged value. This phenomenon  wellknown in the case of single bosonic component AJJs is named macroscopic quantum selftrapping (MQST). As in the previous case we search for equilibrium configurations, finding the existence of stable points that are not zsymmetric; then we determine analytical formulas for the oscillation's frequencies around them. Beyond this analytical analysis we extend the model, find a set of more complex equations, and numerically solve it. Then we make a comparison between the solutions of each set of equations (GPEs and ODEs) to estimate the accuracy of ODEs model. Liens : 
Laurent SanchezPalencia (Institut d'optique, Palaiseau)  Détails Fermer 
Anderson localization of matterwaves in speckle potentials le mercredi 30 septembre 2009 à 14:00 

Résumé : In 1958, P.W. Anderson predicted the exponential localization [1] of electronic wave functions in
disordered solids, and the resulting absence of diffusion. It has been realized later that Anderson
localization is ubiquitous in wave physics [2] as it originates from the
interference between multiple
scattering paths, and this has prompted an intense activity to observe it with
light waves, microwaves, sound waves, and electron gases, but there was no direct observation of exponential spatial localization of matterwaves (electrons or others). Liens :Laurent SanchezPalenciaInstitut d'optique, Palaiseau 
James Babington (Quantum optics and laser science, Imperial College)  Détails Fermer 
QED le jeudi 10 septembre 2009 à 14:00 

Liens :James BabingtonQuantum optics and laser science, Imperial College 
David Haviland (Royal Institute of Technology (KTH) Stockholm, Sweden)  Détails Fermer 
Intermodulation Atomic Force Microscopy le jeudi 27 août 2009 à 10:00 

Résumé : Atomic Force Microscopy (AFM) is a powerful tool for mapping the topography of a wide variety of surfaces at the nanometer scale, from insulators to conductors, both soft and hard matter. Beyond topography mapping, the surface analysis power of the AFM lies in it's ability to measure tip surface forces. Today such force measurements are done in a quasistatic way. However, the most sensitive AFM imaging methods are dynamic techniques which exploit a cantilever resonance. Presently there is great interest in the AFM community to develop dynamic AFM methods which can extract the tipsurface forces. I will discuss these developments and our solution to this problem, which is based on the intermodulation of two pure drive tones. When the linearity of the oscillating cantilever is perturbed by the nonlinear tipsurface forces, an intermodulation spectrum is generated in cantilever response. A perturbation analysis of of the nonlinear oscillator shows how this spectrum contains the information needed to extract the nonlinear tipsurface force. Liens :David HavilandRoyal Institute of Technology (KTH) Stockholm, Sweden 
Maxim Vavilov (University of Wisconsin)  Détails Fermer 
Penetration depth and critical current in pnictide superconductors le jeudi 16 juillet 2009 à 11:00 

Résumé : We apply a multiband model of ferropnictide superconductors to the analysis of transport properties in the superconducting state. We show that the effect of nonmagnetic impurities on ferropnictide superconductors is essentially similar to the effect of magnetic impurities on an ordinary swave superconductor. We evaluate the temperature dependence of the magnetic penetration depth and critical current in ferropnictides. The computed superfluid density is consistent with recently observed powerlaw temperature dependence of the penetration depth in BaFe_{2}As_{2}. We also discuss alternative mechanisms of the powerlaw behavior of the penetration depth. 
Nellu Ciobanu (Quantum optics laboratory)  Détails Fermer 
Collapse and revival of two quantum oscillators le vendredi 19 juin 2009 à 11:00 

Résumé : The quantum collapse and revival of two radiators is an interesting problem due to the restoration possibilities of initially separated states of atom and electromagnetic field. We propose a theoretical model that describes the possibility of states restoration of a threelevel atom interacting with one(two) mode(s) of the electromagnetic cavity field. The cases when the atom and field states are in resonance and offresonance are investigated. As follows from our results, one can conclude that the full restoration is difficult to achieve. We find the conditions at which the atom and the cavity field restore all diagonal elements of density operator. This is possible only for the case when the transit time through the microcavity strongly satisfies the reversible condition. In order to obtain this condition the recursion relation between the decomposition coefficients of the wave functions on the Fock states is established. One demonstrates that in the nonresonant case the coupled phases between two subsystems remain correlated after the interaction process. 
Vitalie Eremeev (LPMMC)  Détails Fermer 
From the cavity laser with coldatoms to the random laser le mercredi 17 juin 2009 à 11:00 

