405 research outputs found
Locally Optimal Control of Quantum Systems with Strong Feedback
For quantum systems with high purity, we find all observables that, when
continuously monitored, maximize the instantaneous reduction in the von Neumann
entropy. This allows us to obtain all locally optimal feedback protocols with
strong feedback, and explicit expressions for the best such protocols for
systems of size N <= 4. We also show that for a qutrit the locally optimal
protocol is the optimal protocol for a given range of control times, and derive
an upper bound on all optimal protocols with strong feedback.Comment: 4 pages, Revtex4. v2: published version (some errors corrected
A numerical study of two-photon ionization of helium using the Pyprop framework
Few-photon induced breakup of helium is studied using a newly developed ab
initio numerical framework for solving the six-dimensional time-dependent
Schroedinger equation. We present details of the method and calculate
(generalized) cross sections for the process of two-photon nonsequential
(direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a
process that has been very much debated in recent years and is not yet fully
understood. In particular, we have studied the convergence property of the
total cross section in the vicinity of the upper threshold (54.4 eV), versus
the pulse duration of the applied laser field. We find that the cross section
exhibits an increasing trend near the threshold, as has also been observed by
others, and show that this rise cannot solely be attributed to an unintended
inclusion of the sequential two-photon double ionization process, caused by the
bandwidth of the applied field.Comment: 7 pages, 3 figure
Physical model for the generation of ideal resources in multipartite quantum networking
We propose a physical model for generating multipartite entangled states of
spin- particles that have important applications in distributed quantum
information processing. Our protocol is based on a process where mobile spins
induce the interaction among remote scattering centers. As such, a major
advantage lies on the management of stationary and well separated spins. Among
the generable states, there is a class of -qubit singlets allowing for
optimal quantum telecloning in a scalable and controllable way. We also show
how to prepare Aharonov, W and Greenberger-Horne-Zeilinger states.Comment: 5 pages, 2 figures. Format revise
Dilute Bose gases interacting via power-law potentials
Neutral atoms interact through a van der Waals potential which asymptotically
falls off as r^{-6}. In ultracold gases, this interaction can be described to a
good approximation by the atom-atom scattering length. However, corrections
arise that depend on the characteristic length of the van der Waals potential.
We parameterize these corrections by analyzing the energies of two- and
few-atom systems under external harmonic confinement, obtained by numerically
and analytically solving the Schrodinger equation. We generalize our results to
particles interacting through a longer-ranged potential which asymptotically
falls off as r^{-4}.Comment: 7 pages, 4 figure
Clusters under strong VUV pulses: A quantum-classical hybrid-description incorporating plasma effects
The quantum-classical hybrid-description of rare-gas clusters interacting
with intense light pulses which we have developed is described in detail. Much
emphasis is put on the treatment of screening electrons in the cluster which
set the time scale for the evolution of the system and form the link between
electrons strongly bound to ions and quasi-free plasma electrons in the
cluster. As an example we discuss the dynamics of an Ar147 cluster exposed to a
short VUV laser pulse of 20eV photon energy.Comment: 8 pages, 9 figure
Attosecond probing of instantaneous AC Stark shifts in helium atoms
Based on numerical solutions of the time-dependent Schr\"odinger equation for
either one or two active electrons, we propose a method for observing
instantaneous level shifts in an oscillating strong infrared (IR) field in
time, using a single tunable attosecond pulse to probe excited states of the
perturbed atom. The ionization probability in the combined fields depends on
both, the frequency of the attosecond pulse and the time delay between both
pulses, since the IR field shifts excited energy levels into and out of
resonance with the attosecond probe pulse. We show that this method (i) allows
the detection of instantaneous atomic energy gaps with sub-laser-cycle time
resolution and (ii) can be applied as an ultrafast gate for more complex
processes such as non-sequential double-ionization
Fermion Dynamics by Internal and Space-Time Symmetries
This manuscript is devoted to introduce a gauge theory of the Lorentz Group
based on the ambiguity emerging in dealing with isometric
diffeo-morphism-induced Lorentz transformations. The behaviors under local
transformations of fermion fields and spin connections (assumed to be ordinary
world vectors) are analyzed in flat space-time and the role of the torsion
field, within the generalization to curved space-time, is briefly discussed.
The fermion dynamics is then analyzed including the new gauge fields and
assuming time-gauge. Stationary solutions of the problem are also analyzed in
the non-relativistic limit, to study the spinor structure of an hydrogen-like
atom.Comment: 10 pages, no figur
Perfect State Transfer, Effective Gates and Entanglement Generation in Engineered Bosonic and Fermionic Networks
We show how to achieve perfect quantum state transfer and construct effective
two-qubit gates between distant sites in engineered bosonic and fermionic
networks. The Hamiltonian for the system can be determined by choosing an
eigenvalue spectrum satisfying a certain condition, which is shown to be both
sufficient and necessary in mirror-symmetrical networks. The natures of the
effective two-qubit gates depend on the exchange symmetry for fermions and
bosons. For fermionic networks, the gates are entangling (and thus universal
for quantum computation). For bosonic networks, though the gates are not
entangling, they allow two-way simultaneous communications. Protocols of
entanglement generation in both bosonic and fermionic engineered networks are
discussed.Comment: RevTeX4, 6 pages, 1 figure; replaced with a more general example and
clarified the sufficient and necessary condition for perfect state transfe
The Accelerated Kepler Problem
The accelerated Kepler problem is obtained by adding a constant acceleration
to the classical two-body Kepler problem. This setting models the dynamics of a
jet-sustaining accretion disk and its content of forming planets as the disk
loses linear momentum through the asymmetric jet-counterjet system it powers.
The dynamics of the accelerated Kepler problem is analyzed using physical as
well as parabolic coordinates. The latter naturally separate the problem's
Hamiltonian into two unidimensional Hamiltonians. In particular, we identify
the origin of the secular resonance in the accelerated Kepler problem and
determine analytically the radius of stability boundary of initially circular
orbits that are of particular interest to the problem of radial migration in
binary systems as well as to the truncation of accretion disks through stellar
jet acceleration.Comment: 16 pages, 9 figures, in press at Celestial Mechanics and Dynamical
Astronom
Heterointerface effects on the charging energy of shallow D- ground state in silicon: the role of dielectric mismatch
Donor states in Si nanodevices can be strongly modified by nearby insulating
barriers and metallic gates. We report here experimental results indicating a
strong reduction in the charging energy of isolated As dopants in Si FinFETs
relative to the bulk value. By studying the problem of two electrons bound to a
shallow donor within the effective mass approach, we find that the measured
small charging energy may be due to a combined effect of the insulator
screening and the proximity of metallic gates.Comment: 7 pages, 6 figure
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