975 research outputs found
Coherent electronic transfer in quantum dot systems using adiabatic passage
We describe a scheme for using an all-electrical, rapid, adiabatic population
transfer between two spatially separated dots in a triple-quantum dot system.
The electron spends no time in the middle dot and does not change its energy
during the transfer process. Although a coherent population transfer method,
this scheme may well prove useful in incoherent electronic computation (for
example quantum-dot cellular automata) where it may provide a coherent
advantage to an otherwise incoherent device. It can also be thought of as a
limiting case of type II quantum computing, where sufficient coherence exists
for a single gate operation, but not for the preservation of superpositions
after the operation. We extend our analysis to the case of many intervening
dots and address the issue of transporting quantum information through a
multi-dot system.Comment: Replaced with (approximately) the published versio
Making Classical Ground State Spin Computing Fault-Tolerant
We examine a model of classical deterministic computing in which the ground
state of the classical system is a spatial history of the computation. This
model is relevant to quantum dot cellular automata as well as to recent
universal adiabatic quantum computing constructions. In its most primitive
form, systems constructed in this model cannot compute in an error free manner
when working at non-zero temperature. However, by exploiting a mapping between
the partition function for this model and probabilistic classical circuits we
are able to show that it is possible to make this model effectively error free.
We achieve this by using techniques in fault-tolerant classical computing and
the result is that the system can compute effectively error free if the
temperature is below a critical temperature. We further link this model to
computational complexity and show that a certain problem concerning finite
temperature classical spin systems is complete for the complexity class
Merlin-Arthur. This provides an interesting connection between the physical
behavior of certain many-body spin systems and computational complexity.Comment: 24 pages, 1 figur
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Function reconstruction as a classical moment problem: A maximum entropy approach
We present a systematic study of the reconstruction of a non-negative
function via maximum entropy approach utilizing the information contained in a
finite number of moments of the function. For testing the efficacy of the
approach, we reconstruct a set of functions using an iterative entropy
optimization scheme, and study the convergence profile as the number of moments
is increased. We consider a wide variety of functions that include a
distribution with a sharp discontinuity, a rapidly oscillatory function, a
distribution with singularities, and finally a distribution with several spikes
and fine structure. The last example is important in the context of the
determination of the natural density of the logistic map. The convergence of
the method is studied by comparing the moments of the approximated functions
with the exact ones. Furthermore, by varying the number of moments and
iterations, we examine to what extent the features of the functions, such as
the divergence behavior at singular points within the interval, is reproduced.
The proximity of the reconstructed maximum entropy solution to the exact
solution is examined via Kullback-Leibler divergence and variation measures for
different number of moments.Comment: 20 pages, 17 figure
Adsorption induced reconstruction of the Cu(110) surface
The formation of the O/Cu(110)-(2 × 1) and H/Cu(110)-(1 × 2) superstructures has been investigated by a LEED beam profile analysis. The oxygen induced reconstruction proceeds at later stages by creation of holes on flat terraces. This could not be observed at the hydrogen induced missing row reconstruction. The formation of the missing row structure proceeds most probably via nucleation at steps and subsequent growth of (1 × 2) islands. The influence of different distributions of steps and islands on beam profiles is discussed
Bound States and Threshold Resonances in Quantum Wires with Circular Bends
We study the solutions to the wave equation in a two-dimensional tube of unit
width comprised of two straight regions connected by a region of constant
curvature. We introduce a numerical method which permits high accuracy at high
curvature. We determine the bound state energies as well as the transmission
and reflection matrices, and and focus on the nature of
the resonances which occur in the vicinity of channel thresholds. We explore
the dependence of these solutions on the curvature of the tube and angle of the
bend and discuss several limiting cases where our numerical results confirm
analytic predictions.Comment: 24 pages, revtex file, one style file and 17 PostScript figures
include
Social cognitive predictors of well-being in African college students
Lent and Brown's (2006, 2008) social cognitive model of work well-being was tested in two samples of African college students, one from Angola (N = 241) and one from Mozambique (N = 425). Participants completed domain-specific measures of academic self-efficacy, environmental support, goal progress, and satisfaction, along with measures of global positive affect and life satisfaction. Path analyses indicated that the model fit the data well overall, both in the full sample and in separate sub-samples by country and gender. Contrary to expectations, however, self-efficacy predicted academic satisfaction only indirectly, via goal progress; and goal progress predicted life satisfaction only indirectly, via academic satisfaction. The predictors accounted for substantial portions of the variance in both academic domain satisfaction and life satisfaction. Implications for research and practice involving the social cognitive model are considered.info:eu-repo/semantics/publishedVersio
From quantum cellular automata to quantum lattice gases
A natural architecture for nanoscale quantum computation is that of a quantum
cellular automaton. Motivated by this observation, in this paper we begin an
investigation of exactly unitary cellular automata. After proving that there
can be no nontrivial, homogeneous, local, unitary, scalar cellular automaton in
one dimension, we weaken the homogeneity condition and show that there are
nontrivial, exactly unitary, partitioning cellular automata. We find a one
parameter family of evolution rules which are best interpreted as those for a
one particle quantum automaton. This model is naturally reformulated as a two
component cellular automaton which we demonstrate to limit to the Dirac
equation. We describe two generalizations of this automaton, the second of
which, to multiple interacting particles, is the correct definition of a
quantum lattice gas.Comment: 22 pages, plain TeX, 9 PostScript figures included with epsf.tex
(ignore the under/overfull \vbox error messages); minor typographical
corrections and journal reference adde
Two-Bit Gates are Universal for Quantum Computation
A proof is given, which relies on the commutator algebra of the unitary Lie
groups, that quantum gates operating on just two bits at a time are sufficient
to construct a general quantum circuit. The best previous result had shown the
universality of three-bit gates, by analogy to the universality of the Toffoli
three-bit gate of classical reversible computing. Two-bit quantum gates may be
implemented by magnetic resonance operations applied to a pair of electronic or
nuclear spins. A ``gearbox quantum computer'' proposed here, based on the
principles of atomic force microscopy, would permit the operation of such
two-bit gates in a physical system with very long phase breaking (i.e., quantum
phase coherence) times. Simpler versions of the gearbox computer could be used
to do experiments on Einstein-Podolsky-Rosen states and related entangled
quantum states.Comment: 21 pages, REVTeX 3.0, two .ps figures available from author upon
reques
Magnetization of noncircular quantum dots
We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.Comment: 11 RevTeX pages with 6 included Postscript figure
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