472 research outputs found
On Termination for Faulty Channel Machines
A channel machine consists of a finite controller together with several fifo
channels; the controller can read messages from the head of a channel and write
messages to the tail of a channel. In this paper, we focus on channel machines
with insertion errors, i.e., machines in whose channels messages can
spontaneously appear. Such devices have been previously introduced in the study
of Metric Temporal Logic. We consider the termination problem: are all the
computations of a given insertion channel machine finite? We show that this
problem has non-elementary, yet primitive recursive complexity
Guided atom laser : a new tool for guided atom optics
We present a guided atom laser. A Bose-Einstein condensate (BEC) is created
in a crossed hybrid magnetic and an elongated optical trap, which acts as a
matterwave guide. Atoms are extracted from the BEC by radio frequency (rf)
outcoupling and then guided in the horizontal optical matterwave guide. This
method allows to control the acceleration of the beam and to achieve large de
Broglie wavelength. We also measure the longitudinal energy of the guided atom
laser using atom optical elements based on a blue light barrier
A quantum trampoline for ultra-cold atoms
We have observed the interferometric suspension of a free-falling
Bose-Einstein condensate periodically submitted to multiple-order diffraction
by a vertical 1D standing wave. The various diffracted matter waves recombine
coherently, resulting in high contrast interference in the number of atoms
detected at constant height. For long suspension times, multiple-wave
interference is revealed through a sharpening of the fringes. We use this
scheme to measure the acceleration of gravity
How to estimate the differential acceleration in a two-species atom interferometer to test the equivalence principle
We propose a scheme for testing the weak equivalence principle (Universality
of Free Fall) using an atom-interferometric measurement of the local
differential acceleration between two atomic species with a large mass ratio as
test masses. A apparatus in free fall can be used to track atomic free-fall
trajectories over large distances. We show how the differential acceleration
can be extracted from the interferometric signal using Bayesian statistical
estimation, even in the case of a large mass and laser wavelength difference.
We show that this statistical estimation method does not suffer from
acceleration noise of the platform and does not require repeatable experimental
conditions. We specialize our discussion to a dual potassium/rubidium
interferometer and extend our protocol with other atomic mixtures. Finally, we
discuss the performances of the UFF test developed for the free-fall (0-g)
airplane in the ICE project (\verb"http://www.ice-space.fr"
Effect of disorder close to the superfluid transition in a two-dimensional Bose gas
We experimentally study the effect of disorder on trapped quasi
two-dimensional (2D) 87Rb clouds in the vicinity of the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The disorder
correlation length is of the order of the Bose gas characteristic length scales
(thermal de Broglie wavelength, healing length) and disorder thus modifies the
physics at a microscopic level. We analyze the coherence properties of the
cloud through measurements of the momentum distributions, for two disorder
strengths, as a function of its degeneracy. For moderate disorder, the
emergence of coherence remains steep but is shifted to a lower entropy. In
contrast, for strong disorder, the growth of coherence is hindered. Our study
is an experimental realization of the dirty boson problem in a well controlled
atomic system suitable for quantitative analysis
Guided Quasicontinuous Atom Laser
We report the first realization of a guided quasicontinuous atom laser by rf
outcoupling a Bose-Einstein condensate from a hybrid optomagnetic trap into a
horizontal atomic waveguide. This configuration allows us to cancel the
acceleration due to gravity and keep the de Broglie wavelength constant at 0.5
m during 0.1 s of propagation. We also show that our configuration,
equivalent to pigtailing an optical fiber to a (photon) semiconductor laser,
ensures an intrinsically good transverse mode matching.Comment: version published in Phys. Rev. Lett. 97, 200402 (2006
Anderson Localization of Expanding Bose-Einstein Condensates in Random Potentials
We show that the expansion of an initially confined interacting 1D
Bose-Einstein condensate can exhibit Anderson localization in a weak random
potential with correlation length \sigma_R. For speckle potentials the Fourier
transform of the correlation function vanishes for momenta k > 2/\sigma_R so
that the Lyapunov exponent vanishes in the Born approximation for k >
1/\sigma_R. Then, for the initial healing length of the condensate \xi >
\sigma_R the localization is exponential, and for \xi < \sigma_R it changes to
algebraic.Comment: published versioon (no significant change compared to last version
Population and Phase Coherence during the Growth of an Elongated Bose-Einstein Condensate
We study the growth of an elongated phase-fluctuating condensate from a
non-equilibrium thermal cloud obtained by shock-cooling. We compare the growth
of the condensate with numerical simulations, revealing a time delay and a
reduction in the growth rate which we attribute to phase fluctuations. We
measure the phase coherence using momentum Bragg spectroscopy, and thereby
observe the evolution of the phase coherence as a function of time. Combining
the phase coherence results with the numerical simulations, we suggest a simple
model for the reduction of the growth rate based on the reduction of bosonic
stimulation due to phase fluctuations and obtain improved agreement between
theory and experiment
Transport quantique d'atomes ultrafroids dans des potentiels désordonnés
Dans cette thèse, nous étudions le transport quantique d ondes de matière avec des atomes ultrafroids. Ces systèmes d atomes ultrafroids fournissent un très bon contrôle et une grande flexibilité pour les paramètres du système tels que les interactions, sa dimensionnalité et les potentiels externes. Cela les rend un excellent outil pour l étude de plusieurs concepts fondamentaux de la physique de la matière condensée. Nous nous concentrons sur le transport quantique dans les milieux désordonnés. Il diffère du transport classique par le rôle fondamental joué par les phénomènes d inférence, qui peuvent éventuellement conduire à la suppression du transport; connu comme la Localisation d Anderson. Nous étudions l expansion d un condensat de Bose-Einstein dans un désordre fort et montrons des signes de localisation d atomes ultrafroids à trois dimensions. Dans la dernière partie de ce manuscrit, nous discutons l observation de la rétrodiffusion cohérente d atomes ultrafroids, ce qui est un signal direct du rôle de la cohérence quantique dans le transport quantique dans les milieux désordonnés. Nous observons l évolution temporelle de la distribution d impulsions d un nuage de atomes ultrafroids, lancé avec une distribution de vitesse étroite dans un potentiel désordonné. Un pic émerge dans le sens rétrograde, correspondant au signal de CBS.In this thesis we study the quantum transport of matter waves with ultracold atoms. Such ultracold atom systems provide a very good control and a high flexibility of the parameters of the systems like the interactions, its dimensionality and the external potentials. This makes them a great tool for the investigation of several fundamental concepts of condensed matter physics. We focus on the quantum transport in disordered media. It differs to classical transport by the fundamental role played by inference phenomena, which can eventually lead to the suppression of transport; known as Anderson Localization. Observing the expansion of a Bose-Einstein condensate in a strong light disorder, we show evidence for Localization of ultracold atoms in three dimensions. In the last part of this manuscript we discuss the observation of Coherent Backscattering of ultracold atoms, which is a direct signal of the role of quantum coherence in quantum transport in disordered media. We observe the time evolution of the momentum distribution of a cloud of ultra-cold atoms, launched with a narrow velocity distribution in a disordered potential. A peak emerges in the backwards direction, corresponding to the CBS signal.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
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