178 research outputs found
Fidelity decay in trapped Bose-Einstein condensates
The quantum coherence of a Bose-Einstein condensate is studied using the
concept of quantum fidelity (Loschmidt echo). The condensate is confined in an
elongated anharmonic trap and subjected to a small random potential such as
that created by a laser speckle. Numerical experiments show that the quantum
fidelity stays constant until a critical time, after which it drops abruptly
over a single trap oscillation period. The critical time depends
logarithmically on the number of condensed atoms and on the perturbation
amplitude. This behavior may be observable by measuring the interference
fringes of two condensates evolving in slightly different potentials.Comment: 4 pages, to appear in Physical Review Letters, February 200
Autoresonant control of the many-electron dynamics in nonparabolic quantum wells
The optical response of nonparabolic quantum wells is dominated by a strong
peak at the plasmon frequency. When the electrons reach the anharmonic regions,
resonant absorption becomes inefficient. This limitation is overcome by using a
chirped laser pulse in the autoresonant regime. By direct simulations using the
Wigner phase-space approach, the authors prove that, with a sequence of just a
few pulses, electrons can be efficiently detrapped from a nonparabolic well.
For an array of multiple quantum wells, they can create and control an
electronic current by suitably applying an autoresonant laser pulse and a
slowly varying dc electric field.Comment: 3 page
Bose-Einstein condensation of positronium: modification of the s-wave scattering length below the critical temperature
The production of a Bose-Einstein condensate made of positronium may be
feasible in the near future. Below the condensation temperature, the
positronium collision process is modified by the presence of the condensate.
This makes the theoretical description of the positronium kinetics at low
temperature challenging. Based on the quasi-particle Bogoliubov theory, we
describe the many-body particle-particle collision in a simple manner. We find
that, in a good approximation, the full positronium-positronium interaction can
be described by an effective scattering length. Our results are general and
apply to different species of bosons. The correction to the bare scattering
length is expressed in terms of a single dimensionless parameter that
completely characterizes the condensate
Loschmidt echo in a system of interacting electrons
We study the Loschmidt echo for a system of electrons interacting through mean-field Coulomb forces. The electron gas is modeled by a self-consistent set of hydrodynamic equations. It is observed that the quantum fidelity drops abruptly after a time that is proportional to the logarithm of the perturbation amplitude. The fidelity drop is related to the breakdown of the symmetry properties of the wave function
Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors
We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.Comment: Submitted to PR
Fragmentation Phase Transition in Atomic Clusters II - Coulomb Explosion of Metal Clusters -
We discuss the role and the treatment of polarization effects in many-body
systems of charged conducting clusters and apply this to the statistical
fragmentation of Na-clusters. We see a first order microcanonical phase
transition in the fragmentation of for Z=0 to 8. We can
distinguish two fragmentation phases, namely evaporation of large particles
from a large residue and a complete decay into small fragments only. Charging
the cluster shifts the transition to lower excitation energies and forces the
transition to disappear for charges higher than Z=8. At very high charges the
fragmentation phase transition no longer occurs because the cluster
Coulomb-explodes into small fragments even at excitation energy .Comment: 19 text pages +18 *.eps figures, my e-mail adress: [email protected]
submitted to Z. Phys.
Study of the quenched lifetime of an interacting positronium gas
Abstract By using a kinetic approach, we study the evolution of a gas composed of interacting ortho-and para-positronium atoms. We calculate the total lifetime of the gas and study the ortho-positronium quenching effect induced by the bi-atomic scattering mechanisms with spin exchange. We analyze a realistic situation where the positronium is formed by highly energetic positrons impinging on a solid surface. In the case of a spin-polarized source of positrons, the spin-polarization time of the final positronium gas is estimated
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