1,373 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
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
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
Quantum Control of Qubits and Atomic Motion Using Ultrafast Laser Pulses
Pulsed lasers offer significant advantages over CW lasers in the coherent
control of qubits. Here we review the theoretical and experimental aspects of
controlling the internal and external states of individual trapped atoms with
pulse trains. Two distinct regimes of laser intensity are identified. When the
pulses are sufficiently weak that the Rabi frequency is much smaller
than the trap frequency \otrap, sideband transitions can be addressed and
atom-atom entanglement can be accomplished in much the same way as with CW
lasers. By contrast, if the pulses are very strong (\Omega \gg \otrap),
impulsive spin-dependent kicks can be combined to create entangling gates which
are much faster than a trap period. These fast entangling gates should work
outside of the Lamb-Dicke regime and be insensitive to thermal atomic motion.Comment: 16 pages, 15 figure
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