100,786 research outputs found
Adiabatic following criterion, estimation of the nonadiabatic excitation fraction and quantum jumps
An accurate theory describing adiabatic following of the dark, nonabsorbing
state in the three-level system is developed. An analytical solution for the
wave function of the particle experiencing Raman excitation is found as an
expansion in terms of the time varying nonadiabatic perturbation parameter. The
solution can be presented as a sum of adiabatic and nonadiabatic parts. Both
are estimated quantitatively. It is shown that the limiting value to which the
amplitude of the nonadiabatic part tends is equal to the Fourier component of
the nonadiabatic perturbation parameter taken at the Rabi frequency of the
Raman excitation. The time scale of the variation of both parts is found. While
the adiabatic part of the solution varies slowly and follows the change of the
nonadiabatic perturbation parameter, the nonadiabatic part appears almost
instantly, revealing a jumpwise transition between the dark and bright states.
This jump happens when the nonadiabatic perturbation parameter takes its
maximum value.Comment: 33 pages, 8 figures, submitted to PRA on 28 Oct. 200
Pade approximations of solitary wave solutions of the Gross-Pitaevskii equation
Pade approximants are used to find approximate vortex solutions of any
winding number in the context of Gross-Pitaevskii equation for a uniform
condensate and condensates with axisymmetric trapping potentials. Rational
function and generalised rational function approximations of axisymmetric
solitary waves of the Gross-Pitaevskii equation are obtained in two and three
dimensions. These approximations are used to establish a new mechanism of
vortex nucleation as a result of solitary wave interactions.Comment: In press by Journal of Physics: Mathematics and Genera
Second order gradient ascent pulse engineering
We report some improvements to the gradient ascent pulse engineering (GRAPE)
algorithm for optimal control of quantum systems. These include more accurate
gradients, convergence acceleration using the BFGS quasi-Newton algorithm as
well as faster control derivative calculation algorithms. In all test systems,
the wall clock time and the convergence rates show a considerable improvement
over the approximate gradient ascent.Comment: Submitted for publicatio
Clusters of Exceptional Points for a Laser Control of Selective Vibrational Transfer
When a molecule is exposed to a laser field, all field-free vibrational
states become resonances, with complex quasi energies calculated using Floquet
theory. There are many ways to produce the coalescences of pairs of such quasi
energies, with appropriate wavelength-intensity choices which define
Exceptional Points (EP) in the laser parameter plane. We dress for the
molecular ion H an exhaustive map of these exceptional points which
appear in clusters. Such clusters can be used to define several vibrational
transfer scenarios implying more than a single exceptional point, exchanging
single or multiple vibrational quanta. The ultimate goal is molecular
vibrational cooling by transferring an initial (thermal, for instance)
population on a final (ground, for instance) single vibrational state. When a
molecule is exposed to a laser field, all field-free vibrational states become
resonances, with complex quasi energies calculated using Floquet theory. There
are many ways to produce the coalescences of pairs of such quasi energies, with
appropriate wavelength-intensity choices which define Exceptional Points (EP)
in the laser parameter plane. We dress for the molecular ion H an
exhaustive map of these exceptional points which appear in clusters. Such
clusters can be used to define several vibrational transfer scenarios implying
more than a single exceptional point, exchanging single or multiple vibrational
quanta. The ultimate goal is molecular vibrational cooling by transferring an
initial (thermal, for instance) population on a final (ground, for instance)
single vibrational state.Comment: 16 pages, 7 figures, 1 tabl
Analytic pulse design for selective population transfer in many-level quantum systems: maximizing amplitude of population oscillations
State selective preparation and manipulation of discrete-level quantum
systems such as atoms, molecules or quantum dots is a the ultimate tool for
many diverse fields such as laser control of chemical reactions, atom optics,
high-precision metrology and quantum computing. Rabi oscillations are one of
the simplest, yet potentially quite useful mechanisms for achieving such
manipulation. Rabi theory establishes that in the two-level systems resonant
drive leads to the periodic and complete population oscillations between the
two system levels. In this paper an analytic optimization algorithm for
