117 research outputs found
Recurrence of fidelity in near integrable systems
Within the framework of simple perturbation theory, recurrence time of
quantum fidelity is related to the period of the classical motion. This
indicates the possibility of recurrence in near integrable systems. We have
studied such possibility in detail with the kicked rotor as an example. In
accordance with the correspondence principle, recurrence is observed when the
underlying classical dynamics is well approximated by the harmonic oscillator.
Quantum revivals of fidelity is noted in the interior of resonances, while
classical-quantum correspondence of fidelity is seen to be very short for
states initially in the rotational KAM region.Comment: 13 pages, 6 figure
Coherent transport in a two-electron quantum dot molecule
We investigate the dynamics of two interacting electrons confined to a pair
of coupled quantum dots driven by an external AC field. By numerically
integrating the two-electron Schroedinger equation in time, we find that for
certain values of the strength and frequency of the AC field we can cause the
electrons to be localised within the same dot, in spite of the Coulomb
repulsion between them. Reducing the system to an effective two-site model of
Hubbard type and applying Floquet theory leads to a detailed understanding of
this effect. This demonstrates the possibility of using appropriate AC fields
to manipulate entangled states in mesoscopic devices on extremely short
timescales, which is an essential component of practical schemes for quantum
information processing.Comment: 4 pages, 3 figures; the section dealing with the perturbative
treatment of the Floquet states has been substantially expanded to make it
easier to follo
Nonperturbative Coherent Population Trapping: An Analytic Model
Coherent population trapping is shown to occur in a driven symmetric
double-well potential in the strong-field regime. The system parameters have
been chosen to reproduce the transition of the
inversion mode of the ammonia molecule. For a molecule initially prepared in
its lower doublet we find that, under certain circumstances, the level
remains unpopulated, and this occurs in spite of the fact that the laser field
is resonant with the transition and intense enough
so as to strongly mix the and ground states. This
counterintuitive result constitutes a coherent population trapping phenomenon
of nonperturbative origin which cannot be accounted for with the usual models.
We propose an analytic nonperturbative model which accounts correctly for the
observed phenomenon.Comment: 5 pages, 2 figure
Exact exchange-correlation potential for a time-dependent two electron system
We obtain an exact solution of the time-dependent Schroedinger equation for a
two-electron system confined to a plane by an isotropic parabolic potential
whose curvature is periodically modulated in time. From this solution we
compute the exact time-dependent exchange correlation potential v_xc which
enters the Kohn-Sham equation of time-dependent density functional theory. Our
exact result provides a benchmark against which various approximate forms for
v_xc can be compared. Finally v_xc is separated in an adiabatic and a pure
dynamical part and it is shown that, for the particular system studied, the
dynamical part is negligible.Comment: 23 pages, 6 figure
Harmonic generation in ring-shaped molecules
We study numerically the interaction between an intense circularly polarized
laser field and an electron moving in a potential which has a discrete
cylindrical symmetry with respect to the laser pulse propagation direction.
This setup serves as a simple model, e.g., for benzene and other aromatic
compounds. From general symmetry considerations, within a Floquet approach,
selection rules for the harmonic generation [O. Alon Phys. Rev. Lett. 80 3743
(1998)] have been derived recently. Instead, the results we present in this
paper have been obtained solving the time-dependent Schroedinger equation ab
initio for realistic pulse shapes. We find a rich structure which is not always
dominated by the laser harmonics.Comment: 15 pages including 7 figure
High-order Harmonic Generation and Dynamic Localization in a driven two-level system, a non-perturbative solution using the Floquet-Green formalism
We apply the Floquet-Green operator formalism to the case of a
harmonically-driven two-level system. We derive exact expressions for the
quasi-energies and the components of the Floquet eigenstates with the use of
continued fractions. We study the avoided crossings structure of the
quasi-energies as a function of the strength of the driving field and give an
interpretation in terms of resonant multi-photon processes. From the Floquet
eigenstates we obtain the time-evolution operator. Using this operator we study
Dynamic Localization and High-order Harmonic Generation in the non-perturbative
regime
Dynamical control of correlated states in a square quantum dot
In the limit of low particle density, electrons confined to a quantum dot
form strongly correlated states termed Wigner molecules, in which the Coulomb
interaction causes the electrons to become highly localized in space. By using
an effective model of Hubbard-type to describe these states, we investigate how
an oscillatory electric field can drive the dynamics of a two-electron Wigner
molecule held in a square quantum dot. We find that, for certain combinations
of frequency and strength of the applied field, the tunneling between various
charge configurations can be strongly quenched, and we relate this phenomenon
to the presence of anti-crossings in the Floquet quasi-energy spectrum. We
further obtain simple analytic expressions for the location of these
anti-crossings, which allows the effective parameters for a given quantum dot
to be directly measured in experiment, and suggests the exciting possibility of
using ac-fields to control the time evolution of entangled states in mesoscopic
devices.Comment: Replaced with version to be published in Phys. Rev.
Molecular Wires Acting as Coherent Quantum Ratchets
The effect of laser fields on the electron transport through a molecular wire
being weakly coupled to two leads is investigated. The molecular wire acts as a
coherent quantum ratchet if the molecule is composed of periodically arranged,
asymmetric chemical groups. This setup presents a quantum rectifier with a
finite dc-response in the absence of a static bias. The nonlinear current is
evaluated in closed form within the Floquet basis of the isolated, driven wire.
The current response reveals multiple current reversals together with a
nonlinear dependence (reflecting avoided quasi-energy crossings) on both, the
amplitude and the frequency of the laser field. The current saturates for long
wires at a nonzero value, while it may change sign upon decreasing its length.Comment: 4 pages, 4 figures, RevTeX
A triple carboxylic acid-functionalized RAFT agent platform for the elaboration of well-defined telechelic 3-arm star PDMAc
This communication describes the synthesis of a triple acid-functionalized RAFT agent and its use to prepare well-defined 3-arm star polymers of N,N-dimethylacrylamide (DMAc). A simple esterification reaction allowed the convenient integration of three electron-rich naphthalene recognition units on the RAFT agent platform and subsequently the elaboration of a naphthalene end-decorated telechelic 3-arm star PDMAc. This functionalized star polymer was further exploited to build a hydrogel with a complementary homoditopic host unit featuring tetracationic macrocycle cyclobis(paraquat-p-phenylene) units
Floquet Formalism of Quantum Pumps
We review Floquet formalism of quantum electron pumps. In the Floquet
formalism the quantum pump is regarded as a time dependent scattering system,
which allows us to go beyond the adiabatic limit. It can be shown that the
well-known adiabatic formula given by Brouwer can be derived from the adiabatic
limit of Floquet formalism. We compare various physical properties of the
quantum pump both in the adiabatic and in the non-adiabatic regime using the
Floquet theory.Comment: Latex2e 16 pages, 6 figures. A review paper to appear in Int. J. Mod.
Phys.
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