36,290 research outputs found
Optimization of Short Coherent Control Pulses
The coherent control of small quantum system is considered. For a two-level
system coupled to an arbitrary bath we consider a pulse of finite duration. We
derive the leading and the next-leading order corrections to the evolution
operator due to the non-commutation of the pulse and the bath Hamiltonian. The
conditions are computed that make the leading corrections vanish. The pulse
shapes optimized in this way are given for and pulses.Comment: 9 pages, 6 figures; published versio
TDC Chip and Readout Driver Developments for COMPASS and LHC-Experiments
A new TDC-chip is under development for the COMPASS experiment at CERN. The
ASIC, which exploits the 0.6 micrometer CMOS sea-of-gate technology, will allow
high resolution time measurements with digitization of 75 ps, and an
unprecedented degree of flexibility accompanied by high rate capability and low
power consumption. Preliminary specifications of this new TDC chip are
presented.
Furthermore a FPGA based readout-driver and buffer-module as an interface
between the front-end of the COMPASS detector systems and an optical S-LINK is
in development. The same module serves also as remote fan-out for the COMPASS
trigger distribution and time synchronization system. This readout-driver
monitors the trigger and data flow to and from front-ends. In addition, a
specific data buffer structure and sophisticated data flow control is used to
pursue local pre-event building. At start-up the module controls all necessary
front-end initializations.Comment: 5 pages, 4 figure
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen
We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.Comment: 10 pages, 5 figure
An Introduction to Conformal Ricci Flow
We introduce a variation of the classical Ricci flow equation that modifies
the unit volume constraint of that equation to a scalar curvature constraint.
The resulting equations are named the Conformal Ricci Flow Equations because of
the role that conformal geometry plays in constraining the scalar curvature.
These equations are analogous to the incompressible Navier-Stokes equations of
fluid mechanics inasmuch as a conformal pressure arises as a Lagrange
multiplier to conformally deform the metric flow so as to maintain the scalar
curvature constraint. The equilibrium points are Einstein metrics with a
negative Einstein constant and the conformal pressue is shown to be zero at an
equilibrium point and strictly positive otherwise. The geometry of the
conformal Ricci flow is discussed as well as the remarkable analytic fact that
the constraint force does not lose derivatives and thus analytically the
conformal Ricci equation is a bounded perturbation of the classical
unnormalized Ricci equation. That the constraint force does not lose
derivatives is exactly analogous to the fact that the real physical pressure
force that occurs in the Navier-Stokes equations is a bounded function of the
velocity. Using a nonlinear Trotter product formula, existence and uniqueness
of solutions to the conformal Ricci flow equations is proven. Lastly, we
discuss potential applications to Perelman's proposed implementation of
Hamilton's program to prove Thurston's 3-manifold geometrization conjectures.Comment: 52 pages, 1 figur
Electroweak and finite width corrections to top quark decays into transverse and longitudinal -bosons
We calculate the electroweak and finite width corrections to the decay of an
unpolarized top quark into a bottom quark and a -gauge boson where the
helicities of the are specified as longitudinal, transverse-plus and
transverse-minus. Together with the corrections these
corrections may become relevant for the determination of the mass of the top
quark through angular decay measurements.Comment: 4 pages, 7 postscript figures adde
Magnetic bound states in the quarter-filled ladder system }
Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5
shows in the dimerized singlet ground state () an unexpected
sequence of three magnetic bound states. Our results suggest that the recently
proposed mapping onto an effective spin chain for has to be given
up in favor of the full topology and exchange paths of a ladder in the
dimerized phase for . As the new ground state we propose a dynamic
superposition of energetically nearly degenerate dimer configurations on the
ladder.Comment: 5 pages, 4 figures, to be published in PRB, brief reports, Dec. 199
Future asymptotic expansions of Bianchi VIII vacuum metrics
Bianchi VIII vacuum solutions to Einstein's equations are causally
geodesically complete to the future, given an appropriate time orientation, and
the objective of this article is to analyze the asymptotic behaviour of
solutions in this time direction. For the Bianchi class A spacetimes, there is
a formulation of the field equations that was presented in an article by
Wainwright and Hsu, and in a previous article we analyzed the asymptotic
behaviour of solutions in these variables. One objective of this paper is to
give an asymptotic expansion for the metric. Furthermore, we relate this
expansion to the topology of the compactified spatial hypersurfaces of
homogeneity. The compactified spatial hypersurfaces have the topology of
Seifert fibred spaces and we prove that in the case of NUT Bianchi VIII
spacetimes, the length of a circle fibre converges to a positive constant but
that in the case of general Bianchi VIII solutions, the length tends to
infinity at a rate we determine.Comment: 50 pages, no figures. Erronous definition of Seifert fibred spaces
correcte
Localization length of a soliton from a non-magnetic impurity in a general double-spin-chain model
A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ( model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length is found to obey a power law of the
dimerization with an exponent -1/3; .
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.Comment: 4 pages, REVTeX, Figures are in eps-file
Core collapse supernovae in the QCD phase diagram
We compare two classes of hybrid equations of state with a hadron-to-quark
matter phase transition in their application to core collapse supernova
simulations. The first one uses the quark bag model and describes the
transition to three-flavor quark matter at low critical densities. The second
one employs a Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with
parameters describing a phase transition to two-flavor quark matter at higher
critical densities. These models possess a distinctly different temperature
dependence of their transition densities which turns out to be crucial for the
possible appearance of quark matter in supernova cores. During the early post
bounce accretion phase quark matter is found only if the phase transition takes
place at sufficiently low densities as in the study based on the bag model. The
increase critical density with increasing temperature, as obtained for our PNJL
parametrization, prevents the formation of quark matter. The further evolution
of the core collapse supernova as obtained applying the quark bag model leads
to a structural reconfiguration of the central proto-neutron star where, in
addition to a massive pure quark matter core, a strong hydrodynamic shock wave
forms and a second neutrino burst is released during the shock propagation
across the neutrinospheres. We discuss the severe constraints in the freedom of
choice of quark matter models and their parametrization due to the recently
observed 2 solar mass pulsar and their implications for further studies of core
collapse supernovae in the QCD phase diagram.Comment: 19 pages, 4 figures, CPOD2010 conference proceedin
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