7,841 research outputs found
Reconstruction of Hamiltonians from given time evolutions
In this paper we propose a systematic method to solve the inverse dynamical
problem for a quantum system governed by the von Neumann equation: to find a
class of Hamiltonians reproducing a prescribed time evolution of a pure or
mixed state of the system. Our approach exploits the equivalence between an
action of the group of evolution operators over the state space and an adjoint
action of the unitary group over Hermitian matrices. The method is illustrated
by two examples involving a pure and a mixed state.Comment: 14 page
Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case
We present a study on the determination of rotation periods (P) of solar-like
stars from the photometric irregular time-sampling of the ESA Gaia mission,
currently scheduled for launch in 2013, taking into account its dependence on
ecliptic coordinates. We examine the case of solar-twins as well as thousands
of synthetic time-series of solar-like stars rotating faster than the Sun. In
the case of solar twins we assume that the Gaia unfiltered photometric passband
G will mimic the variability of the total solar irradiance (TSI) as measured by
the VIRGO experiment. For stars rotating faster than the Sun, light-curves are
simulated using synthetic spectra for the quiet atmosphere, the spots, and the
faculae combined by applying semi-empirical relationships relating the level of
photospheric magnetic activity to the stellar rotation and the Gaia
instrumental response. The capabilities of the Deeming, Lomb-Scargle, and Phase
Dispersion Minimisation methods in recovering the correct rotation periods are
tested and compared. The false alarm probability (FAP) is computed using Monte
Carlo simulations and compared with analytical formulae. The Gaia scanning law
makes the rate of correct detection of rotation periods strongly dependent on
the ecliptic latitude (beta). We find that for P ~ 1 d, the rate of correct
detection increases with ecliptic latitude from 20-30 per cent at beta ~
0{\deg} to a peak of 70 per cent at beta=45{\deg}, then it abruptly falls below
10 per cent at beta > 45{\deg}. For P > 5 d, the rate of correct detection is
quite low and for solar twins is only 5 per cent on average.Comment: 12 pages, 18 figures, accepted by MNRA
Morals From Rationality Alone? Some Doubts
Contractarians aim to derive moral principles from the dictates of instrumental rationality alone. But it is well-known that contractarian moral theories struggle to identify normative principles that are both uniquely rational and morally compelling. Michael Moehler's recent book, *Minimal Morality* seeks to avoid these difficulties by developing a novel "two-level" social contract theory, which restricts the scope of contractarian morality to cases of deep and persistent moral disagreement. Yet Moehler remains ambitious, arguing that a restricted version of Kant's categorical imperative is a uniquely rational principle of conflict resolution. We develop a formal model of Moehler's informal game-theoretic argument, which reconstructs a valid argument for Moehler's conclusion. This model, in turn, enables us to expose how a successful argument for Moehler's contractarian principle rests on assumptions that can only be justified by subtle yet significant departures from the standard conception of rationality. We thus extend our understanding of familiar contractarian difficulties by showing how they arise even if we restrict the scope of contractarian morality to a domain where its application seems both promising and necessary. We show that the problem lies not in contractarians' immodest ambitions but in the modest resources rationality can offer to satisfy them
Hydrogen-Related Conversion Processes of Ge-Related Point Defects in Silica Triggered by UV Laser Irradiation
The conversion processes of Ge-related point defects triggered in amorphous
SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the
interplay between the (=Ge•-H) H(II) center and the twofold coordinated
Ge defect (=Ge••). The former is generated in the post-irradiation
stage, while the latter decays both during and after exposure. The
post-irradiation decay kinetics of =Ge•• is isolated and found to
be anti-correlated to the growth of H(II), at least at short times. From this
finding it is suggested that both processes are due to trapping of radiolytic
H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is
preserved also under repeated irradiation: light at 4.7eV destroys the already
formed H(II) centers and restore their precursors =Ge••. This
process leads to repeatability of the post-irradiation kinetics of the two
species after multiple laser exposures. A comprehensive scheme of chemical
reactions explaining the observed post-irradiation processes is proposed and
tested against experimental data.Comment: 25 pages, 7 figures, submitted to Phys. Rev.
Quantum theory of heating of a single trapped ion
The heating of trapped ions due to the interaction with a {\it quantized
environment} is studied {\it without performing the Born-Markov approximation}.
A generalized master equation local in time is derived and a novel theoretical
approach to solve it analytically is proposed. Our master equation is in the
Lindblad form with time dependent coefficients, thus allowing the simulation of
the dynamics by means of the Monte Carlo Wave Function (MCWF) method.Comment: 4 pages, 3 figure
Misbelief and misunderstandings on the non--Markovian dynamics of a damped harmonic oscillator
We use the exact solution for the damped harmonic oscillator to discuss some
relevant aspects of its open dynamics often mislead or misunderstood. We
compare two different approximations both referred to as Rotating Wave
Approximation. Using a specific example, we clarify some issues related to
non--Markovian dynamics, non--Lindblad type dynamics, and positivity of the
density matrix.Comment: 6 pages, 2 figures, added info: submitted to J. Opt. B: Quantum and
Semiclass. Opt., Special Issue of the 10th Central European Workshop on
Quantum Optics, reference added, discussion clarifie
Macroion adsorption: The crucial role of excluded volume and coions
The adsorption of charged colloids (macroions) onto an oppositely charged
planar substrate is investigated theoretically. Taking properly into account
the finite size of the macroions, unusual behaviors are reported. It is found
that the role of the coions (the little salt-ions carrying the same sign of
charge as that of the substrate) is crucial to understand the mechanisms
involved in the process of macroion adsorption. In particular, the coions can
accumulate near the substrate's surface and lead to a counter-intuitive {\it
surface charge amplification}.Comment: 11 pages - 4 figures. To appear in JC
N-qubit states as points on the Bloch sphere
We show how the Majorana representation can be used to express the pure
states of an N-qubit system as points on the Bloch sphere. We compare this
geometrical representation of N-qubit states with an alternative one, proposed
recently by the present authors.Comment: 9 pages, 2 figures, contribution to CEWQO 2009 proceedings. v2: Minor
changes, published versio
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
Reentrant transitions in colloidal or dusty plasma bilayers
The phase diagram of crystalline bilayers of particles interacting via a
Yukawa potential is calculated for arbitrary screening lengths and particle
densities. Staggered rectangular, square, rhombic and triangular structures are
found to be stable including a first-order transition between two different
rhombic structures. For varied screening length at fixed density, one of these
rhombic phases exhibits both a single and even a double reentrant transition.
Our predictions can be verified experimentally in strongly confined charged
colloidal suspensions or dusty plasma bilayers.Comment: 4 pages, 3 eps figs - revtex4. PRL - in pres
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