36,708 research outputs found
Measurements continuous in time and a posteriori states in quantum
Measurements continuous in time were consistently introduced in quantum
mechanics and applications worked out, mainly in quantum optics. In this
context a quantum filtering theory has been developed giving the reduced state
after the measurement when a certain trajectory of the measured observables is
registered (the a posteriori states). In this paper a new derivation of
filtering equations is presented, in the cases of counting processes and of
measurement processes of diffusive type. It is also shown that the equation for
the a posteriori dynamics in the diffusive case can be obtained, by a suitable
limit, from that one in the counting case. Moreover, the paper is intended to
clarify the meaning of the various concepts involved and to discuss the
connections among them. As an illustration of the theory, simple models are
worked out.Comment: 31 page. See also related papers at
http://www.maths.nott.ac.uk/personal/vpb/research/mes_fou.html and
http://www.maths.nott.ac.uk/personal/vpb/research/fil_con.htm
Quantum wires in magnetic field: A comparative study of the Hartree-Fock and the spin density functional approaches
We present a detailed comparison of the self-consistent calculations based on
the Hartree-Fock and the spin density functional theory for a spit-gate quantum
wire in the IQH regime. We demonstrate that both approaches provide
qualitatively (and in most cases quantitatively) similar results for the
spin-resolved electron density, spin polarization, spatial spin separation at
the edges and the effective factor. The both approach give the same values
of the magnetic fields corresponding to the successive subband depopulation and
qualitatively similar evolution of the magnetosubbands. Quantitatively,
however, the HF and the DFT subbands are different (even though the
corresponding total electron densities are practically the same). In contrast
to the HF approach, the DFT calculations predict much larger spatial spin
separation near the wire edge for the low magnetic fields (when the
compressible strips for spinless electrons are not formed yet). In the opposite
limit of the large fields, the Hatree-Fock and the DFT approaches give very
similar values for the spatial spin separation.Comment: 5 pages, 3 figure
Statistics of Neutron Stars at the Stage of Supersonic Propeller
We analyze the statistical distribution of neutron stars at the stage of a
supersonic propeller. An important point of our analysis is allowance for the
evolution of the angle of inclination of the magnetic axis to the spin axis of
the neutron star for the boundary of the transition to the supersonic propeller
stage for two models: the model with hindered particle escape from the stellar
surface and the model with free particle escape. As a result, we have shown
that a consistent allowance for the evolution of the inclination angle in the
region of extinct radio pulsars for the two models leads to an increase in the
total number of neutron stars at the supersonic propeller stage. This increase
stems from he fact that when allowing for the evolution of the inclination
angle for neutron stars in the region of extinct radio pulsars and,
hence, for the boundary of the transition to the propeller stage, this
transition is possible at shorter spin periods (P~5-10 s) than assumed in the
standard model.Comment: 15 pages, 6 figures; scale corrected for figures 3-
The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star
The propeller regime of disk accretion to a rapidly rotating magnetized star
is investigated here for the first time by axisymmetric 2.5D
magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in
which the magnetic field is predominantly toroidal. A smaller fraction of the
star's poloidal magnetic flux inflates vertically, forming a magnetically
dominated tower. Matter accumulates in the equatorial region outside
magnetosphere and accretes to the star quasi-periodically through elongated
funnel streams which cause the magnetic field to reconnect. The star spins-down
owing to the interaction of the closed magnetosphere with the disk. For the
considered conditions, the spin-down torque varies with the angular velocity of
the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller
stage may be important in the evolution of X-ray pulsars, cataclysmic variables
and young stars. In particular, it may explain the present slow rotation of the
classical T Tauri stars.Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are
available at http://www.astro.cornell.edu/us-russia/disk_prop.ht
A semantic web service-based architecture for the interoperability of e-government services
We propose a semantically-enhanced architecture to address the issues of interoperability and service integration in e-government web information systems. An architecture for a life event portal based on Semantic Web Services (SWS) is described. The architecture includes loosely-coupled modules organized in three distinct layers: User Interaction, Middleware and Web Services. The Middleware provides the semantic infrastructure for ontologies and SWS. In particular a conceptual model for integrating domain knowledge (Life Event Ontology), application knowledge (E-government Ontology) and service description (Service Ontology) is defined. The model has been applied to a use case scenario in e-government and the results of a system prototype have been reported to demonstrate some relevant features of the proposed approach
Supervised learning of an opto-magnetic neural network with ultrashort laser pulses
The explosive growth of data and its related energy consumption is pushing
the need to develop energy-efficient brain-inspired schemes and materials for
data processing and storage. Here, we demonstrate experimentally that Co/Pt
films can be used as artificial synapses by manipulating their magnetization
state using circularly-polarized ultrashort optical pulses at room temperature.
We also show an efficient implementation of supervised perceptron learning on
an opto-magnetic neural network, built from such magnetic synapses.
Importantly, we demonstrate that the optimization of synaptic weights can be
achieved using a global feedback mechanism, such that the learning does not
rely on external storage or additional optimization schemes. These results
suggest there is high potential for realizing artificial neural networks using
optically-controlled magnetization in technologically relevant materials, that
can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure
Generic suppression of conductance quantization of interacting electrons in graphene nanoribbons in a perpendicular magnetic field
The effects of electron interaction on the magnetoconductance of graphene
nanoribbons (GNRs) are studied within the Hartree approximation. We find that a
perpendicular magnetic field leads to a suppression instead of an expected
improvement of the quantization. This suppression is traced back to
interaction-induced modifications of the band structure leading to the
formation of compressible strips in the middle of GNRs. It is also shown that
the hard wall confinement combined with electron interaction generates overlaps
between forward and backward propagating states, which may significantly
enhance backscattering in realistic GNRs. The relation to available experiments
is discussed.Comment: 4 pages, 3 figure
Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics
The mechanism of ablation of solids by intense femtosecond laser pulses is
described in an explicit analytical form. It is shown that at high intensities
when the ionization of the target material is complete before the end of the
pulse, the ablation mechanism is the same for both metals and dielectrics. The
physics of this new ablation regime involves ion acceleration in the
electrostatic field caused by charge separation created by energetic electrons
escaping from the target. The formulae for ablation thresholds and ablation
rates for metals and dielectrics, combining the laser and target parameters,
are derived and compared to experimental data. The calculated dependence of the
ablation thresholds on the pulse duration is in agreement with the experimental
data in a femtosecond range, and it is linked to the dependence for nanosecond
pulses.Comment: 27 pages incl.3 figs; presented at CLEO-Europe'2000 11-15 Sept.2000;
papers QMD6 and CTuK11
Non-Weyl asymptotics for quantum graphs with general coupling conditions
Inspired by a recent result of Davies and Pushnitski, we study resonance
asymptotics of quantum graphs with general coupling conditions at the vertices.
We derive a criterion for the asymptotics to be of a non-Weyl character. We
show that for balanced vertices with permutation-invariant couplings the
asymptotics is non-Weyl only in case of Kirchhoff or anti-Kirchhoff conditions,
while for graphs without permutation numerous examples of non-Weyl behaviour
can be constructed. Furthermore, we present an insight helping to understand
what makes the Kirchhoff/anti-Kirchhoff coupling particular from the resonance
point of view. Finally, we demonstrate a generalization to quantum graphs with
nonequal edge weights.Comment: minor changes, to appear in Pierre Duclos memorial issue of J. Phys.
A: Math. Theo
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