5,348 research outputs found
Parametric Competition in non-autonomous Hamiltonian Systems
In this work we use the formalism of chord functions (\emph{i.e.}
characteristic functions) to analytically solve quadratic non-autonomous
Hamiltonians coupled to a reservoir composed by an infinity set of oscillators,
with Gaussian initial state. We analytically obtain a solution for the
characteristic function under dissipation, and therefore for the determinant of
the covariance matrix and the von Neumann entropy, where the latter is the
physical quantity of interest. We study in details two examples that are known
to show dynamical squeezing and instability effects: the inverted harmonic
oscillator and an oscillator with time dependent frequency. We show that it
will appear in both cases a clear competition between instability and
dissipation. If the dissipation is small when compared to the instability, the
squeezing generation is dominant and one can see an increasing in the von
Neumann entropy. When the dissipation is large enough, the dynamical squeezing
generation in one of the quadratures is retained, thence the growth in the von
Neumann entropy is contained
Interband polarized absorption in InP polytypic superlattices
Recent advances in growth techniques have allowed the fabrication of
semiconductor nanostructures with mixed wurtzite/zinc-blende crystal phases.
Although the optical characterization of these polytypic structures is well
eported in the literature, a deeper theoretical understanding of how crystal
phase mixing and quantum confinement change the output linear light
polarization is still needed. In this paper, we theoretically investigate the
mixing effects of wurtzite and zinc-blende phases on the interband absorption
and in the degree of light polarization of an InP polytypic superlattice. We
use a single 88 kp Hamiltonian that describes both crystal
phases. Quantum confinement is investigated by changing the size of the
polytypic unit cell. We also include the optical confinement effect due to the
dielectric mismatch between the superlattice and the vaccum and we show it to
be necessary to match experimental results. Our calculations for large wurtzite
concentrations and small quantum confinement explain the optical trends of
recent photoluminescence excitation measurements. Furthermore, we find a high
sensitivity to zinc-blende concentrations in the degree of linear polarization.
This sensitivity can be reduced by increasing quantum confinement. In
conclusion, our theoretical analysis provides an explanation for optical trends
in InP polytypic superlattices, and shows that the interplay of crystal phase
mixing and quantum confinement is an area worth exploring for light
polarization engineering.Comment: 9 pages, 6 figures and 1 tabl
Atomic Focusing by Quantum Fields: Entanglement Properties
The coherent manipulation of the atomic matter waves is of great interest
both in science and technology. In order to study how an atom optic device
alters the coherence of an atomic beam, we consider the quantum lens proposed
by Averbukh et al [1] to show the discrete nature of the electromagnetic field.
We extend the analysis of this quantum lens to the study of another essentially
quantum property present in the focusing process, i.e., the atom-field
entanglement, and show how the initial atomic coherence and purity are affected
by the entanglement. The dynamics of this process is obtained in closed form.
We calculate the beam quality factor and the trace of the square of the reduced
density matrix as a function of the average photon number in order to analyze
the coherence and purity of the atomic beam during the focusing process.Comment: 10 pages, 4 figure
Comment on the Adiabatic Condition
The experimental observation of effects due to Berry's phase in quantum
systems is certainly one of the most impressive demonstrations of the
correctness of the superposition principle in quantum mechanics. Since Berry's
original paper in 1984, the spin 1/2 coupled with rotating external magnetic
field has been one of the most studied models where those phases appear. We
also consider a special case of this soluble model. A detailed analysis of the
coupled differential equations and comparison with exact results teach us why
the usual procedure (of neglecting nondiagonal terms) is mathematically sound.Comment: 9 page
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