1,046 research outputs found
Evaluation of accuracy of complete-arch multiple-unit abutment-level dental implant impressions using different impression and splinting materials.
Purpose: This in vitro study evaluated the accuracy of multiple-unit dental implant casts obtained from splinted or nonsplinted direct impression techniques using various splinting materials by comparing the casts to the reference models. The effect of two different impression materials on the accuracy of the implant casts was also evaluated for abutment-level impressions. Materials and Methods: A reference model with six internal-connection implant replicas placed in the completely edentulous mandibular arch and connected to multi-base abutments was fabricated from heat-curing acrylic resin. Forty impressions of the reference model were made, 20 each with polyether (PE) and polyvinylsiloxane (PVS) impression materials using the open tray technique. The PE and PVS groups were further subdivided into four subgroups of five each on the bases of splinting type: no splinting, bite registration PE, bite registration addition silicone, or autopolymerizing acrylic resin. The positional accuracy of the implant replica heads was measured on the poured casts using a coordinate measuring machine to assess linear differences in interimplant distances in all three axes. The collected data (linear and three-dimensional [3D] displacement values) were compared with the measurements calculated on the reference resin model and analyzed with nonparametric tests (Kruskal-Wallis and Mann-Whitney). Results: No significant differences were found between the various splinting groups for both PE and PVS impression materials in terms of linear and 3D distortions. However, small but significant differences were found between the two impression materials (PVS, 91 mu m; PE, 103 mu m) in terms of 3D discrepancies, irrespective of the splinting technique employed. Conclusions: Casts obtained from both impression materials exhibited differences from the reference model. The impression material influenced impression inaccuracy more than the splinting material for multiple-unit abutment-level impressions.Article Link : http://www.ncbi.nlm.nih.gov/pubmed/2427891
The Phases and Triviality of Scalar Quantum Electrodynamics
The phase diagram and critical behavior of scalar quantum electrodynamics are
investigated using lattice gauge theory techniques. The lattice action fixes
the length of the scalar (``Higgs'') field and treats the gauge field as
non-compact. The phase diagram is two dimensional. No fine tuning or
extrapolations are needed to study the theory's critical behovior. Two lines of
second order phase transitions are discovered and the scaling laws for each are
studied by finite size scaling methods on lattices ranging from through
. One line corresponds to monopole percolation and the other to a
transition between a ``Higgs'' and a ``Coulomb'' phase, labelled by divergent
specific heats. The lines of transitions cross in the interior of the phase
diagram and appear to be unrelated. The monopole percolation transition has
critical indices which are compatible with ordinary four dimensional
percolation uneffected by interactions. Finite size scaling and histogram
methods reveal that the specific heats on the ``Higgs-Coulomb'' transition line
are well-fit by the hypothesis that scalar quantum electrodynamics is
logarithmically trivial. The logarithms are measured in both finite size
scaling of the specific heat peaks as a function of volume as well as in the
coupling constant dependence of the specific heats measured on fixed but large
lattices. The theory is seen to be qualitatively similar to .
The standard CRAY random number generator RANF proved to be inadequateComment: 25pages,26figures;revtex;ILL-(TH)-94-#12; only hardcopy of figures
availabl
Secrecy content of two-qubit states
We analyze the set of two-qubit states from which a secret key can be
extracted by single-copy measurements plus classical processing of the
outcomes. We introduce a key distillation protocol and give the corresponding
necessary and sufficient condition for positive key extraction. Our results
imply that the critical error rate derived by Chau, Phys. Rev. A {\bf 66},
060302 (2002), for a secure key distribution using the six-state scheme is
tight. Remarkably, an optimal eavesdropping attack against this protocol does
not require any coherent quantum operation.Comment: 5 pages, RevTe
Scaling laws for the 2d 8-state Potts model with Fixed Boundary Conditions
We study the effects of frozen boundaries in a Monte Carlo simulation near a
first order phase transition. Recent theoretical analysis of the dynamics of
first order phase transitions has enabled to state the scaling laws governing
the critical regime of the transition. We check these new scaling laws
performing a Monte Carlo simulation of the 2d, 8-state spin Potts model. In
particular, our results support a pseudo-critical beta finite-size scaling of
the form beta(infinity) + a/L + b/L^2, instead of beta(infinity) + c/L^d +
d/L^{2d}. Moreover, our value for the latent heat is 0.294(11), which does not
coincide with the latent heat analytically derived for the same model if
periodic boundary conditions are assumed, which is 0.486358...Comment: 10 pages, 3 postscript figure
Thermodynamically guided nonequilibrium Monte Carlo method for generating realistic shear flows in polymeric systems
A thermodynamically guided atomistic MonteCarlo methodology is presented for simulating systems beyond equilibrium by expanding the statistical ensemble to include a tensorial variable accounting for the overall structure of the system subjected to flow. For a given shear rate, the corresponding tensorial conjugate field is determined iteratively through independent nonequilibrium molecular dynamics simulations. Test simulations for the effect of flow on the conformation of a C50H102 polyethylene liquid show that the two methods (expanded MonteCarlo and nonequilibrium molecular dynamics) provide identical results.open181
Communication of Spin Directions with Product States and Finite Measurements
Total spin eigenstates can be used to intrinsically encode a direction, which
can later be decoded by means of a quantum measurement. We study the optimal
strategy that can be adopted if, as is likely in practical applications, only
product states of -spins are available. We obtain the asymptotic behaviour
of the average fidelity which provides a proof that the optimal states must be
entangled. We also give a prescription for constructing finite measurements for
general encoding eigenstates.Comment: 4 pages, minor changes, version to appear in PR
Optimal strategies for sending information through a quantum channel
Quantum states can be used to encode the information contained in a
direction, i.e., in a unit vector. We present the best encoding procedure when
the quantum state is made up of spins (qubits). We find that the quality of
this optimal procedure, which we quantify in terms of the fidelity, depends
solely on the dimension of the encoding space. We also investigate the use of
spatial rotations on a quantum state, which provide a natural and less
demanding encoding. In this case we prove that the fidelity is directly related
to the largest zeros of the Legendre and Jacobi polynomials. We also discuss
our results in terms of the information gain.Comment: 4 pages, RevTex, final version to appear in Phys.Rev.Let
Minimal measurements of the gate fidelity of a qudit map
We obtain a simple formula for the average gate fidelity of a linear map
acting on qudits. It is given in terms of minimal sets of pure state
preparations alone, which may be interesting from the experimental point of
view. These preparations can be seen as the outcomes of certain minimal
positive operator valued measures. The connection of our results with these
generalized measurements is briefly discussed
On the Logarithmic Triviality of Scalar Quantum Electrodynamics
Using finite size scaling and histogram methods we obtain numerical results
from lattice simulations indicating the logarithmic triviality of scalar
quantum electrodynamics, even when the bare gauge coupling is chosen large.
Simulations of the non-compact formulation of the lattice abelian Higgs model
with fixed length scalar fields on lattices with ranging from
through indicate a line of second order critical points.
Fluctuation-induced first order transitions are ruled out. Runs of over ten
million sweeps for each produce specific heat peaks which grow
logarithmically with and whose critical couplings shift with picking
out a correlation length exponent of consistent with mean field
theory. This behavior is qualitatively similar to that found in pure
.Comment: 9 page
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