1,857 research outputs found
Overcoming device unreliability with continuous learning in a population coding based computing system
The brain, which uses redundancy and continuous learning to overcome the
unreliability of its components, provides a promising path to building
computing systems that are robust to the unreliability of their constituent
nanodevices. In this work, we illustrate this path by a computing system based
on population coding with magnetic tunnel junctions that implement both neurons
and synaptic weights. We show that equipping such a system with continuous
learning enables it to recover from the loss of neurons and makes it possible
to use unreliable synaptic weights (i.e. low energy barrier magnetic memories).
There is a tradeoff between power consumption and precision because low energy
barrier memories consume less energy than high barrier ones. For a given
precision, there is an optimal number of neurons and an optimal energy barrier
for the weights that leads to minimum power consumption
Underachievement of Creatively Gifted High School Students
Underachievement is a pervasive problem for gifted students, and creatively gifted students may be at greater risk for underachievement due to personality traits, lack of challenge in strength areas, a mismatch between school environment and student needs, low status associated with creative achievements and behaviors in the school system, and other factors. This study focused on six creatively gifted, underachieving high school students from an urban-cluster area in the western United States. A hermeneutic phenomenological approach was used to gather data in the form of interviews with underachieving, creatively gifted students, their parents, and teachers; observation of classrooms; and creative artifacts to uncover the essence of the experience of underachievement for these stakeholders. These data groups were then compared to each other and existing literature to help generate recommendations for changes in school programming and practice for helping this student population
Smooth quantum-classical transition in photon subtraction and addition processes
Recently Parigi et al. [Science 317, 1890 (2007)] implemented experimentally
the photon subtraction and addition processes from/to a light field in a
conditional way, when the required operations were produced successfully only
upon the positive outcome of a separate measurement. It was verified that for a
low intensity beam (quantum regime) the bosonic annihilation operator does
indeed describe a single photon subtraction, while the creation operator
describes a photon addition. Nonetheless, the exact formal expressions for
these operations do not always reduce to these simple identifications, and in
this connection here we deduce the general superoperators for multiple photons
subtraction and addition processes and analyze the statistics of the resulting
states for classical field states having an arbitrary intensity. We obtain
closed analytical expressions and verify that for classical fields with high
intensity (classical regime) the operators that describe photon subtraction and
addition processes deviate significantly from simply annihilation and creation
operators. Complementarily, we analyze in details such a smooth
quantum-classical transition as function of beam intensity for both processes.Comment: 7 pages, 5 figures. To appear in Phys. Rev.
Collisional Semiclassical Aproximations in Phase-Space Representation
The Gaussian Wave-Packet phase-space representation is used to show that the
expansion in powers of of the quantum Liouville propagator leads, in
the zeroth order term, to results close to those obtained in the statistical
quasiclassical method of Lee and Scully in the Weyl-Wigner picture. It is also
verified that propagating the Wigner distribution along the classical
trajectories the amount of error is less than that coming from propagating the
Gaussian distribution along classical trajectories.Comment: 20 pages, REVTEX, no figures, 3 tables include
Control of the geometric phase and pseudo-spin dynamics on coupled Bose-Einstein condensates
We describe the behavior of two coupled Bose-Einstein condensates in
time-dependent (TD) trap potentials and TD Rabi (or tunneling) frequency, using
the two-mode approach. Starting from Bloch states, we succeed to get analytical
solutions for the TD Schroedinger equation and present a detailed analysis of
the relative and geometric phases acquired by the wave function of the
condensates, as well as their population imbalance. We also establish a
connection between the geometric phases and constants of motion which
characterize the dynamic of the system. Besides analyzing the affects of
temporality on condensates that differs by hyperfine degrees of freedom
(internal Josephson effect), we also do present a brief discussion of a one
specie condensate in a double-well potential
(external Josephson effect).Comment: 1 tex file and 11 figures in pdf forma
Semiconductor quantum dot - a quantum light source of multicolor photons with tunable statistics
We investigate the intensity correlation properties of single photons emitted
from an optically excited single semiconductor quantum dot. The second order
temporal coherence function of the photons emitted at various wavelengths is
measured as a function of the excitation power. We show experimentally and
theoretically, for the first time, that a quantum dot is not only a source of
correlated non-classical monochromatic photons but is also a source of
correlated non-classical \emph{multicolor} photons with tunable correlation
properties. We found that the emitted photon statistics can be varied by the
excitation rate from a sub-Poissonian one, where the photons are temporally
antibunched, to super-Poissonian, where they are temporally bunched.Comment: 4 pages, 2 figure
Engineering Quantum Jump Superoperators for Single Photon Detectors
We study the back-action of a single photon detector on the electromagnetic
field upon a photodetection by considering a microscopic model in which the
detector is constituted of a sensor and an amplification mechanism. Using the
quantum trajectories approach we determine the Quantum Jump Superoperator (QJS)
that describes the action of the detector on the field state immediately after
the photocount. The resulting QJS consists of two parts: the bright counts
term, representing the real photoabsorptions, and the dark counts term,
representing the amplification of intrinsic excitations inside the detector.
First we compare our results for the counting rates to experimental data,
showing a good agreement. Then we point out that by modifying the field
frequency one can engineer the form of QJS, obtaining the QJS's proposed
previously in an ad hoc manner
Multi-Dimensional Hermite Polynomials in Quantum Optics
We study a class of optical circuits with vacuum input states consisting of
Gaussian sources without coherent displacements such as down-converters and
squeezers, together with detectors and passive interferometry (beam-splitters,
polarisation rotations, phase-shifters etc.). We show that the outgoing state
leaving the optical circuit can be expressed in terms of so-called
multi-dimensional Hermite polynomials and give their recursion and
orthogonality relations. We show how quantum teleportation of photon
polarisation can be modelled using this description.Comment: 10 pages, submitted to J. Phys. A, removed spurious fil
Spin coherent manipulation in Josephson weak links
Novel designs of Josephson weak links based on semiconducting nanowires combined with circuit QED
techniques have enabled the resolution of their fine structure due to spin-orbit interactions, opening a path
towards Andreev spin qubits. Nevertheless, direct manipulation of the spin within a given Andreev state is
in general suppressed compared to interdoublet manipulation in the absence of Zeeman effects. In addition,
noisy spin-flip mechanisms limit any coherent manipulation protocol to spin postselection. We propose a
combination of a spin polarization protocol analogous to sideband cooling with stimulated Raman adiabatic
passage specifically tailored for these systems. We show this approach is robust for a large range of design
parameters, including the currently rather stringent coherence time
Entanglement of Atomic Qubits using an Optical Frequency Comb
We demonstrate the use of an optical frequency comb to coherently control and
entangle atomic qubits. A train of off-resonant ultrafast laser pulses is used
to efficiently and coherently transfer population between electronic and
vibrational states of trapped atomic ions and implement an entangling quantum
logic gate with high fidelity. This technique can be extended to the high field
regime where operations can be performed faster than the trap frequency. This
general approach can be applied to more complex quantum systems, such as large
collections of interacting atoms or molecules.Comment: 4 pages, 5 figure
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