195 research outputs found
DeepJSCC-Q: Channel Input Constrained Deep Joint Source-Channel Coding
Recent works have shown that the task of wireless transmission of images can be learned with the use of machine learning techniques. Very promising results in end-to-end image quality, superior to popular digital schemes that utilize source and channel coding separation, have been demonstrated through the training of an autoencoder, with a non-trainable channel layer in the middle. However, these methods assume that any complex value can be transmitted over the channel, which can prevent the application of the algorithm in scenarios where the hardware or protocol can only admit certain sets of channel inputs, such as the use of a digital constellation. Herein, we propose DeepJSCC-Q, an end-to-end optimized joint source-channel coding scheme for wireless image transmission, which is able to operate with a fixed channel input alphabet. We show that DeepJSCC-Q can achieve similar performance to models that use continuous-valued channel input. Importantly, it preserves the graceful degradation of image quality observed in prior work when channel conditions worsen, making DeepJSCC-Q much more attractive for deployment in practical systems
Photoluminescence quenching mechanisms in type II InAs/GaInSb QWs on InAs substrates
We would like to acknowledge the National Science Centre of Poland for support within Grant No. 2014/15/B/ST7/04663.Optical properties of AlSb/InAs/GaInSb/InAs/AlSb quantum wells (QWs) grown on an InAs substrate were investigated from the point of view of room temperature emission in the mid- and long-wavelength infrared ranges. By means of two independent techniques of optical spectroscopy, photoreflectance and temperature-dependent photoluminescence, it was proven that the main process limiting the performance of such InAs substrate-based type II structures is related to the escape of carriers from the hole ground state of the QW. Two nonradiative recombination channels were identified. The main process was attributed to holes tunneling to the valence band of the GaAsSb spacing layer and the second one with trapping of holes by native defects located in the same layer.Publisher PDFPeer reviewe
Density classification on infinite lattices and trees
Consider an infinite graph with nodes initially labeled by independent
Bernoulli random variables of parameter p. We address the density
classification problem, that is, we want to design a (probabilistic or
deterministic) cellular automaton or a finite-range interacting particle system
that evolves on this graph and decides whether p is smaller or larger than 1/2.
Precisely, the trajectories should converge to the uniform configuration with
only 0's if p1/2. We present solutions to that problem
on the d-dimensional lattice, for any d>1, and on the regular infinite trees.
For Z, we propose some candidates that we back up with numerical simulations
A Full Computation-relevant Topological Dynamics Classification of Elementary Cellular Automata
Cellular automata are both computational and dynamical systems. We give a
complete classification of the dynamic behaviour of elementary cellular
automata (ECA) in terms of fundamental dynamic system notions such as
sensitivity and chaoticity. The "complex" ECA emerge to be sensitive, but not
chaotic and not eventually weakly periodic. Based on this classification, we
conjecture that elementary cellular automata capable of carrying out complex
computations, such as needed for Turing-universality, are at the "edge of
chaos"
Time-Resolved Measurement of Interatomic Coulombic Decay in Ne_2
The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al.,
Phys. Rev. Lett. 79, 4778 (1997)] in Ne_2 is determined via an extreme
ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The
pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms,
whereupon the ionized dimer undergoes ICD resulting in a repulsive
Ne^{+}(2p^{-1}) - Ne^{+}(2p^{-1}) state, which is probed with a second pulse,
removing a further electron. The yield of coincident Ne^{+} - Ne^{2+} pairs is
recorded as a function of the pump-probe delay, allowing us to deduce the ICD
lifetime of the Ne_{2}^{+}(2s^{-1}) state to be (150 +/- 50) fs in agreement
with quantum calculations.Comment: 5 pages, 3 figures, accepted by PRL on July 11th, 201
Electron Rearrangement Dynamics in Dissociating I 2 n Molecules Accessed by Extreme Ultraviolet Pump Probe Experiments
The charge rearrangement in dissociating In 2 molecules is measured as a function of the internuclear distance R using extreme ultraviolet pulses delivered by the free electron laser in Hamburg. Within an extreme ultraviolet pump probe scheme, the first pulse initiates dissociation by multiply ionizing I2, and the delayed probe pulse further ionizes one of the two fragments at a given time, thus triggering charge rearrangement at a well defined R. The electron transfer between the fragments is monitored by analyzing the delay dependent ion kinetic energies and charge states. The experimental results are in very good agreement with predictions of the classical over the barrier model demonstrating its validity in a thus far unexplored quasimolecular regime relevant for free electron laser, plasma, and chemistry application
Watching the acetylene vinylidene intramolecular reaction in real time
It is a long-standing dream of scientists to capture the ultra-fast dynamics
of molecular or chemical reactions in real time and to make a molecular movie.
With free-electron lasers delivering extreme ultraviolet (XUV) light at
unprecedented intensities, in combination with pump-probe schemes, it is now
possible to visualize structural changes on the femtosecond time scale in
photo-excited molecules. In hydrocarbons the absorption of a single photon may
trigger the migration of a hydrogen atom within the molecule. Here, such a
reaction was filmed in acetylene molecules (C2H2) showing a partial migration
of one of the protons along the carbon backbone which is consistent with
dynamics calculations on ab initio potential energy surfaces. Our approach
opens attractive perspectives and potential applications for a large variety of
XUV-induced ultra-fast phenomena in molecules relevant to physics, chemistry,
and biology.Comment: 21 pages, 3 figures, submitte
Multiple ionization and fragmentation dynamics of molecular iodine studied in IR-XUV pump-probe experiments
The ionization and fragmentation dynamics of iodine molecules (I-2) are traced using very intense (similar to 10(14) W cm(-2)) ultra-short (similar to 60 fs) light pulses with 87 eV photons of the Free-electron LASer at Hamburg (FLASH) in combination with a synchronized femtosecond optical laser. Within a pump-probe scheme the IR pulse initiates a molecular fragmentation and then, after an adjustable time delay, the system is exposed to an intense FEL pulse. This way we follow the creation of highly-charged molecular fragments as a function of time, and probe the dynamics of multi-photon absorption during the transition from a molecule to individual atoms
Regularity of maximal operators: recent progress and some open problems
This is an expository paper on the regularity theory of maximal operators,
when these act on Sobolev and BV functions, with a special focus on some of the
current open problems in the topic. Overall, a list of fifteen research
problems is presented. It summarizes the contents of a talk delivered by the
author at the CIMPA 2017 Research School - Harmonic Analysis, Geometric Measure
Theory and Applications, in Buenos Aires, Argentina.Comment: 19 pages. Expository paper with the contents of a lecture given at
the in the CIMPA 2017 Research School - Harmonic Analysis, Geometric Measure
Theory and Applications, in Buenos Aires, Argentin
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