52 research outputs found
The quantum speed up as advanced knowledge of the solution
With reference to a search in a database of size N, Grover states: "What is
the reason that one would expect that a quantum mechanical scheme could
accomplish the search in O(square root of N) steps? It would be insightful to
have a simple two line argument for this without having to describe the details
of the search algorithm". The answer provided in this work is: "because any
quantum algorithm takes the time taken by a classical algorithm that knows in
advance 50% of the information that specifies the solution of the problem".
This empirical fact, unnoticed so far, holds for both quadratic and exponential
speed ups and is theoretically justified in three steps: (i) once the physical
representation is extended to the production of the problem on the part of the
oracle and to the final measurement of the computer register, quantum
computation is reduction on the solution of the problem under a relation
representing problem-solution interdependence, (ii) the speed up is explained
by a simple consideration of time symmetry, it is the gain of information about
the solution due to backdating, to before running the algorithm, a
time-symmetric part of the reduction on the solution; this advanced knowledge
of the solution reduces the size of the solution space to be explored by the
algorithm, (iii) if I is the information acquired by measuring the content of
the computer register at the end of the algorithm, the quantum algorithm takes
the time taken by a classical algorithm that knows in advance 50% of I, which
brings us to the initial statement.Comment: 23 pages, to be published in IJT
Electronic Structure of Transition-Metal Dicyanamides Me[N(CN)] (Me = Mn, Fe, Co, Ni, Cu)
The electronic structure of Me[N(CN)] (Me=Mn, Fe, Co, Ni, Cu)
molecular magnets has been investigated using x-ray emission spectroscopy (XES)
and x-ray photoelectron spectroscopy (XPS) as well as theoretical
density-functional-based methods. Both theory and experiments show that the top
of the valence band is dominated by Me 3d bands, while a strong hybridization
between C 2p and N 2p states determines the valence band electronic structure
away from the top. The 2p contributions from non-equivalent nitrogen sites have
been identified using resonant inelastic x-ray scattering spectroscopy with the
excitation energy tuned near the N 1s threshold. The binding energy of the Me
3d bands and the hybridization between N 2p and Me 3d states both increase in
going across the row from Me = Mn to Me = Cu. Localization of the Cu 3d states
also leads to weak screening of Cu 2p and 3s states, which accounts for shifts
in the core 2p and 3s spectra of the transition metal atoms. Calculations
indicate that the ground-state magnetic ordering, which varies across the
series is largely dependent on the occupation of the metal 3d shell and that
structural differences in the superexchange pathways for different compounds
play a secondary role.Comment: 20 pages, 11 figures, 2 table
Fermionic SK-models with Hubbard interaction: Magnetism and electronic structure
Models with range-free frustrated Ising spin- and Hubbard interaction are
treated exactly by means of the discrete time slicing method. Critical and
tricritical points, correlations, and the fermion propagator, are derived as a
function of temperature T, chemical potential \mu, Hubbard coupling U, and spin
glass energy J. The phase diagram is obtained. Replica symmetry breaking
(RSB)-effects are evaluated up to four-step order (4RSB). The use of exact
relations together with the 4RSB-solutions allow to model exact solutions by
interpolation. For T=0, our numerical results provide strong evidence that the
exact density of states in the spin glass pseudogap regime obeys \rho(E)=const
|E-E_F| for energies close to the Fermi level. Rapid convergence of \rho'(E_F)
under increasing order of RSB is observed. The leading term resembles the
Efros-Shklovskii Coulomb pseudogap of localized disordered fermionic systems in
2D. Beyond half filling we obtain a quadratic dependence of the fermion filling
factor on the chemical potential. We find a half filling transition between a
phase for U>\mu, where the Fermi level lies inside the Hubbard gap, into a
phase where \mu(>U) is located at the center of the upper spin glass pseudogap
(SG-gap). For \mu>U the Hubbard gap combines with the lower one of two SG-gaps
(phase I), while for \mu<U it joins the sole SG-gap of the half-filling regime
