497 research outputs found
Ab-initio determination of Bose-Hubbard parameters for two ultracold atoms in an optical lattice using a three-well potential
We calculate numerically the exact energy spectrum of the six dimensional
problem of two interacting Bosons in a three-well optical lattice. The
particles interact via a full Born-Oppenheimer potential which can be adapted
to model the behavior of the s-wave scattering length at Feshbach resonances.
By adjusting the parameters of the corresponding Bose-Hubbard (BH) Hamiltonian
the deviation between the numerical energy spectrum and the BH spectrum is
minimized. This defines the optimal BH parameter set which we compare to the
standard parameters of the BH model. The range of validity of the BH model with
these parameter sets is examined, and an improved analytical prediction of the
interaction parameter is introduced. Furthermore, an extended BH model and
implications due to the energy dependence of the scattering length and
couplings to higher Bloch bands at a Feshbach resonance are discussed.Comment: 14 pages, 11 figures; typos and minor errors corrected, five
references added, next-to-nearest neighbor hopping included in extended
Bose-Hubbard mode
Nonperturbative Corrections to One Gluon Exchange Quark Potentials
The leading nonperturbative QCD corrections to the one gluon exchange
quark-quark, quark-antiquark and pair-excitation potentials are
derived by using a covariant form of nonlocal two-quark and two-gluon vacuum
expectation values. Our numerical calculation indicates that the correction of
quark and gluon condensates to the quark-antiquark potential improves the heavy
quarkonium spectra to some degree.Comment: LaTex, 16 pages, three figures, to appear in Nucl. Phys.
Error matrices in quantum process tomography
We discuss characterization of experimental quantum gates by the error
matrix, which is similar to the standard process matrix in the Pauli
basis, except the desired unitary operation is factored out, by formally
placing it either before or after the error process. The error matrix has only
one large element, which is equal to the process fidelity, while other elements
are small and indicate imperfections. The imaginary parts of the elements along
the left column and/or top row directly indicate the unitary imperfection and
can be used to find the needed correction. We discuss a relatively simple way
to calculate the error matrix for a composition of quantum gates. Similarly, it
is rather straightforward to find the first-order contribution to the error
matrix due to the Lindblad-form decoherence. We also discuss a way to identify
and subtract the tomography procedure errors due to imperfect state preparation
and measurement. In appendices we consider several simple examples of the
process tomography and also discuss an intuitive physical interpretation of the
Lindblad-form decoherence.Comment: 21 pages (slightly revised version
Selection-bias correction based on the multinomial logit: An application to the Mexican labor market
In this presentation, we illustrate an application of a relatively new selection-bias correction methodology based on the multinomial logit model using the selmlog Stata command (Bourguignon, Fournier, and Gurgand, 2007, Journal of Economic Surveys 21: 174–205). selmlog allows for getting both consistent and efficient estimates of the selection process and a fairly good correction for the outcome equation, even when the independence of irrelevant alternatives (IIA) assumption is not achieved. The exercise depicts the current pattern of the occupational choices for the individuals in the Mexican labor market using a longitudinal panel with microdata from the Encuesta Nacional de Ocupación y Empleo (ENOE) during February 2008 to March 2009. We estimate an equation over an endogenously selected population. The command grants simplicity for both distributional and IIA assumptions for parametric models.
Volume changes during active shape fluctuations in cells
Cells modify their volume in response to changes in osmotic pressure but it
is usually assumed that other active shape variations do not involve
significant volume fluctuations. Here we report experiments demonstrating that
water transport in and out of the cell is needed for the formation of blebs,
commonly observed protrusions in the plasma membrane driven by cortex
contraction. We develop and simulate a model of fluid mediated membrane-cortex
deformations and show that a permeable membrane is necessary for bleb formation
which is otherwise impaired. Taken together our experimental and theoretical
results emphasize the subtle balance between hydrodynamics and elasticity in
actively driven cell morphological changes.Comment: Phys. Rev. Lett. in press. 13 pages 4 figures, 9 supplementary
figure
The Dynamics of School and Work in Rural Bangladesh
This paper investigates the causes underlying the poor school performance of children in rural Bangladesh, while focusing on the effect of work on school progress. To this end, a dynamic switching model is presented for the sequence of school and work outcomes up to the end of secondary school, where the switching in each school level considered is determined by the endogenous work sequence up to that level. This approach allow us to characterize the full sequence of school and work choices of children, and to evaluate the dynamic effects of work on schooling. We find that work has a negative and sizable effect on school progress for the entire population, as well as for all the subpopulations considered, including the different groups of working children. We are also able to identify at each school level the observable and unobservable characteristics of working children relative to those of non-working children. This characterization and the magnitude of the estimated effects of work have important policy implications.dynamics of school and work, dynamic evaluation, selection
STRUCTURAL CHANGES AND CONVERGENCE IN EU AND IN ADRIATIC-BALKANS REGION
Coming from standard economic growth theory and empirical evidences, we concentrated on the convergence process as a result of structural changes in economy. We investigate the differences among countries in EU in terms of the share in total economy of main sectors. Then, based on the spatial (empirical) distribution of such shares in EU we are proposing a model to estimate a typology of the convergence process in the European area. Taking into account the existing differences among sectors in matter of productivity, there are two versions of the model: considering the share of sectors in total employment and the share of sectors in GDP respectively. Moreover, we developed several modelling schemes that could be useful to improve the strategies oriented to achieve a real convergence in EU and further in Adriatic-Balkans region. In this way, we can obtain simulations from a country or group of countries (European Union, for example) on long term and quantifying the impact of structural changes on the convergence process. Indeed, the actual global crisis seems to influence negatively the convergence process in EU. As a rule, just new adhered countries were more affected by the actual crisis. Today all forecasts are suffering by uncertainty. Thus, further efforts must be allocated to evaluate the negative impact of actual crisis on the convergence process.
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