354 research outputs found
Electronic Energy Transfer: Localized Operator Partitioning of Electronic Energy in Composite Quantum Systems
A Hamiltonian based approach using spatially localized projection operators
is introduced to give precise meaning to the chemically intuitive idea of the
electronic energy on a quantum subsystem. This definition facilitates the study
of electronic energy transfer in arbitrarily coupled quantum systems. In
particular, the decomposition scheme can be applied to molecular components
that are strongly interacting (with significant orbital overlap) as well as to
isolated fragments. The result leads to the proper electronic energy at all
internuclear distances, including the case of separated fragments, and reduces
to the well-known Forster and Dexter results in their respective limits.
Numerical calculations of coherent energy and charge transfer dynamics in
simple model systems are presented and the effect of collisionally induced
decoherence is examined
Decoherence Effects in Reactive Scattering
Decoherence effects on quantum and classical dynamics in reactive scattering
are examined using a Caldeira-Leggett type model. Through a study of dynamics
of the collinear H+H2 reaction and the transmission over simple one-dimensional
barrier potentials, we show that decoherence leads to improved agreement
between quantum and classical reaction and transmission probabilities,
primarily by increasing the energy dispersion in a well defined way. Increased
potential nonlinearity is seen to require larger decoherence in order to attain
comparable quantum-classical agreement.Comment: 25 pages, 6 figures, to be published in J. Chem. Phy
Advances in decoherence control
I address the current status of dynamical decoupling techniques in terms of
required control resources and feasibility. Based on recent advances in both
improving the theoretical design and assessing the control performance for
specific noise models, I argue that significant progress may still be possible
on the road of implementing decoupling under realistic constraints.Comment: 14 pages, 3 encapsulated eps figures. To appear in Journal of Modern
Optics, Special Proceedings Volume of the XXXIV Winter Colloquium on the
Physics of Quantum Electronics, Snowbird, Jan 200
Cold Atomic Collisions: Coherent Control of Penning and Associative Ionization
Coherent Control techniques are computationally applied to cold (1mK < T < 1
K) and ultracold (T < 1 microK) Ne*(3s,3P2) + Ar(1S0) collisions. We show that
by using various initial superpositions of the Ne*(3s,3P2) M = {-2,-1,0,1,2}
Zeeman sub-levels it is possible to reduce the Penning Ionization (PI) and
Associative Ionization (AI) cross sections by as much as four orders of
magnitude. It is also possible to drastically change the ratio of these two
processes. The results are based on combining, within the "Rotating Atom
Approximation", empirical and ab-initio ionization-widths.Comment: 4 pages, 2 tables, 2 figure
Coherent Control of Quantum Chaotic Diffusion
Extensive coherent control over quantum chaotic diffusion using the kicked
rotor model is demonstrated and its origin in deviations from random matrix
theory is identified. Further, the extent of control in the presence of
external decoherence is established. The results are relevant to both areas of
quantum chaos and coherent control.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Aspects of quantum coherence in the optical Bloch equations
Aspects of coherence and decoherence are analyzed within the optical Bloch
equations. By rewriting the analytic solution in an alternate form, we are able
to emphasize a number of unusual features: (a) despite the Markovian nature of
the bath, coherence at long times can be retained; (b) the long-time asymptotic
degree of coherence in the system is intertwined with the asymptotic difference
in level populations; (c) the traditional population-relaxation and decoherence
times, and , lose their meaning when the system is in the presence
of an external field, and are replaced by more general overall timescales; (d)
increasing the field strength, quantified by the Rabi frequency, ,
increases the rate of decoherence rather than reducing it, as one might expect;
and (e) maximum asymptotic coherence is reached when the system parameters
satisfy .Comment: 18 pages, 6 figures; to appear in J Chem Phy
Quantum Control of the Hyperfine Spin of a Cs Atom Ensemble
We demonstrate quantum control of a large spin-angular momentum associated
with the F=3 hyperfine ground state of 133Cs. A combination of time dependent
magnetic fields and a static tensor light shift is used to implement
near-optimal controls and map a fiducial state to a broad range of target
states, with yields in the range 0.8-0.9. Squeezed states are produced also by
an adiabatic scheme that is more robust against errors. Universal control
facilitates the encoding and manipulation of qubits and qudits in atomic ground
states, and may lead to improvement of some precision measurements.Comment: 4 pages, 4 figures (color
Entanglement and Timing-Based Mechanisms in the Coherent Control of Scattering Processes
The coherent control of scattering processes is considered, with electron
impact dissociation of H used as an example. The physical mechanism
underlying coherently controlled stationary state scattering is exposed by
analyzing a control scenario that relies on previously established entanglement
requirements between the scattering partners. Specifically, initial state
entanglement assures that all collisions in the scattering volume yield the
desirable scattering configuration. Scattering is controlled by preparing the
particular internal state wave function that leads to the favored collisional
configuration in the collision volume. This insight allows coherent control to
be extended to the case of time-dependent scattering. Specifically, we identify
reactive scattering scenarios using incident wave packets of translational
motion where coherent control is operational and initial state entanglement is
unnecessary. Both the stationary and time-dependent scenarios incorporate
extended coherence features, making them physically distinct. From a
theoretical point of view, this work represents a large step forward in the
qualitative understanding of coherently controlled reactive scattering. From an
experimental viewpoint, it offers an alternative to entanglement-based control
schemes. However, both methods present significant challenges to existing
experimental technologies
The Importance of DNA Repair in Tumor Suppression
The transition from a normal to cancerous cell requires a number of highly
specific mutations that affect cell cycle regulation, apoptosis,
differentiation, and many other cell functions. One hallmark of cancerous
genomes is genomic instability, with mutation rates far greater than those of
normal cells. In microsatellite instability (MIN tumors), these are often
caused by damage to mismatch repair genes, allowing further mutation of the
genome and tumor progression. These mutation rates may lie near the error
catastrophe found in the quasispecies model of adaptive RNA genomes, suggesting
that further increasing mutation rates will destroy cancerous genomes. However,
recent results have demonstrated that DNA genomes exhibit an error threshold at
mutation rates far lower than their conservative counterparts. Furthermore,
while the maximum viable mutation rate in conservative systems increases
indefinitely with increasing master sequence fitness, the semiconservative
threshold plateaus at a relatively low value. This implies a paradox, wherein
inaccessible mutation rates are found in viable tumor cells. In this paper, we
address this paradox, demonstrating an isomorphism between the conservatively
replicating (RNA) quasispecies model and the semiconservative (DNA) model with
post-methylation DNA repair mechanisms impaired. Thus, as DNA repair becomes
inactivated, the maximum viable mutation rate increases smoothly to that of a
conservatively replicating system on a transformed landscape, with an upper
bound that is dependent on replication rates. We postulate that inactivation of
post-methylation repair mechanisms are fundamental to the progression of a
tumor cell and hence these mechanisms act as a method for prevention and
destruction of cancerous genomes.Comment: 7 pages, 5 figures; Approximation replaced with exact calculation;
Minor error corrected; Minor changes to model syste
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