23,509 research outputs found
Temporal stimulated intersubband emission of photoexcited electrons
We have studied the transient evolution of electrons distributed over two
levels in a wide quantum well, with the two levels below the optical phonon
energy, after an ultrafast interband excitation and cascade emission of optical
phonons. If electrons are distributed near the top of the passive region, a
temporal negative absorption appears to be dominant in the intersubband
response. This is due to the effective broadening of the upper level state
under the optical phonon emission. We have then considered the amplification of
the ground mode in a THz waveguide with a multiquantum well placed at the
center of the cavity. A huge increase of the probe signal is obtained, which
permits the temporal stimulated emission regime of the photoexcited electrons
in the THz spectral region.Comment: 5 pages, 5 figures, brief repor
Scaling and Formulary cross sections for ion-atom impact ionization
The values of ion-atom ionization cross sections are frequently needed for
many applications that utilize the propagation of fast ions through matter.
When experimental data and theoretical calculations are not available,
approximate formulas are frequently used. This paper briefly summarizes the
most important theoretical results and approaches to cross section calculations
in order to place the discussion in historical perspective and offer a concise
introduction to the topic. Based on experimental data and theoretical
predictions, a new fit for ionization cross sections is proposed. The range of
validity and accuracy of several frequently used approximations (classical
trajectory, the Born approximation, and so forth) are discussed using, as
examples, the ionization cross sections of hydrogen and helium atoms by various
fully stripped ions.Comment: 46 pages, 8 figure
Collisions of low-energy antiprotons with molecular hydrogen: ionization, excitation and stopping power
A time-dependent coupled-channel approach was used to calculate ionization,
excitation, and energy-loss cross sections as well as energy spectra for
antiproton and proton collisions with molecular hydrogen for impact energies 8
keV < E < 4000 keV.Comment: 4 pages, 4 figures, conference LEAP0
Coherent Optical Spectroscopy Of Molecules And Molecular Beams
This paper presents our recent work on coherent optical spectroscopy of molecules and molecular beams. The theory for these nonlinear optical effects is summarized and related to the measurements in the gas phase and in the condensed phase. Finally, we discuss the importance of these methods, which disentangle the inhomogeneous optical resonances, in understanding nonradiative and optical dephasing processes
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
Dissipation and Decoherence in Nanodevices: a Generalized Fermi's Golden Rule
We shall revisit the conventional adiabatic or Markov approximation, which
--contrary to the semiclassical case-- does not preserve the positive-definite
character of the corresponding density matrix, thus leading to highly
non-physical results. To overcome this serious limitation, originally pointed
out and partially solved by Davies and co-workers almost three decades ago, we
shall propose an alternative more general adiabatic procedure, which (i) is
physically justified under the same validity restrictions of the conventional
Markov approach, (ii) in the semiclassical limit reduces to the standard
Fermi's golden rule, and (iii) describes a genuine Lindblad evolution, thus
providing a reliable/robust treatment of energy-dissipation and dephasing
processes in electronic quantum devices. Unlike standard master-equation
formulations, the dependence of our approximation on the specific choice of the
subsystem (that include the common partial trace reduction) does not threaten
positivity, and quantum scattering rates are well defined even in case the
subsystem is infinitely extended/has continuous spectrum.Comment: 6 pages, 0 figure
Impurity and quaternions in nonrelativistic scattering from a quantum memory
Models of quantum computing rely on transformations of the states of a
quantum memory. We study mathematical aspects of a model proposed by Wu in
which the memory state is changed via the scattering of incoming particles.
This operation causes the memory content to deviate from a pure state, i.e.
induces impurity. For nonrelativistic particles scattered from a two-state
memory and sufficiently general interaction potentials in 1+1 dimensions, we
express impurity in terms of quaternionic commutators. In this context, pure
memory states correspond to null hyperbolic quaternions. In the case with point
interactions, the scattering process amounts to appropriate rotations of
quaternions in the frequency domain. Our work complements a previous analysis
by Margetis and Myers (2006 J. Phys. A 39 11567--11581).Comment: 16 pages, no figure
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