270 research outputs found
Multilayer gas cells for sub-Doppler spectroscopy
We have carried out theoretical research on ultra-high resolution
spectroscopy of atoms (or molecules) in the suggested cell with a series of
plane-parallel thin gas layers between spatially separated gas regions of this
cell for optical pumping and probing. It is shown the effective velocity
selection of optically pumped atoms because of their specific transit time and
collisional relaxation in such a cell, which lead to narrow sub-Doppler
resonances in absorption of the probe monochromatic light beam. Resolution of
this spectroscopic method is analyzed in cases of stationary and definite
nonstationary optical pumping of atoms by the broadband radiation versus
geometrical parameters of given cells and pumping intensity. The suggested
multilayer gas cell is the compact analog of many parallel atomic (molecular)
beams and may be used also as the basis of new compact optical frequency
standards of high accuracy.Comment: 12 pages, 4 figure
Migrations and formation of the Volga Bulgarian State (VIII-X C.): Results of the complex analysis
© Serials Publications. The formation of the medieval Bulgarian state has always attracted the attention of researchers, as this event provided the Volga-Ural region's breakthrough to civilization and became an important stage in social, cultural and ethno-political history of the region. Relocation of the Bulgarian tribes living on the right bank of the Middle Don at the end of VII century to the Middle Volga was determining in this process. There they contacted with different communities and tribes, both local and from the Central Asia. Bulgarians sought to create their own statehood. With the weakening of the Khazars' power and the sharp increasing of the international trade on the Great Volga route in the IX century the Bulgarians were gradually establishing their hegemony over the population of the Middle Volga in the confrontation with two other tribal associations. A new phase of folding the foundations of the Bulgarian feudal state began in early X century. During this period there was an active urban growth and international trade along the Great Volga route. At the same time Islam was adopted as the state religion. Bulgarians led by the elteber Almysh (who became a supreme corporate landowner) started to be the military-service class in the new state.922 was the year of diplomatic recognition of Bulgaria. It exchanged embassies with the Baghdad Caliphate. Cities were developing as political, administrative, trade and handicraft centers. During this period a number of elements from the other ethnic groups (Slavic, Finno-Ugrian and Scandinavian) enter into the Bulgarian squad and its own syncretic culture is produced. But in 940two emirates Bulgar and Suvar were formed. Their state institutions strengthened and Islam spread. The defeat of the Khazar Khaganate (in 980) led to strengthening of the Bulgarians united in a Bulgarian state
Tetra-AML: Automatic Machine Learning via Tensor Networks
Neural networks have revolutionized many aspects of society but in the era of
huge models with billions of parameters, optimizing and deploying them for
commercial applications can require significant computational and financial
resources. To address these challenges, we introduce the Tetra-AML toolbox,
which automates neural architecture search and hyperparameter optimization via
a custom-developed black-box Tensor train Optimization algorithm, TetraOpt. The
toolbox also provides model compression through quantization and pruning,
augmented by compression using tensor networks. Here, we analyze a unified
benchmark for optimizing neural networks in computer vision tasks and show the
superior performance of our approach compared to Bayesian optimization on the
CIFAR-10 dataset. We also demonstrate the compression of ResNet-18 neural
networks, where we use 14.5 times less memory while losing just 3.2% of
accuracy. The presented framework is generic, not limited by computer vision
problems, supports hardware acceleration (such as with GPUs and TPUs) and can
be further extended to quantum hardware and to hybrid quantum machine learning
models
Theoretical study of dark resonances in micro-metric thin cells
We investigate theoretically dark resonance spectroscopy for a dilute atomic
vapor confined in a thin (micro-metric) cell. We identify the physical
parameters characterizing the spectra and study their influence. We focus on a
Hanle-type situation, with an optical irradiation under normal incidence and
resonant with the atomic transition. The dark resonance spectrum is predicted
to combine broad wings with a sharp maximum at line-center, that can be singled
out when detecting a derivative of the dark resonance spectrum. This narrow
signal derivative, shown to broaden only sub-linearly with the cell length, is
a signature of the contribution of atoms slow enough to fly between the cell
windows in a time as long as the characteristic ground state optical pumping
time. We suggest that this dark resonance spectroscopy in micro-metric thin
cells could be a suitable tool for probing the effective velocity distribution
in the thin cell arising from the atomic desorption processes, and notably to
identify the limiting factors affecting desorption under a grazing incidence.Comment: 12 pages, 11 figures theoretical articl
A Nonsingular Brans Wormhole: An Analogue to Naked Black Holes
In a recent paper, we showed the Jordan frame vacuum Brans Class I solution
provided a wormhole analogue to Horowitz-Ross naked black hole in the wormhole
range -3/2<{\omega}<-4/3. Thereafter, the solution has been criticized by some
authors that, because of the presence of singularity in that solution within
this range, a wormhole interpretation of it is untenable. While the criticism
is correct, we show here that (i) a singularity-free wormhole can actually be
obtained from Class I solution by performing a kind of Wick rotation on it,
resulting into what Brans listed as his independent Class II solution (ii) the
Class II solution has all the necessary properties of a regular wormhole in a
revised range -2<{\omega}<-3/2 and finally, (iii) naked black holes, as
described by Horowitz and Ross, are spacetimes where the tidal forces attain
their maxima above the black hole horizon. We show that in the non-singular
Class II spacetime this maxima is attained above the throat and thus can be
treated as a wormhole analogue. Some related issues are also addressed.Comment: 20 pages, 4 figure
A Brownian Model for Crystal Nucleation
In this work a phenomenological stochastic differential equation is proposed
to model the time evolution of the radius of a pre-critical molecular cluster
during nucleation (the classical order parameter). Such a stochastic
differential equation constitutes the basis for the calculation of the
(nucleation) induction time under Kramers' theory of thermally activated escape
processes. Considering the nucleation stage as a Poisson rare-event, analytical
expressions for the induction time statistics are deduced for both steady and
unsteady conditions, the latter assuming the semiadiabatic limit. These
expressions can be used to identify the underlying mechanism of molecular
cluster formation (distinguishing between homogeneous or heterogeneous
nucleation from the nucleation statistics is possible) as well as to predict
induction times and induction time distributions. The predictions of this model
are in good agreement with experimentally measured induction times at constant
temperature, unlike the values obtained from the classical equation, but
agreement is not so good for induction time statistics. Stochastic simulations
truncated to the maximum waiting time of the experiments confirm that this fact
is due to the time constraints imposed by experiments. Correcting for this
effect, the experimental and predicted curves fit remarkably well. Thus, the
proposed model seems to be a versatile tool to predict cluster size
distributions, nucleation rates, (nucleation) induction time and induction time
statistics for a wide range of conditions (e.g. time-dependent temperature,
supersaturation, pH, etc.) where classical nucleation theory is of limited
applicability.Comment: 20 pages, 3 figure
Dynamics of false vacuum bubbles in Brans-Dicke theory
We study the dynamics of false vacuum bubbles in the Brans-Dicke theory of
gravity by using the thin shell or thin wall approximation. We consider a false
vacuum bubble that has a different value for the Brans-Dicke field between the
inside false vacuum region and the outside true vacuum region. Within a certain
limit of field values, the difference of field values makes the effective
tension of the shell negative. This allows new expanding false vacuum bubbles
to be seen by the outside observer, which are disallowed in Einstein gravity.Comment: 29 pages, 20 figure
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