10,028 research outputs found
An exact representation of the fermion dynamics in terms of Poisson processes and its connection with Monte Carlo algorithms
We present a simple derivation of a Feynman-Kac type formula to study
fermionic systems. In this approach the real time or the imaginary time
dynamics is expressed in terms of the evolution of a collection of Poisson
processes. A computer implementation of this formula leads to a family of
algorithms parametrized by the values of the jump rates of the Poisson
processes. From these an optimal algorithm can be chosen which coincides with
the Green Function Monte Carlo method in the limit when the latter becomes
exact.Comment: 4 pages, 1 PostScript figure, REVTe
Exact ground state for a class of matrix Hamiltonian models: quantum phase transition and universality in the thermodynamic limit
By using a recently proposed probabilistic approach, we determine the exact
ground state of a class of matrix Hamiltonian models characterized by the fact
that in the thermodynamic limit the multiplicities of the potential values
assumed by the system during its evolution are distributed according to a
multinomial probability density. The class includes i) the uniformly fully
connected models, namely a collection of states all connected with equal
hopping coefficients and in the presence of a potential operator with arbitrary
levels and degeneracies, and ii) the random potential systems, in which the
hopping operator is generic and arbitrary potential levels are assigned
randomly to the states with arbitrary probabilities. For this class of models
we find a universal thermodynamic limit characterized only by the levels of the
potential, rescaled by the ground-state energy of the system for zero
potential, and by the corresponding degeneracies (probabilities). If the
degeneracy (probability) of the lowest potential level tends to zero, the
ground state of the system undergoes a quantum phase transition between a
normal phase and a frozen phase with zero hopping energy. In the frozen phase
the ground state condensates into the subspace spanned by the states of the
system associated with the lowest potential level.Comment: 31 pages, 13 figure
Comment on "Why quantum mechanics cannot be formulated as a Markov process"
In the paper with the above title, D. T. Gillespie [Phys. Rev. A 49, 1607,
(1994)] claims that the theory of Markov stochastic processes cannot provide an
adequate mathematical framework for quantum mechanics. In conjunction with the
specific quantum dynamics considered there, we give a general analysis of the
associated dichotomic jump processes. If we assume that Gillespie's
"measurement probabilities" \it are \rm the transition probabilities of a
stochastic process, then the process must have an invariant (time independent)
probability measure. Alternatively, if we demand the probability measure of the
process to follow the quantally implemented (via the Born statistical
postulate) evolution, then we arrive at the jump process which \it can \rm be
interpreted as a Markov process if restricted to a suitable duration time.
However, there is no corresponding Markov process consistent with the
event space assumption, if we require its existence for all times .Comment: Latex file, resubm. to Phys. Rev.
Analytical probabilistic approach to the ground state of lattice quantum systems: exact results in terms of a cumulant expansion
We present a large deviation analysis of a recently proposed probabilistic
approach to the study of the ground-state properties of lattice quantum
systems. The ground-state energy, as well as the correlation functions in the
ground state, are exactly determined as a series expansion in the cumulants of
the multiplicities of the potential and hopping energies assumed by the system
during its long-time evolution. Once these cumulants are known, even at a
finite order, our approach provides the ground state analytically as a function
of the Hamiltonian parameters. A scenario of possible applications of this
analyticity property is discussed.Comment: 26 pages, 5 figure
Analytical approximations of the dispersion relation of a linear chain ofmetal nanoparticles
We find some useful analytical approximations of the dispersion relation of a linear chain of metal nanoparticles in the subwavelength limit where the dipolar approximation can be used. We also approximate the group velocity without a direct estimation of the derivative of the dispersion relation, that carries unavoidable error amplifications. In the end we use these results in order to get some simple recipes that evaluate the sensitivity of the dispersion relation and the propagation losses with respect to the main
parameters of the chain
"All on short" prosthetic-implant supported rehabilitations
Objectives. Short implants are increasing their popularity among clinicians who want to fulfill the constant demanding of fixed prosthetic solutions in edentulous jaws. The aim of this report was to propose a new possibility to project and realize an occlusal guided implant cross-arch prosthesis supported by ultra-short implants, describing it presented an edentulous mandible case report. Methods. A 61-year-old, Caucasian, female patient who attended the dental clinic of the University of L’Aquila presented with edentulous posterior inferior jaw and periodontitis and periimplantitis processes in the anterior mandible. The remaining tooth and the affected implant were removed. Six 4-mm-long implants were placed to support a cross-arch metal-resin prosthesis. Results. At 1-year follow-up clinical and radiological assessment showed a good osseointegration of the fixtures and the patient was satisfied with the prosthesis solution. Conclusion. The method, even if it requires further validation, seems to be a valid aid in solving lower edentulous clinical cases, and appears less complex and with more indications of other proposals presented in the current clinical literature. Our case report differs from the current technique All-on-Four, which uses four implants in the mandible to support overdenture prosthesis, assuring a very promising clinical resul
Resonant, broadband and highly efficient optical frequency conversion in semiconductor nanowire gratings at visible and UV wavelengths
Using a hydrodynamic approach we examine bulk- and surface-induced second and
third harmonic generation from semiconductor nanowire gratings having a
resonant nonlinearity in the absorption region. We demonstrate resonant,
broadband and highly efficient optical frequency conversion: contrary to
conventional wisdom, we show that harmonic generation can take full advantage
of resonant nonlinearities in a spectral range where nonlinear optical
coefficients are boosted well beyond what is achievable in the transparent,
long-wavelength, non-resonant regime. Using femtosecond pulses with
approximately 500 MW/cm2 peak power density, we predict third harmonic
conversion efficiencies of approximately 1% in a silicon nanowire array, at
nearly any desired UV or visible wavelength, including the range of negative
dielectric constant. We also predict surface second harmonic conversion
efficiencies of order 0.01%, depending on the electronic effective mass,
bistable behavior of the signals as a result of a reshaped resonance, and the
onset fifth order nonlinear effects. These remarkable findings, arising from
the combined effects of nonlinear resonance dispersion, field localization, and
phase-locking, could significantly extend the operational spectral bandwidth of
silicon photonics, and strongly suggest that neither linear absorption nor skin
depth should be motivating factors to exclude either semiconductors or metals
from the list of useful or practical nonlinear materials in any spectral range.Comment: 12 pages, 4 figure
Experimental test of the no signaling theorem
In 1981 N. Herbert proposed a gedanken experiment in order to achieve by the
''First Laser Amplified Superluminal Hookup'' (FLASH) a faster than light
communication (FTL) by quantum nonlocality. The present work reports the first
experimental realization of that proposal by the optical parametric
amplification of a single photon belonging to an entangled EPR pair into an
output field involving 5 x 10^3 photons. A thorough theoretical and
experimental analysis explains in general and conclusive terms the precise
reasons for the failure of the FLASH program as well as of any similar FTL
proposals.Comment: 4 pages, 4 figure
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