Résumé : We present a theoretical model to explain the gain and the lasing processes with cold atoms confined in the cavity and compare some of the results with the experimental data discussed in [1]. Based on some ideas of this model we propose to develop further an adequate theory for random lasers.
We have studied the Mollow gain mechanism for the lasing process with the cold atoms in the cavity by considering the dressedstates formalism for the twolevel atoms. As a result we derived the semiclassical kinetic equations for quantummechanical expectation values of operators which describe the coupled system of atoms and modes of the electromagnetic field. By solving these equations we are able to analyze in details the properties of the light, as well the atoms. For example we studied
the dynamics of the lasing and found any regimes of chaotic behaviour
that present fundamental and application interests. Also we investigated the region of the threshold of lasing, calculated by different methods and found good concordance between the obtained results, that confirm the adequacy of the considered approaches.
The next step in our investigations is the proposal of the consistent approach for the process of random lasing by using the similarities with the physics of the lasing in the cavity, i.e. to develop and solve the master equation for the system of twolevel atoms with pumping without the cavity and considering the multiscattering processes between the atoms and gain mechanism respectively. Concerning to this problem we present the general methodology and anticipate some results since the study is still in progressing. Liens :Vitalie EremeevLPMMC 
Nicolas Pauget (LPMMC)  Détails Fermer 
Currentphase relation and metastability of a SSET coupled to a mechanical oscillator le lundi 8 juin 2009 à 14:00 

Résumé : We consider a superconducting single electrons transistor (SSET) coupled to a mechanical degree of freedom in the quantum regime. When the coupling between the oscillator and the SSET exceeds a threshold, the mechanical oscillator modifies strongly the currentphase relation of the SSET. In the semiclassical limit, a bistable and hysteretic behavior of the device is predicted. This is the analogous of the so called FrankCondon blockade predicted and recently observed for a normal metal SET. We study in some details the behavior of the metastable state of the system, and the decay rate to the foundamental state. The decoherence prevents the decay in the semiclassical limit (Zeno effect), validating the semiclassical picture. When the quantum of oscillator can be resolved we study the decay time near an avoided crossing of the two relevant quantum levels within a quantum master equation approach. We find that the supercurrent will depend on time and can be used to measure the decay time of the mechanical state. Liens :Nicolas PaugetLPMMC 
Nicolas Cherroret (LPMMC)  Détails Fermer 
Dynamic conductance fluctuations le mardi 26 mai 2009 à 11:00 

Résumé : Disordered conductors are known to exhibit sampletosample fluctuations
of the static conductance [1]. These fluctuations are due to interference
effects which take place during the coherent multiple scattering of
electrons inside the conductor, and therefore lie in the heart of
mesoscopic physics. We studied timedependent sampletosample conductance fluctuations of a system with static disorder and irradiated by a short time pulse, by means of diagrammatic theory and microwave pulsed
transmission measurements. The fluctuations of timedependent conductance are not universal, i.e., depend on sample parameters, in contrast to the
steadystate case. The variance of normalized conductance is found to
increase as a third power of delay time t from an exciting pulse [2]. Liens :Nicolas CherroretLPMMC 
Giulia Ferrini (LPMMC)  Détails Fermer 
Probing mesoscopic superpositions in BoseJosephson junctions le lundi 4 mai 2009 à 11:00 

Résumé : A single Josephson junction has been experimentally implemented with ultracold bosonic gases by confining atoms in a doublewell trap. While the first experiments were performed in the classical limit, i.e. are well described by the meanfield GrossPitaevskii equation, there is an increasing interest in exploring the quantum regime both theoretically and experimentally. I will report on our theoretical work on the dynamics of a mesoscopic BoseJosephson junction. For a constant number of atoms, we give a fully quantum description of the junction in terms of a simple spinboson model, which allows to determine its ground state properties, its momentum distribution as well as its dynamical evolution. After a sudden rise of the barrier between the two wells, an initially phasecoherent state evolves into a coherent superposition of phase states  a Schroedinger's cat state. We propose a strategy to observe such “ phase cats ” : we show that their phase distribution is related to the full probability distribution of the spinboson operator J_{x}; such probability distribution, which can be obtained by repeated measurements of J_{x}, can distinguish between coherent superpositions and incoherent mixtures of phase states. Liens :Giulia FerriniLPMMC 
Afifa Yedjour (LPMMC)  Détails Fermer 
Optical disorder in a BoseEinstein condensate released from a trap le lundi 27 avril 2009 à 11:00 