producing Rabi-like oscillations in the general discrete many-level quantum
systems is presented.Comment: Published in Phys.Rev.A. This is the final published versio
Theory of coherent Bragg spectroscopy of a trapped Bose-Einstein condensate
We present a detailed theoretical analysis of Bragg spectroscopy from a
Bose-Einstein condensate at T=0K. We demonstrate that within the linear
response regime, both a quantum field theory treatment and a meanfield
Gross-Pitaevskii treatment lead to the same value for the mean evolution of the
quasiparticle operators. The observable for Bragg spectroscopy experiments,
which is the spectral response function of the momentum transferred to the
condensate, can therefore be calculated in a meanfield formalism. We analyse
the behaviour of this observable by carrying out numerical simulations in
axially symmetric three-dimensional cases and in two dimensions. An approximate
analytic expression for the observable is obtained and provides a means for
identifying the relative importance of three broadening and shift mechanisms
(meanfield, Doppler, and finite pulse duration) in different regimes. We show
that the suppression of scattering at small values of q observed by
Stamper-Kurn et al. [Phys. Rev. Lett. 83, 2876 (1999)] is accounted for by the
meanfield treatment, and can be interpreted in terms of the interference of the
u and v quasiparticle amplitudes. We also show that, contrary to the
assumptions of previous analyses, there is no regime for trapped condensates
for which the spectral response function and the dynamic structure factor are
equivalent. Our numerical calculations can also be performed outside the linear
response regime, and show that at large laser intensities a significant
decrease in the shift of the spectral response function can occur due to
depletion of the initial condensate.Comment: RevTeX4 format, 16 pages plus 7 eps figures; Update to published
version: minors changes and an additional figure. (To appear in Phys. Rev. A
Power Optimizations in MTJ-based Neural Networks through Stochastic Computing
Artificial Neural Networks (ANNs) have found widespread applications in tasks
such as pattern recognition and image classification. However, hardware
implementations of ANNs using conventional binary arithmetic units are
computationally expensive, energy-intensive and have large area overheads.
Stochastic Computing (SC) is an emerging paradigm which replaces these
conventional units with simple logic circuits and is particularly suitable for
fault-tolerant applications. Spintronic devices, such as Magnetic Tunnel
Junctions (MTJs), are capable of replacing CMOS in memory and logic circuits.
In this work, we propose an energy-efficient use of MTJs, which exhibit
probabilistic switching behavior, as Stochastic Number Generators (SNGs), which
forms the basis of our NN implementation in the SC domain. Further, error
resilient target applications of NNs allow us to introduce Approximate
Computing, a framework wherein accuracy of computations is traded-off for
substantial reductions in power consumption. We propose approximating the
synaptic weights in our MTJ-based NN implementation, in ways brought about by
properties of our MTJ-SNG, to achieve energy-efficiency. We design an algorithm
that can perform such approximations within a given error tolerance in a
single-layer NN in an optimal way owing to the convexity of the problem
formulation. We then use this algorithm and develop a heuristic approach for
approximating multi-layer NNs. To give a perspective of the effectiveness of
our approach, a 43% reduction in power consumption was obtained with less than
1% accuracy loss on a standard classification problem, with 26% being brought
about by the proposed algorithm.Comment: Accepted in the 2017 IEEE/ACM International Conference on Low Power
Electronics and Desig
Bragg spectroscopy of an accelerating condensate with solitary-wave behaviour
We present a theoretical treatment of Bragg spectroscopy of an accelerating
condensate in a solitary-wave state. Our treatment is based on the
Gross-Pitaevskii equation with an optical potential representing the Bragg
pulse and an additional external time-dependent potential generating the
solitary-wave behaviour. By transforming to a frame translating with the
condensate, we derive an approximate set of equations that can be readily
solved to generate approximate Bragg spectra. Our analytic method is accurate
within a well defined parameter regime and provides physical insight into the
structure of the spectra. We illustrate our formalism using the example of
Bragg spectroscopy of a condensate in a time-averaged orbiting potential trap.Comment: 9 pages, 3 figure
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