12,768 research outputs found
Monte Carlo Simulation of Laser Diodes Sub-Poissonian Light Generation
When laser diodes are driven by high-impedance electrical sources the
variance of the number of photo-detection events counted over large time
durations is less than the average number of events (sub-Poissonian light). The
paper presents a Monte Carlo simulation that keeps track of each level
occupancy (0 or 1) in the conduction and valence bands, and of the number of
light quanta in the optical cavity. When there is good electron-lattice thermal
contact the electron and hole temperatures remain equal to that of the lattice.
In that case, elementary laser-diode noise theory results are accurately
reproduced by the simulation. But when the thermal contact is poor (or, almost
equivalently, at high power levels) new effects occur (spectral-hole burning,
temperature fluctuations, statistical fluctuations of the optical gain) that
are difficult to handle theoretically. Our numerical simulation shows that the
frequency domain over which the photo-current spectral density is below the
shot-noise level becomes narrower as the optical power increases.Comment: 22 pages, 3 figures, 1 table, submitted to Optical and Quantum
Electronic
Voltage-sensitive dye imaging reveals tonotopic organization of auditory cortex spontaneous activity
Imaging neural activity across a large (several mm) cortical area with high temporal and spatial resolution is desirable, for example in the auditory system to measure cortical processing across a broad frequency spectrum. Voltage-sensitive dye imaging (VSDI) has a unique combination of properties making this possible, but so far studies have been limited to studying simple sparsely-presented sensory stimuli. We demonstrate the feasibility of long-acquisition VSDI (using the dye RH-1691) in auditory cortex while presenting complex time-varying acoustic stimuli or silence. Using a dense array of partially-overlapping 50 ms tone pips (8 frequencies per octave spanning six octaves), we obtained high-resolution spectrotemporal receptive fields (STRFs) simultaneously across the majority of the guinea pig primary auditory cortical fields (A1 and DC). Long epochs of spontaneous activity were also measured, permitting a comparison of spontaneous activity patterns with functional architecture. By grouping all pixels in areas A1 and DC according to sound frequency preference (obtained from STRFs), we reveal that spontaneous activity (such as cortical spindles) show complex spatial patterns, which are organized according to sound frequency preference within and across cortical areas. More specifically, spontaneous activity correlation decreases as frequency preference diverges within A1 or DC; but additionally, pixels in A1 are also highly correlated with (even far-away) pixels in DC sharing similar frequency preference. These properties of patterned cortical spontaneous activity constrain mechanistic hypotheses regarding their genesis. Beyond these observations, the feasibility of VSDI with continuous stimulation or silence permits measuring population activity during long-lasting sound patterns, which is necessary for examining cortical dynamics and sensory-context dependent processing
From Imprinting to Adaptation: Building a History of Affective Interaction
We present a Perception-Action architecture and experiments to simulate imprintingâthe establishment of strong attachment links with a âcaregiverââin a robot. Following recent theories, we do not consider imprinting as rigidly timed and irreversible, but as a more flexible phenomenon that allows for further adaptation as a result of reward-based learning through experience. Our architecture reconciles these two types of perceptual learning traditionally considered as different and even incompatible. After the initial imprinting, adaptation is achieved in the context of a history of âaffectiveâ interactions between the robot and a human, driven by âdistressâ and âcomfortâ responses in the robot
Exploring pure quantum states with maximally mixed reductions
We investigate multipartite entanglement for composite quantum systems in a
pure state. Using the generalized Bloch representation for n-qubit states, we
express the condition that all k-qubit reductions of the whole system are
maximally mixed, reflecting maximum bipartite entanglement across all k vs. n-k
bipartitions. As a special case, we examine the class of balanced pure states,
which are constructed from a subset of the Pauli group P_n that is isomorphic
to Z_2^n. This makes a connection with the theory of quantum error-correcting
codes and provides bounds on the largest allowed k for fixed n. In particular,
the ratio k/n can be lower and upper bounded in the asymptotic regime, implying
that there must exist multipartite entangled states with at least k=0.189 n
when . We also analyze symmetric states as another natural class
of states with high multipartite entanglement and prove that, surprisingly,
they cannot have all maximally mixed k-qubit reductions with k>1. Thus,
measured through bipartite entanglement across all bipartitions, symmetric
states cannot exhibit large entanglement. However, we show that the permutation
symmetry only constrains some components of the generalized Bloch vector, so
that very specific patterns in this vector may be allowed even though k>1 is
forbidden. This is illustrated numerically for a few symmetric states that
maximize geometric entanglement, revealing some interesting structures.Comment: 10 pages, 2 figure
Tracing transient charges in expanding clusters
We study transient charges formed in methane clusters following ionization by intense near-infrared laser pulses. Cluster ionization by 400-fs (I=1Ă1014 W/cm2) pulses is highly efficient, resulting in the observation of a dominant C3+ ion contribution. The C4+ ion yield is very small but is strongly enhanced by applying a time-delayed weak near-infrared pulse. We conclude that most of the valence electrons are removed from their atoms during the laser-cluster interaction and that electrons from the nanoplasma recombine with ions and populate Rydberg states when the cluster expands, leading to a decrease of the average charge state of individual ions. Furthermore, we find clear bound-state signatures in the electron kinetic energy spectrum, which we attribute to Auger decay taking place in expanding clusters. Such nonradiative processes lead to an increase of the final average ion charge state that is measured in experiments. Our results suggest that it is crucial to include both recombination and nonradiative decay processes for the understanding of recorded ion charge spectra
Fluorescence from a few electrons
Systems containing few Fermions (e.g., electrons) are of great current
interest. Fluorescence occurs when electrons drop from one level to another
without changing spin. Only electron gases in a state of equilibrium are
considered. When the system may exchange electrons with a large reservoir, the
electron-gas fluorescence is easily obtained from the well-known Fermi-Dirac
distribution. But this is not so when the number of electrons in the system is
prevented from varying, as is the case for isolated systems and for systems
that are in thermal contact with electrical insulators such as diamond. Our
accurate expressions rest on the assumption that single-electron energy levels
are evenly spaced, and that energy coupling and spin coupling between electrons
are small. These assumptions are shown to be realistic for many systems.
Fluorescence from short, nearly isolated, quantum wires is predicted to drop
abruptly in the visible, a result not predicted by the Fermi-Dirac
distribution. Our exact formulas are based on restricted and unrestricted
partitions of integers. The method is considerably simpler than the ones
proposed earlier, which are based on second quantization and contour
integration.Comment: 10 pages, 3 figures, RevTe
An SZ/X-ray galaxy cluster model and the X-ray follow-up of the Planck clusters
Sunyaev-Zel'dovich (SZ) cluster surveys will become an important cosmological
tool over next few years, and it will be essential to relate these new surveys
to cluster surveys in other wavebands. We present an empirical model of cluster
SZ and X-ray observables constructed to address this question and to motivate,
dimension and guide X-ray follow-up of SZ surveys. As an example application of
the model, we discuss potential XMM-Newton follow-up of Planck clusters.Comment: 4 pages, 5 figures. To appear in the proceedings of the XXXXIIIrd
Rencontres de Morion
Is MS1054-03 an exceptional cluster? A new investigation of ROSAT/HRI X-ray data
We reanalyzed the ROSAT/HRI observation of MS1054-03, optimizing the channel
HRI selection and including a new exposure of 68 ksec. From a wavelet analysis
of the HRI image we identify the main cluster component and find evidence for
substructure in the west, which might either be a group of galaxies falling
onto the cluster or a foreground source. Our 1-D and 2-D analysis of the data
show that the cluster can be fitted well by a classical betamodel centered only
20arcsec away from the central cD galaxy. The core radius and beta values
derived from the spherical model(beta = 0.96_-0.22^+0.48) and the elliptical
model (beta = 0.73+/-0.18) are consistent. We derived the gas mass and total
mass of the cluster from the betamodel fit and the previously published ASCA
temperature (12.3^{+3.1}_{-2.2} keV). The gas mass fraction at the virial
radius is fgas = (14[-3,+2.5]+/-3)% for Omega_0=1, where the errors in brackets
come from the uncertainty on the temperature and the remaining errors from the
HRI imaging data. The gas mass fraction computed for the best fit ASCA
temperature is significantly lower than found for nearby hot clusters,
fgas=20.1pm 1.6%. This local value can be matched if the actual virial
temperature of MS1054-032 were close to the lower ASCA limit (~10keV) with an
even lower value of 8 keV giving the best agreement. Such a bias between the
virial and measured temperature could be due to the presence of shock waves in
the intracluster medium stemming from recent mergers. Another possibility, that
reconciles a high temperature with the local gas mass fraction, is the
existence of a non zero cosmological constant.Comment: 12 pages, 5 figures, accepted for publication in Ap
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