504 research outputs found
Semiclassical Description of Wavepacket Revival
We test the ability of semiclassical theory to describe quantitatively the
revival of quantum wavepackets --a long time phenomena-- in the one dimensional
quartic oscillator (a Kerr type Hamiltonian). Two semiclassical theories are
considered: time-dependent WKB and Van Vleck propagation. We show that both
approaches describe with impressive accuracy the autocorrelation function and
wavefunction up to times longer than the revival time. Moreover, in the Van
Vleck approach, we can show analytically that the range of agreement extends to
arbitrary long times.Comment: 10 pages, 6 figure
Semiquantal dynamics of fluctuations: Ostensible quantum chaos
The time-dependent variational principle using generalized Gaussian trial
functions yields a finite dimensional approximation to the full quantum
dynamics and is used in many disciplines. It is shown how these 'semi-quantum'
dynamics may be derived via the Ehrenfest theorem and recast as an extended
classical gradient system with the fluctuation variables coupled to the average
variables. An extended potential is constructed for a one-dimensional system.
The semiquantal behavior is shown to be chaotic even though the system has
regular classical behavior and the quantum behavior had been assumed regular.Comment: 9 pages, TeX, 2 figures (not attached; hard copies available
immediately on request). To appear in Physical Review Letter
Dirac open quantum system dynamics: formulations and simulations
We present an open system interaction formalism for the Dirac equation.
Overcoming a complexity bottleneck of alternative formulations, our framework
enables efficient numerical simulations (utilizing a typical desktop) of
relativistic dynamics within the von Neumann density matrix and Wigner phase
space descriptions. Employing these instruments, we gain important insights
into the effect of quantum dephasing for relativistic systems in many branches
of physics. In particular, the conditions for robustness of Majorana spinors
against dephasing are established. Using the Klein paradox and tunneling as
examples, we show that quantum dephasing does not suppress negative energy
particle generation. Hence, the Klein dynamics is also robust to dephasing
Photoionization of helium by attosecond pulses: extraction of spectra from correlated wave functions
We investigate the photoionization spectrum of helium by attosecond XUV
pulses both in the spectral region of doubly excited resonances as well as
above the double ionization threshold. In order to probe for convergence, we
compare three techniques to extract photoelectron spectra from the wavepacket
resulting from the integration of the time-dependent Schroedinger equation in
a finite-element discrete variable representation basis. These techniques are:
projection on products of hydrogenic bound and continuum states, projection
onto multi-channel scattering states computed in a B-spline close-coupling
basis, and a technique based on exterior complex scaling (ECS) implemented in
the same basis used for the time propagation. These methods allow to monitor
the population of continuum states in wavepackets created with ultrashort
pulses in different regimes. Applications include photo cross sections and
anisotropy parameters in the spectral region of doubly excited resonances,
time-resolved photoexcitation of autoionizing resonances in an attosecond
pump-probe setting, and the energy and angular distribution of correlated
wavepackets for two-photon double ionization.Comment: 19 pages, 12 figure
Light propagation in atomic Mott Insulators
We study radiation-matter interaction in a system of ultracold atoms trapped
in an optical lattice in a Mott insulator phase. We develop a fully general
quantum model, and we perform calculations for a one-dimensional geometry at
normal incidence. Both two- and three-level atomic configurations are
studied. The polariton dispersion and the reflectivity spectra are
characterized in the different regimes, for both semi-infinite and finite-size
geometries. We apply this model to propose a photon energy lifter experiment: a
device which is able to shift the carrier frequency of a slowly travelling
wavepacket without affecting the pulse shape nor its coherence
Self-similar Radiation from Numerical Rosenau-Hyman Compactons
The numerical simulation of compactons, solitary waves with compact support,
is characterized by the presence of spurious phenomena, as numerically-induced
radiation, which is illustrated here using four numerical methods applied to
the Rosenau-Hyman K(p,p) equation. Both forward and backward radiations are
emitted from the compacton presenting a self-similar shape which has been
illustrated graphically by the proper scaling. A grid refinement study shows
that the amplitude of the radiations decreases as the grid size does,
confirming its numerical origin. The front velocity and the amplitude of both
radiations have been studied as a function of both the compacton and the
numerical parameters. The amplitude of the radiations decreases exponentially
in time, being characterized by a nearly constant scaling exponent. An ansatz
for both the backward and forward radiations corresponding to a self-similar
function characterized by the scaling exponent is suggested by the present
numerical results.Comment: To be published in Journal of Computational Physic
Suppression of dephasing and decoherence in open quantum systems
In this thesis, we discuss the possibilities for suppression of decoherence, the loss of purity in an open quantum
system, and the related process of dephasing. In the first chapter, we review the literature on the subject
of open system dynamics, decoherence and methods of decoherence suppression. In the second chapter, we
focus on the specific case of a rotationally hot diatomic molecule as an example of an open quantum system,
where molecular vibrational wavepackets are subject to dephasing due to rovibrational coupling. We report
analytical and numerical results addressing whether the dephasing rate can be controlled by adjustment
of the initial wavepacket phases. It appears that over long timescales, phase-only control is not possible,
but for earlier timescales the possibility of phase-only control of dephasing remains. In addition, we point
out that the time-dependence of the dephasing process depends significantly upon the degeneracy of the
rotational environment states. In the final chapter, we discuss the same system, but apply a qualitatively
different method of dephasing suppression based on the nonlinear resonance effect in a driving field. We
extend previous work on the topic (Shapiro et al. [1]) by considering the effect of using a train of laser pulses
as the driving field, in place of a two-colour continuous-wave laser field. We demonstrate that the pulse train
method can be more effective at suppressing dephasing than the two-colour CW case
- …