131 research outputs found
Quantum memory for light: large efficiency at telecom wavelength
We implement the ROSE protocol in an erbium doped solid, compatible with the
telecom range. The ROSE scheme is an adaptation of the standard 2-pulse photon
echo to make it suitable for a quantum memory. We observe an efficiency of 40%
in a forward direction by using specific orientations of the light
polarizations, magnetic field and crystal axes
Selective optical addressing of nuclear spins through superhyperfine interaction in rare-earth doped solids
In Er:YSiO, we demonstrate the selective optical addressing of
the Y nuclear spins through their superhyperfine coupling with
the Er electronic spins possessing large Land\'e -factors. We
experimentally probe the electron-nuclear spin mixing with photon echo
techniques and validate our model. The site-selective optical addressing of the
Y nuclear spins is designed by adjusting the magnetic field strength and
orientation. This constitutes an important step towards the realization of
long-lived solid-state qubits optically addressed by telecom photons.Comment: 5 pages, 4 figures, supplementary material (3 pages
Threshold selection, mitosis and dual mutation in cooperative co-evolution: application to medical 3d tomography
International audienceWe present and analyse the behaviour of specialised operators designed for cooperative coevolution strategy in the framework of 3D tomographic PET reconstruction. The basis is a simple cooperative co-evolution scheme (the "fly algorithm"), which embeds the searched solution in the whole population, letting each individual be only a part of the solution. An individual, or fly, is a 3D point that emits positrons. Using a cooperative co-evolution scheme to optimize the position of positrons, the population of flies evolves so that the data estimated from flies matches measured data. The final population approximates the radioactivity concentration. In this paper, three operators are proposed, threshold selection, mitosis and dual mutation, and their impact on the algorithm efficiency is experimentally analysed on a controlled test-case. Their extension to other cooperative co-evolution schemes is discussed
Applying Evolutionary Optimisation to Robot Obstacle Avoidance
This paper presents an artificial evolutionbased method for stereo image
analysis and its application to real-time obstacle detection and avoidance for
a mobile robot. It uses the Parisian approach, which consists here in splitting
the representation of the robot's environment into a large number of simple
primitives, the "flies", which are evolved following a biologically inspired
scheme and give a fast, low-cost solution to the obstacle detection problem in
mobile robotics
A Two-Threshold Model for Scaling Laws of Non-Interacting Snow Avalanches
The sizes of snow slab failure that trigger snow avalanches are power-law
distributed. Such a power-law probability distribution function has also been
proposed to characterize different landslide types. In order to understand this
scaling for gravity driven systems, we introduce a two-threshold 2-d cellular
automaton, in which failure occurs irreversibly. Taking snow slab avalanches as
a model system, we find that the sizes of the largest avalanches just
preceeding the lattice system breakdown are power law distributed. By tuning
the maximum value of the ratio of the two failure thresholds our model
reproduces the range of power law exponents observed for land-, rock- or snow
avalanches. We suggest this control parameter represents the material cohesion
anisotropy.Comment: accepted PR
Voxelisation in the 3-D Fly Algorithm for PET
International audienceThe Fly Algorithm was initially developed for 3-D robot vision applications. It consists in solving the inverse problem of shape reconstruction from projections by evolving a population of 3-D points in space (the 'flies'), using an evolutionary optimisation strategy. Here, in its version dedicated to tomographic reconstruction in medical imaging, the flies are mimicking radioactive photon sources. Evolution is controlled using a fitness function based on the discrepancy of the projections simulated by the flies with the actual pattern received by the sensors. The reconstructed radioactive concentration is derived from the population of flies, i.e. a collection of points in the 3-D Euclidean space, after convergence. 'Good' flies were previously binned into voxels. In this paper, we study which flies to include in the final solution and how this information can be sampled to provide more accurate datasets in a reduced computation time. We investigate the use of density fields, based on Metaballs and on Gaussian functions respectively, to obtain a realistic output. The spread of each Gaussian kernel is modulated in function of the corresponding fly fitness. The resulting volumes are compared with previous work in terms of normalised-cross correlation. In our test-cases, data fidelity increases by more than 10% when density fields are used instead of binning. Our method also provides reconstructions comparable to those obtained using well-established techniques used in medicine (filtered back-projection and ordered subset expectation-maximisation)
Time reversal of light by linear dispersive filtering near atomic resonance
Based on the similarity of paraxial diffraction and dispersion mathematical
descriptions, the temporal imaging of optical pulses combines linear dispersive
filters and quadratic phase modulations operating as time lenses. We consider
programming a dispersive filter near atomic resonance in rare earth ion doped
crystals, which leads to unprecedented high values of dispersive power. This
filter is used in an approximate imaging scheme, combining a single time lens
and a single dispersive section and operating as a time reversing device, with
potential applications in radio-frequency signal processing. This scheme is
closely related to three-pulse photon echo with chirped pulses but the
connection with temporal imaging and dispersive filtering emphasizes new
features.Comment: 21 pages, 11 figure
Optical Excitation of Nuclear Spin Coherence in Tm3+:YAG
A thulium-doped crystal is experimentally shown to be an excellent candidate
for broadband quantum storage in a solid-state medium. For the first time,
nuclear spin coherence is optically excited, detected and characterized in such
a crystal. The lifetime of the spin coherence -- the potential storage entity
-- is measured by means of Raman echo to be about 300 s over a wide range
of ground state splittings. This flexibility, attractive for broadband
operation, and well fitted to existing quantum sources, results from the simple
hyperfine structure, contrasting with Pr- and Eu- doped crystals
Optical Excitation of Nuclear Spin Coherence in Tm3+:YAG
A thulium-doped crystal is experimentally shown to be an excellent candidate
for broadband quantum storage in a solid-state medium. For the first time,
nuclear spin coherence is optically excited, detected and characterized in such
a crystal. The lifetime of the spin coherence -- the potential storage entity
-- is measured by means of Raman echo to be about 300 s over a wide range
of ground state splittings. This flexibility, attractive for broadband
operation, and well fitted to existing quantum sources, results from the simple
hyperfine structure, contrasting with Pr- and Eu- doped crystals
Stimulated Raman adiabatic passage in Tm3+:YAG
International audienceWe report on the experimental demonstration of stimulated Raman adiabatic passage in a Tm 3+ : YAG crystal. Tm 3+ : YAG is a promising material for use in quantum information processing applications, but as yet there are few experimental investigations of coherent Raman processes in this material. We investigate the effect of inhomogeneous broadening and Rabi frequency on the transfer efficiency and the width of the two-photon spectrum. Simulations of the complete Tm 3+ : YAG system are presented along with the corresponding experimental results
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