24 research outputs found
Quantum versus classical effects in two-photon speckle patterns
We discuss quantum and classical aspects of two-photon interference in light
transmission through disordered media. We show that disorder is the main factor
that suppresses the interference, whatever the quantum state of the incident
light. Secondarily, the two-photon interference is affected by the quantum
nature of light (i.e., the well-defined number of photons in the two-photon
entangled and Fock states as compared to the coherent state). And finally,
entanglement is a resource that allows to prepare two-photon states with
special symmetries with respect to the interchange of the photons and, in
particular, the states with bosonic and fermionic symmetries. The two-photon
interference is more robust for the latter states and its sign can be inverted
for the fermionic state.Comment: 13 pages, 10 figures, revised tex
Time-dependent reflection at the localization transition
A short quasi-monochromatic wave packet incident on a semi-infinite
disordered medium gives rise to a reflected wave. The intensity of the latter
decays as a power law in the long-time limit. Using the
one-dimensional Aubry-Andr\'{e} model, we show that in the vicinity of the
critical point of Anderson localization transition, the decay slows down and
the power-law exponent becomes smaller than both found in
the Anderson localization regime and expected for a
one-dimensional random walk of classical particles.Comment: 9 pages, 6 figures. Revised tex
Photonic topological Anderson insulator in a two-dimensional atomic lattice
Disorder in atomic positions can induce a topologically nontrivial phase -
topological Anderson insulator (TAI) - for transverse electric optical
quasimodes of a two-dimensional honeycomb lattice of immobile atoms. TAI
requires both time-reversal and inversion symmetries to be broken to similar
extents. It is characterized by a nonzero topological invariant, a reduced
density of states and spatially localized quasimodes in the bulk, as well as
propagating edge states. A transition from TAI to the topological insulator
(TI) phase can take place at a constant value of the topological invariant,
showing that TAI and TI represent the same topological phase. The interplay
between topology and disorder for light in the considered atomic lattice is
strongly affected by the suppression of Anderson localization due to
longitudinal optical fields, which makes it different from the corresponding
interplay in electronic systems and calls for a separate detailed study.Comment: Submitted to Comptes Rendus Physiqu
Thermal signatures of Little-Parks effect in the heat capacity of mesoscopic superconducting rings
We present the first measurements of thermal signatures of the Little-Parks
effect using a highly sensitive nanocalorimeter. Small variations of the heat
capacity of 2.5 millions of non interacting micrometer-sized
superconducting rings threaded by a magnetic flux have been measured by
attojoule calorimetry. This non-invasive method allows the measurement of
thermodynamic properties -- and hence the probing of the energy levels -- of
nanosystems without perturbing them electrically. It is observed that is
strongly influenced by the fluxoid quantization (Little-Parks effect) near the
critical temperature . The jump of at the superconducting phase
transition is an oscillating function of with a period ,
the magnetic flux quantum, which is in agreement with the Ginzburg-Landau
theory of superconductivity.Comment: To be published in Physical Review B, Rapid Communication
Recurrent scattering and memory effect at the Anderson localization transition
We report on ultrasonic measurements of the propagation operator in a
strongly scattering mesoglass. The backscattered field is shown to display a
deterministic spatial coherence due to a remarkably large memory effect induced
by long recurrent trajectories. Investigation of the recurrent scattering
contribution directly yields the probability for a wave to come back close to
its starting spot. The decay of this quantity with time is shown to change
dramatically near the Anderson localization transition. The singular value
decomposition of the propagation operator reveals the dominance of very intense
recurrent scattering paths near the mobility edge.Comment: 5 pages, 4 figure
Towards a random laser with cold atoms
Atoms can scatter light and they can also amplify it by stimulated emission.