Résumé : We present our results concerning the effect of optical disorder potentiel on the expansion of a BoseEinstein condensate released from a trap. Using the selfconsistent Born approximation, we determine the microscopic transport parameters that are neccessary to describe the diffusion process of the atomic gas in optical speckle. Our results shows that the selfconsistent Born approximation introduces significant modifications in the spectral function and in the energy distribution of the atoms. We need then to calculate a current density which given by solving the BetheSalpeter equation. Liens :Afifa YedjourLPMMC 
Vladimir Fedorov (LPMMC)  Détails Fermer 
Photon noise in a random laser amplifier with fluctuating properties le mardi 21 avril 2009 à 11:00 

Résumé : We present a theoretical study of light propagation in an amplifying random medium (random laser amplifier) with fluctuating properties. A simple example of such a medium is a suspension of Brownian particles in a laser dye. Noise in the number of photocounts measured by an ideal, fast photodetector, illuminated by a singlemode light emerging from the medium is investigated. General expressions for the normalized variance and autocorrelation function of photocount fluctuations are derived. For weak amplification, the result is similar to the one found in the absence of amplification. For strong amplification or in the absence of external illumination, the amplified spontaneous emission of the random medium dominates the signal. Variance of photocount number that we find in this case exhibits a series of interesting features, which allows us to make a link between physics of random amplifying media and Anderson localization. Useful information about the motion of scattering centers in the medium can be obtained by studying statistical properties of the amplified spontaneous emission. Liens :Vladimir FedorovLPMMC 
Sébastien Kawka (LPMMC)  Détails Fermer 
Quantum theory for Casimir momentum le mardi 24 mars 2009 à 11:00 

Résumé : A full quantum calculation is presented for the Casimir momentum, using second order perturbation theory. Casimir momentum refers to the momentum that appears when matter couples to electromagnetic vacuum fluctuations. We consider an harmonic dipole, whose magnetoelectric activity is governed by external applied fields. We show that the Casimir momentum is nonzero and finite, and we give quantitative estimates. Liens :Sébastien KawkaLPMMC 
Dimitri Gangardt (University of Birmingham)  Détails Fermer 
Mobile impurities in 1D quantum liquids: Bloch oscillations and dissipation le lundi 9 mars 2009 à 14:00 

Résumé : Dynamical properties of impurity particles immersed in a 1D quantum liquid are strongly modified by the interactions. The collective nature of the excitations manifests itself in periodic dispersion typical for lattices and may lead to Bloch oscillations. Experiments with ultracold atoms are currently underway to observe this phenomenon. At the same time the quantum and thermal fluctuations always present in quantum liquids provide a mechanism for dissipation and lead to viscous friction force acting on the moving impurity. A naive estimate shows that typically the dissipation is rather large and may even prevent observation of Bloch oscillations. We evaluate viscosity in terms of the quantum fluid parameters and show how the dissipation is drastically suppressed for nearly integrable systems due to quantum interference. 
Nicolas Pavloff (LPTMS Orsay)  Détails Fermer 
Compétition entre la superfluidité et la localisation d'Anderson dans les condensats de BoseEinstein le lundi 9 mars 2009 à 11:00 

Résumé : Le développement des techniques de manipulation cohérente d'atomes permet d'étudier sur des vapeurs atomiques des propriétés de transport spécifiques aux systèmes quantiques (comme la localisation d'Anderson, les oscillations de Bloch, la quantification de la conductance...). Les systèmes condensés de BoseEinstein sont particulièrement intéressants pour cette problématique car ils ont une cohérence de phase quasi parfaite, et l'interaction entre leurs composants peut être facilement controlée (et modélisée).
Liens :Nicolas PavloffLPTMS Orsay 
Mikhail Zvonarev (DPMC, Université de Genève)  Détails Fermer 
Dynamical properties of onedimensional itinerant ferromagnets le lundi 2 mars 2009 à 11:00 

Résumé : I will discuss how spin and charge excitations propagate in onedimensional quantum systems whose ground state is ferromagnetic. In particular, I will derive the propagator of transverse spin waves and analyze the structure of the magnon peak in the corresponding spectral function. To do so I will employ several analytical methods: bosonization, integrability, and first quantized path integrals formalism. Some of the obtained results do not have analogues in known lowenergy universality classes of quantum 1D systems. 
Iacobo Carusotto (ETH Zürich)  Détails Fermer 
Nonequilibrium manybody physics with optical systems le lundi 23 février 2009 à 11:00 