(phase II). We predict scaling behaviour at the continuous half filling
transition. Implications of the half-filling transition between the deeper
insulating phase II and phase I for delocalization due to hopping processes in
itinerant model extensions are discussed and metal-insulator transition
scenarios described.Comment: 29 pages, 26 Figures, 4 jpeg- and 3 gif-Fig-files include
Ultrafast Nonlinear Optical Response of Strongly Correlated Systems: Dynamics in the Quantum Hall Effect Regime
We present a theoretical formulation of the coherent ultrafast nonlinear
optical response of a strongly correlated system and discuss an example where
the Coulomb correlations dominate. We separate out the correlated contributions
to the third-order nonlinear polarization, and identify non-Markovian dephasing
effects coming from the non-instantaneous interactions and propagation in time
of the collective excitations of the many-body system. We discuss the
signatures, in the time and frequency dependence of the four-wave-mixing (FWM)
spectrum, of the inter-Landau level magnetoplasmon (MP) excitations of the
two-dimensional electron gas (2DEG) in a perpendicular magnetic field. We
predict a resonant enhancement of the lowest Landau level (LL) FWM signal, a
strong non-Markovian dephasing of the next LL magnetoexciton (X), a symmetric
FWM temporal profile, and strong oscillations as function of time delay, of
quantum kinetic origin. We show that the correlation effects can be controlled
experimentally by tuning the central frequency of the optical excitation
between the two lowest LLs.Comment: 21 pages, 10 figure
Studies of Hadronic Event Structure in e+e- Annihilation from 30 GeV to 209 GeV with the L3 Detector
In this Report, QCD results obtained from a study of hadronic event structure
in high energy e^+e^- interactions with the L3 detector are presented. The
operation of the LEP collider at many different collision energies from 91 GeV
to 209 GeV offers a unique opportunity to test QCD by measuring the energy
dependence of different observables. The main results concern the measurement
of the strong coupling constant, \alpha_s, from hadronic event shapes and the
study of effects of soft gluon coherence through charged particle multiplicity
and momentum distributions.Comment: To appear in Physics Report
Polymorphism: an evaluation of the potential risk to the quality of drug products from the FarmĂĄcia Popular Rede PrĂłpria
Predicting intensive care unit readmissions using probabilistic fuzzy systems
We propose the application of probabilistic fuzzy systems (PFS) to model the prediction of early readmission in intensive care unit patients and compare it with the gold-standard method - logistic regression based on the APACHE II score. PFS are characterized by the combination of the linguistic description of the system with the statistical properties of data. On one hand, results point that PFS models perform comparably to the gold-standard method, with AUC values of 0.66±0.03. On the other hand, results also show that PFS models use a significant lower number of variables which, from the clinical practice point of view, suggests improved gains in terms of simplicit
Mortality prediction of septic shock patients using probabilistic fuzzy systems
Mortality scores based on multiple regressions are common in critical care medicine for prognostic stratification of patients. However, to be used at the point of care, they need to be both accurate and easily interpretable. In this work, we propose the application of one existent type of rule base system using statistical information â probabilistic fuzzy systems (PFS) â to predict mortality of septic shock patients. To assess its accuracy and interpretability, these models are compared to methodologies previously proposed in this domain: Takagi-Sugeno fuzzy models and logistic regression models. The methods are tested using a retrospective cohort study including ICU patients with abdominal septic shock. Regarding accuracy, PFS models are comparable to fuzzy modeling and logistic regression. In terms of interpretability, results indicate that PFS models increase the transparency of the learned system (using fuzzy rules), but at the same time, provide additional means for validating the fuzzy classifier using expert knowledge (from physicians in this paper). By providing accurate and interpretable estimates for the mortality risk, results suggest the usefulness of PFS to develop scores for critical care medicine
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