From this simple starting point, we examine the possibility of realizing a
random laser in a cloud of laser-cooled atoms. The answer is not obvious as
both processes (elastic scattering and stimulated emission) seem to exclude one
another: pumping atoms to make them behave as amplifier reduces drastically
their scattering cross-section. However, we show that even the simplest atom
model allows the efficient combination of gain and scattering. Moreover,
supplementary degrees of freedom that atoms offer allow the use of several gain
mechanisms, depending on the pumping scheme. We thus first study these
different gain mechanisms and show experimentally that they can induce
(standard) lasing. We then present how the constraint of combining scattering
and gain can be quantified, which leads to an evaluation of the random laser
threshold. The results are promising and we draw some prospects for a practical
realization of a random laser with cold atoms.Comment: Accepcted for publication by J. Opt. A for the special issue on
nanolasers and random lasers (to be published early 2010
Detection of highly conductive surface electron states in topological crystalline insulators Pb1−xSnxSe using laser terahertz radiation
We suggest a method for detection of highly conductive surface electron states including topological ones. The method is based on measurements of the photoelectromagnetic effect using terahertz laser pulses. In contrast to conventional transport measurements, the method is not sensitive to the bulk conductivity. The method is demonstrated on an example of topological crystalline insulators Pb1−xSnxSe. It is shown that highly conductive surface electron states are present in Pb1−xSnxSe both in the inverse and direct electron energy spectrum
Transport cohérent en milieu aléatoire (des corrélations mésoscopiques à la localisation d'Anderson)
Cette thèse est consacrée à l'étude des effets cohérents qui surviennent lorsqu'une onde se propage dans un milieu désordonné. Différents aspects de leur dynamique sont traités, et un intérêt particulier est accordé aux corrélations mésoscopiques et à la localisation d'Anderson. Dans un premier temps, nous démontrons une version généralisée de la théorie auto-cohérente de la localisation, adaptée à la description des milieux finis et ouverts. Celle-ci introduit un coefficient de diffusion dépendant de la position. La théorie est appliquée à l'étude du phénomène de confinement transverse des ondes, et confrontée aux prédictions de la théorie d'échelle de la localisation. Dans une seconde partie, nous explorons la dynamique des figures de tavelures résultant de la propagation d'impulsions courtes dans un guide d'ondes désordonné. En particulier, nous étudions la fonction de corrélation de l'intensité, qui contient l'information sur les fluctuations universelles de conductance. Dans le cas dynamique, ces fluctuations acquièrent une dépendance temporelle, augmentent avec le temps et perdent leur caractère universel. Ces résultats sont confirmés par une expérience micro-ondes. Pour finir, nous étudions la physique des figures de tavelures résultant de l'expansion d'un condensat de Bose-Einstein dans un potentiel aléatoire. Celles-ci présentent des corrélations de longue portée qui augmentent avec le temps, et peuvent éventuellement prendre des valeurs négatives. Ces résultats sont interprétés en termes d'un déplacement aléatoire du centre de masse du condensat. Le rôle des interactions atomiques lors de l'expansion du condensat est discuté.This thesis is devoted to the study of coherent effects that arise when a wave propagates in a strongly disordered medium. Several aspects of their dynamics are addressed, with special interest in mesoscopic correlations and Anderson localization. First, we demonstrate a generalized form of the self-consistent theory of Anderson localization, adapted to the description of finite open media. This theory introduces a diffusion coefficient that depends on position. The importance of this property is highlighted through the investigation of the phenomenon of transverse confinement of waves in disordered media. The theory is also confronted to predictions of the scaling theory of localization. The second part of the thesis focuses on the dynamics of intensity speckle patterns that arises from the propagation of short pulses in a disordered waveguide. We study the two-point intensity correlation function, which contains the information about the universal conductance fluctuations, well known in mesoscopic conductors. In the dynamic situation, conductance fluctuations acquire a time dependence, are enhanced at long times and lose their universal nature. These results are confirmed by a microwave experiment. Finally, we investigate the physics of matter-wave speckle patterns, resulting from the expansion of a Bose-Einstein condensate in a random potential. They are found to exhibit long-range correlations that grow with time, and can take negative values. We interpret these results in terms of a random displacement of the center of mass of the condensate. The role of atomic interactions during the expansion of the condensate is discussed.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF