277 research outputs found
Local phase space control and interplay of classical and quantum effects in dissociation of a driven Morse oscillator
This work explores the possibility of controlling the dissociation of a
monochromatically driven one-dimensional Morse oscillator by recreating
barriers, in the form of invariant tori with irrational winding ratios, at
specific locations in the phase space. The control algorithm proposed by Huang
{\it et al.} (Phys. Rev. A {\bf 74}, 053408 (2006)) is used to obtain an
analytic expression for the control field. We show that the control term,
approximated as an additional weaker field, is efficient in recreating the
desired tori and suppresses the classical as well as the quantum dissociation.
However, in the case when the field frequency is tuned close to a two-photon
resonance the local barriers are not effective in suppressing the dissociation.
We establish that in the on-resonant case quantum dissociation primarily occurs
via resonance-assisted tunneling and controlling the quantum dynamics requires
a local perturbation of the specific nonlinear resonance in the underlying
phase space.Comment: 12 pages, 6 figures (reduced quality), submitted to Phys. Rev.
Tunable coupling scheme for flux qubits at the optimal point
We discuss a practical design for tunably coupling a pair of flux qubits via
the quantum inductance of a third high-frequency qubit. The design is
particularly well suited for realizing a recently proposed microwave-induced
parametric coupling scheme. This is attractive because the qubits can always
remain at their optimal points. Furthermore, we will show that the resulting
coupling also has an optimal point where it is insensitive to low-frequency
flux noise. This is an important feature for the coherence of coupled qubits.
The presented scheme is an experimentally realistic way of carrying out
two-qubit gates and should be easily extended to multiqubit systems.Comment: 8 pages, 6 figures, minor change
Optical RKKY Interaction between Charged Semiconductor Quantum Dots
We show how a spin interaction between electrons localized in neighboring
quantum dots can be induced and controlled optically. The coupling is generated
via virtual excitation of delocalized excitons and provides an efficient
coherent control of the spins. This quantum manipulation can be realized in the
adiabatic limit and is robust against decoherence by spontaneous emission.
Applications to the realization of quantum gates, scalable quantum computers,
and to the control of magnetization in an array of charged dots are proposed.Comment: 4 pages, 2 figure
Femtosecond transparency in the extreme ultraviolet
Electromagnetically induced transparency-like behavior in the extreme
ultraviolet (XUV) is studied theoretically, including the effect of intense 800
nm laser dressing of He 2s2p (1Po) and 2p^2 (1Se) autoionizing states. We
present an ab initio solution of the time-dependent Schrodinger equation (TDSE)
in an LS-coupling configuration interaction basis set. The method enables a
rigorous treatment of optical field ionization of these coupled autoionizing
states into the N = 2 continuum in addition to N = 1. Our calculated transient
absorption spectra show encouraging agreement with experiment.Comment: 25 pages, 7 figures, 1 tabl
Many-body theory for systems with particle conversion: Extending the multiconfigurational time-dependent Hartree method
We derive a multiconfigurational time-dependent Hartree theory for systems
with particle conversion. In such systems particles of one kind can convert to
another kind and the total number of particles varies in time. The theory thus
extends the scope of the available and successful multiconfigurational
time-dependent Hartree methods -- which were solely formulated for and applied
to systems with a fixed number of particles -- to new physical systems and
problems. As a guiding example we treat explicitly a system where bosonic atoms
can combine to form bosonic molecules and vise versa. In the theory for
particle conversion, the time-dependent many-particle wavefunction is written
as a sum of configurations made of a different number of particles, and
assembled from sets of atomic and molecular orbitals. Both the expansion
coefficients and the orbitals forming the configurations are time-dependent
quantities that are fully determined according to the Dirac-Frenkel
time-dependent variational principle. Particular attention is paid to the
reduced density matrices of the many-particle wavefunction that appear in the
theory and enter the equations of motion. There are two kinds of reduced
density matrices: particle-conserving reduced density matrices which directly
only couple configurations with the same number of atoms and molecules, and
particle non-conserving reduced density matrices which couple configurations
with a different number of atoms and molecules. Closed-form and compact
equations of motion are derived for contact as well as general two-body
interactions, and their properties are analyzed and discussed.Comment: 46 page
Is the Bee louse Braula coeca (Diptera) using chemical camouflage to survive within honeybee colonies?
The bee louse, Braula coeca is a highly specialised flattened, wingless fly that spends its entire adult life on adult honeybees. It feeds by stealing food directly from bees during social feeding (trophallaxis). The Braula fly has a preference to infest the honeybee queen. The queen is the most attended individual in the colony but despite this the adult flies remain undetected by the workers. This is due to Braula possessing a cuticular hydrocarbon profile that mirrors that of their host honeybee colony, despite Diptera and Hymenoptera orders having separated over 290 million years ago. This chemical camouflage is most likely through odour acquisition from the honeybee host since even small colony specific differences in the alkene isomer patterns present in the honeybees were also detected in the Braula’s profile. This finding further supports the idea that the honeybee recognition cues are contained within the alkene part of their hydrocarbon profile and Braula exploit this to remain undetected within an otherwise hostile colony
Unification of the conditional probability and semiclassical interpretations for the problem of time in quantum theory
We show that the time-dependent Schr\"odinger equation (TDSE) is the
phenomenological dynamical law of evolution unraveled in the classical limit
from a timeless formulation in terms of probability amplitudes conditioned by
the values of suitably chosen internal clock variables, thereby unifying the
conditional probability interpretation (CPI) and the semiclassical approach for
the problem of time in quantum theory. Our formalism stems from an exact
factorization of the Hamiltonian eigenfunction of the clock plus system
composite, where the clock and system factors play the role of marginal and
conditional probability amplitudes, respectively. Application of the Variation
Principle leads to a pair of exact coupled pseudoeigenvalue equations for these
amplitudes, whose solution requires an iterative self-consistent procedure. The
equation for the conditional amplitude constitutes an effective "equation of
motion" for the quantum state of the system with respect to the clock
variables. These coupled equations also provide a convenient framework for
treating the back-reaction of the system on the clock at various levels of
approximation. At the lowest level, when the WKB approximation for the marginal
amplitude is appropriate, in the classical limit of the clock variables the
TDSE for the system emerges as a matter of course from the conditional
equation. In this connection, we provide a discussion of the characteristics
required by physical systems to serve as good clocks. This development is seen
to be advantageous over the original CPI and semiclassical approach since it
maintains the essence of the conventional formalism of quantum mechanics,
admits a transparent interpretation, avoids the use of the Born-Oppenheimer
approximation, and resolves various objections raised about them.Comment: 10 pages. Typographical errors correcte
Dynamical Localization: Hydrogen Atoms in Magnetic and Microwave fields
We show that dynamical localization for excited hydrogen atoms in magnetic
and microwave fields takes place at quite low microwave frequency much lower
than the Kepler frequency. The estimates of localization length are given for
different parameter regimes, showing that the quantum delocalization border
drops significantly as compared to the case of zero magnetic field. This opens
up broad possibilities for laboratory investigations.Comment: revtex, 11 pages, 3 figures, to appear in Phys. Rev. A, Feb (1997
Electron transfer experiments and atomic magnetism values. Progress report, February 1, 1975--September 30, 1975
Progress in the first seven months of this new research is described. A new apparatus was constructed, tested and moved to Oak Ridge National Laboratory for studies using the Penning ion source test facility. Preliminary electron transfer cross section results for N, N, He and C ions on gases were obtained. Energy loss measurements made to date support expectations that single electron transfer for multiply-charged ions colliding with gas atoms produces excited final state ions. (auth
Kicked Bose-Hubbard systems and kicked tops -- destruction and stimulation of tunneling
In a two-mode approximation, Bose-Einstein condensates (BEC) in a double-well
potential can be described by a many particle Hamiltonian of Bose-Hubbard type.
We focus on such a BEC whose interatomic interaction strength is modulated
periodically by -kicks which represents a realization of a kicked top.
In the (classical) mean-field approximation it provides a rich mixed phase
space dynamics with regular and chaotic regions. By increasing the
kick-strength a bifurcation leads to the appearance of self-trapping states
localized on regular islands. This self-trapping is also found for the many
particle system, however in general suppressed by coherent many particle
tunneling oscillations. The tunneling time can be calculated from the
quasi-energy splitting of the corresponding Floquet states. By varying the
kick-strength these quasi-energy levels undergo both avoided and even actual
crossings. Therefore stimulation or complete destruction of tunneling can be
observed for this many particle system
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