349 research outputs found
Classical Diffusion of a quantum particle in a noisy environment
We study the spreading of a quantum-mechanical wavepacket in a
one-dimensional tight-binding model with a noisy potential, and analyze the
emergence of classical diffusion from the quantum dynamics due to decoherence.
We consider a finite correlation time of the noisy environment, and treat the
system by utilizing the separation of fast (dephasing) and slow (diffusion)
processes. We show that classical diffusive behavior emerges at long times, and
we calculate analytically the dependence of the classical diffusion coefficient
on the noise magnitude and correlation time. This method provides a general
solution to this problem for arbitrary conditions of the noisy environment. The
results are relevant to a large variety of physical systems, from electronic
transport in solid state physics, to light transmission in optical devices,
diffusion of excitons, and quantum computation
Black-Hole Mass and Growth Rate at High Redshift
We present new H and K bands spectroscopy of 15 high luminosity active
galactic nuclei (AGNs) at redshifts 2.3-3.4 obtained on Gemini South. We
combined the data with spectra of additional 29 high-luminosity sources to
obtain a sample with 10^{45.2}<\lambda L_{\lambda}(5100A)<10^{47.3} ergs/sec
and black hole (BH) mass range, using reverberation mapping relationships based
on the H_beta method, of 10^{8.8}-10^{10.7} M_sun. We do not find a correlation
of L/L_Edd with M_BH but find a correlation with \lambda L_{\lambda}(5100A)
which might be due to selection effects. The L/L_Edd distribution is broad and
covers the range ~0.07-1.6, similar to what is observed in lower redshift,
lower luminosity AGNs. We suggest that this consistently measured and
calibrated sample gives the best representation of L/L_Edd at those redshifts
and note potential discrepancies with recent theoretical and observational
studies. The lower accretion rates are not in accord with growth scenarios for
BHs at such redshifts and the growth times of many of the sources are longer
than the age of the universe at the corresponding epochs. This suggests earlier
episodes of faster growth at z>~3 for those sources. The use of the C IV method
gives considerably different results and a larger scatter; this method seems to
be a poor M_BH and L/L_Edd estimator at very high luminosity.Comment: 8 pages (emulateapj), 4 figures. Accepted for publication in Ap
Realization of quantum walks with negligible decoherence in waveguide lattices
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects
Wind-shearing in gaseous protoplanetary disks and the evolution of binary planetesimals
One of the first stages of planet formation is the growth of small
planetesimals. This early stage occurs much before the dispersal of most of the
gas from the protoplanetary disk. Due to their different aerodynamic
properties, planetesimals of different sizes and shapes experience different
drag forces from the gas during this time. Such differential forces produce a
wind-shearing (WISH) effect between close by, different size planetesimals. For
any two planetesimals, a WISH radius can be considered, at which the
differential acceleration due to the wind becomes greater than the mutual
gravitational pull between the planetesimals. We find that the WISH radius
could be much smaller than the Hill radius, i.e. WISH could play a more
important role than tidal perturbations by the star. Here we study the WISH
radii for planetesimal pairs of different sizes and compare the effects of wind
and gravitational shearing (drag force vs. gravitational tidal force). We then
discuss the role of WISH for the stability and survival of binary
planetesimals. Binaries are sheared apart by the wind if they are wider than
their WISH radius. WISH-stable binaries can inspiral and possibly coalesce due
to gas drag. Here, we calculate the WISH radius and the gas drag-induced merger
timescale, providing stability and survival criteria for gas-embedded binary
planetesimals. Our results suggest that even WISH-stable binaries may merge in
times shorter than the lifetime of the gaseous disk. This may constrain
currently observed binary planetesimals to have formed far from the star or at
a late stage after the dispersal of most of the disk gas. We note that the WISH
radius may also be important for other processes such as planetesimal erosion
and planetesimal encounters and collisions in a gaseous environment.Comment: ApJ, in pres
Pengukuran Kesenjangan Kinerja Antara Supplier Dengan Customer Pada Rantai Pasokan Jasa Pendidikan Tinggi Menggunakan Metode Servqual
Performance is the key to measure the quality of service. On the other side an increase in the quality of service is one of the keys to winning the competition.The purpose of this study is measuring the service quality of services at the college using SERVQUAL. SERVQUAL method itself is a method to measure services through five dimensions namely Tangibels, Empathy, Reliability, Responsiveness and Assurance.The results of using SERVQUAL method from 25 variables obtained dimensions that has the biggest gap is Tangibels, Empathy, and Responsiveness
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei
We reinvestigate the relationship between the characteristic broad-line
region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical
continuum luminosities. Our study makes use of the best available
determinations of R_blr for a large number of active galactic nuclei (AGNs)
from Peterson et al. Using their determinations of R_blr for a large sample of
AGNs and two different regression methods, we investigate the robustness of our
correlation results as a function of data sub-sample and regression technique.
Though small systematic differences were found depending on the method of
analysis, our results are generally consistent. Assuming a power-law relation
R_blr \propto L^\alpha, we find the mean best-fitting \alpha is about
0.67+/-0.05 for the optical continuum and the broad H\beta luminosity, about
0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the
X-ray luminosity. We also find an intrinsic scatter of about 40% in these
relations. The disagreement of our results with the theoretical expected slope
of 0.5 indicates that the simple assumption of all AGNs having on average same
ionization parameter, BLR density, column density, and ionizing spectral energy
distribution, is not valid and there is likely some evolution of a few of these
characteristics along the luminosity scale.Comment: 11 pages, 2 figures, emulateapj, accepted for publication in The
Astrophysical Journa
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