1,698 research outputs found
Dynamical Semigroups for Unbounded Repeated Perturbation of Open System
We consider dynamical semigroups with unbounded Kossakowski-Lindblad-Davies
generators which are related to evolution of an open system with a tuned
repeated harmonic perturbation. Our main result is the proof of existence of
uniquely determined minimal trace-preserving strongly continuous dynamical
semigroups on the space of density matrices. The corresponding dual W
*-dynamical system is shown to be unital quasi-free and completely positive
automorphisms of the CCR-algebra. We also comment on the action of dynamical
semigroups on quasi-free states
Relation between the Dynamics of the Reduced Purity and Correlations
A general property of the relation between the dynamics of the reduced purity
and correlations is investigated in quantum mechanical systems. We show that a
non-zero time-derivative of the reduced purity of a system implies the
existence of non-zero correlations with its environment under any unbounded
Hamiltonians with finite variance. This shows the role of local dynamical
information on the correlations, as well as the role of correlations in the
mechanism of purity change.Comment: 7 page
Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy
The excitonic recombinations in hexagonal boron nitride (hBN) are
investigated with spatially resolved cathodoluminescence spectroscopy in the UV
range. Cathodoluminescence images of an individual hBN crystallite reveals that
the 215 nm free excitonic line is quite homogeneously emitted along the
crystallite whereas the 220 nm and 227 nm excitonic emissions are located in
specific regions of the crystallite. Transmission electron microscopy images
show that these regions contain a high density of crystalline defects. This
suggests that both the 220 nm and 227 nm emissions are produced by the
recombination of excitons bound to structural defects
Non-equilibrium states of a photon cavity pumped by an atomic beam
We consider a beam of two-level randomly excited atoms that pass one-by-one
through a one-mode cavity. We show that in the case of an ideal cavity, i.e. no
leaking of photons from the cavity, the pumping by the beam leads to an
unlimited increase in the photon number in the cavity. We derive an expression
for the mean photon number for all times. Taking into account leaking of the
cavity, we prove that the mean photon number in the cavity stabilizes in time.
The limiting state of the cavity in this case exists and it is independent of
the initial state. We calculate the characteristic functional of this
non-quasi-free non-equilibrium state. We also calculate the energy flux in both
the ideal and open cavity and the entropy production for the ideal cavity.Comment: Corrected energy production calculations and made some changes to
ease the readin
Homogeneous Open Quantum Random Walks on a lattice
We study Open Quantum Random Walks for which the underlying graph is a
lattice, and the generators of the walk are translation-invariant. We consider
the quantum trajectory associated with the OQRW, which is described by a
position process and a state process. We obtain a central limit theorem and a
large deviation principle for the position process, and an ergodic result for
the state process. We study in detail the case of homogeneous OQRWs on a
lattice, with internal space
Non Markovian Quantum Repeated Interactions and Measurements
A non-Markovian model of quantum repeated interactions between a small
quantum system and an infinite chain of quantum systems is presented. By
adapting and applying usual pro jection operator techniques in this context,
discrete versions of the integro-differential and time-convolutioness Master
equations for the reduced system are derived. Next, an intuitive and rigorous
description of the indirect quantum measurement principle is developed and a
discrete non Markovian stochastic Master equation for the open system is
obtained. Finally, the question of unravelling in a particular model of
non-Markovian quantum interactions is discussed.Comment: 22 page
Optical Transitions in Single-Wall Boron Nitride Nanotubes
Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range
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