Résumé : Several recent experiments have provided evidence for the appearance of macroscopic coherence in optical systems with a spontaneous symmetry breaking mechanism which is the nonequilibrium analog of the BoseEinstein condensation phase transtion. Liens :Iacobo CarusottoETH Zürich 
Giovanni Mazzarella (Dipartimento di Fisica G. Galilei  Universita di Padova)  Détails Fermer 
Hidden order in bosonic gases confined in one dimensional optical lattices le lundi 9 février 2009 à 11:00 

Résumé : We analyze the effective Hamiltonian arising from a suitable power series expansion of the overlap integrals of Wannier functions for confined bosonic atoms in a 1d optical lattice. For certain constraints between the coupling constants, we construct an explicit relation between such an effective bosonic Hamiltonian and the integrable spinS anisotropic Heisenberg model. Therefore the former results to be integrable by construction. The field theory is governed by an anisotropic non linear σmodel with singlet and triplet massive excitations; such a result holds also in the generic onintegrable cases. The criticality of the bosonic system is investigated. The schematic phase diagram is drawn. Our study is shedding light on the hidden symmetry of the Haldane type for one dimensional bosons. Liens :Dipartimento di Fisica G. Galilei  Universita di Padova 
Mauro Antezza (LKB)  Détails Fermer 
Propagation of Light in a Periodic Ultracold Atomic System le lundi 2 février 2009 à 11:00 

Résumé : We study the propagation of light in a periodic system of atoms located in the lowest vibrational state of an optical lattice. The proper modes of the atomic gas interacting via the electromagnetic field is analytically investigated by taking into account both the quantum atomic motion and the vectorial character of the light. We show that the atomic motion naturally leads to a divergency free electrodynamic model, and that the photonic spectrum is gapless. Liens :Mauro AntezzaLKB 
Bess Fang (Centre for Quantum Technologies, Université de Singapour)  Détails Fermer 
Fermionization of a strongly interacting BoseFermi mixture in a onedimensional harmonic trap le lundi 26 janvier 2009 à 11:00 

Résumé : We consider a strongly interacting onedimensional (1D) BoseFermi mixture confined in a harmonic trap. It consists of a TonksGirardeau (TG) gas (1D Bose gas with repulsive hardcore interactions) and of a noninteracting Fermi gas (1D spinaligned Fermi gas), both species interacting through hardcore repulsive interactions. Using a generalized BoseFermi mapping, we determine the exact particle density profiles, momentum distributions and behaviour of the mixture under 1D expansion when opening the trap. In real space, bosons and fermions do not display any phase separation: the respective density profiles extend over the same region and they both present a number of peaks equal to the total number of particles in the trap. In momentum space the bosonic component has the typical narrow TG profile, while the fermionic component shows a broad distribution with fermionic oscillations at small momenta. Due to the large bosonfermion repulsive interactions, both the bosonic and the fermionic momentum distributions decay as C p^{4} at large momenta, like in the case of a pure bosonic TG gas. The coefficient C is related to the twobody density matrix and to the bosonic concentration in the mixture. When opening the trap, both momentum distributions "fermionize" under expansion and turn into that of a Fermi gas with a particle number equal to the total number of particles in the mixture. Liens :Bess FangCentre for Quantum Technologies, Université de Singapour 
JeanLouis Barrat (LPMCN, Lyon)  Détails Fermer 
Glissement de vitesse et de température aux interfaces liquidesolide le lundi 19 janvier 2009 à 11:00 

Liens :JeanLouis BarratLPMCN, Lyon 
Frédéric Faure (Institut Fourier, Grenoble)  Détails Fermer 
Introduction au chaos quantique. Résurgence d'un paquet d'onde. le lundi 12 janvier 2009 à 11:00 

Résumé : Le chaos quantique est l'étude de l'évolution d'une onde piégée dans une cavité chaotique. Son comportement est complexe à cause de phénomènes de dispersion et d'interférences créés par l'instabilité des trajectoires. Nous donnerons une introduction au chaos quantique à travers des exemples physiques et des modèles mathématiques simples. Nous présentons les temps caractéristiques et quelques phénomènes associés, comme le temps d'Ehrenfest TE, très court et à partir duquel les phénomènes d'interférences complexes se produisent ; le théorème d'ergodicité quantique de Schnirelmann concernant l'équidistribution des ondes stationnaires ; la théorie des matrices aléatoires, qui est une approche statistique heuristique, et qui concerne les temps longs. On présentera un exemple étonnant (et exceptionnel) de résurgence exacte d'un paquet d'onde à la date 2*TE et ses conséquences comme l'existence d'ondes stationnaires non équidistribuées. On mentionnera quelques problèmes ouverts, du côté physique et mathématique. 