150 research outputs found
Optimal laser control of molecular photoassociation along with vibrational stabilization
AbstractThis work explores the optimization of laser pulses for the control of photoassociation and vibrational stabilization. Simulations are presented within a model system for the electronic ground-state collision of O+H. The goal is to drive the transition from a wavepacket representing the colliding atoms to the vibrational ground level of the diatomic molecule. The optimized fields resulting from two distinct trial pulses are analyzed and compared. Very high yields were obtained in the molecular vibrational ground-level
Energy-scales convergence for optimal and robust quantum transport in photosynthetic complexes
Underlying physical principles for the high efficiency of excitation energy
transfer in light-harvesting complexes are not fully understood. Notably, the
degree of robustness of these systems for transporting energy is not known
considering their realistic interactions with vibrational and radiative
environments within the surrounding solvent and scaffold proteins. In this
work, we employ an efficient technique to estimate energy transfer efficiency
of such complex excitonic systems. We observe that the dynamics of the
Fenna-Matthews-Olson (FMO) complex leads to optimal and robust energy transport
due to a convergence of energy scales among all important internal and external
parameters. In particular, we show that the FMO energy transfer efficiency is
optimum and stable with respect to the relevant parameters of environmental
interactions and Frenkel-exciton Hamiltonian including reorganization energy
, bath frequency cutoff , temperature , bath spatial
correlations, initial excitations, dissipation rate, trapping rate, disorders,
and dipole moments orientations. We identify the ratio of \lambda T/\gamma\*g
as a single key parameter governing quantum transport efficiency, where g is
the average excitonic energy gap.Comment: minor revisions, removing some figures, 19 pages, 19 figure
Volume Fractions of the Kinematic "Near-Critical" Sets of the Quantum Ensemble Control Landscape
An estimate is derived for the volume fraction of a subset in the neighborhood
of the critical set
of the kinematic quantum ensemble control landscape J(U) = Tr(U\rho U' O),
where represents the unitary time evolution operator, {\rho} is the initial
density matrix of the ensemble, and O is an observable operator. This estimate
is based on the Hilbert-Schmidt geometry for the unitary group and a
first-order approximation of . An upper bound on these
near-critical volumes is conjectured and supported by numerical simulation,
leading to an asymptotic analysis as the dimension of the quantum system
rises in which the volume fractions of these "near-critical" sets decrease to
zero as increases. This result helps explain the apparent lack of influence
exerted by the many saddles of over the gradient flow.Comment: 27 pages, 1 figur
Unified analysis of terminal-time control in classical and quantum systems
Many phenomena in physics, chemistry, and biology involve seeking an optimal
control to maximize an objective for a classical or quantum system which is
open and interacting with its environment. The complexity of finding an optimal
control for maximizing an objective is strongly affected by the possible
existence of sub-optimal maxima. Within a unified framework under specified
conditions, control objectives for maximizing at a terminal time physical
observables of open classical and quantum systems are shown to be inherently
free of sub-optimal maxima. This attractive feature is of central importance
for enabling the discovery of controls in a seamless fashion in a wide range of
phenomena transcending the quantum and classical regimes.Comment: 10 page
Wigner phase space distribution as a wave function
We demonstrate that the Wigner function of a pure quantum state is a wave
function in a specially tuned Dirac bra-ket formalism and argue that the Wigner
function is in fact a probability amplitude for the quantum particle to be at a
certain point of the classical phase space. Additionally, we establish that in
the classical limit, the Wigner function transforms into a classical
Koopman-von Neumann wave function rather than into a classical probability
distribution. Since probability amplitude need not be positive, our findings
provide an alternative outlook on the Wigner function's negativity.Comment: 6 pages and 2 figure
Calculation of the unitary part of the Bures measure for N-level quantum systems
We use the canonical coset parameterization and provide a formula with the
unitary part of the Bures measure for non-degenerate systems in terms of the
product of even Euclidean balls. This formula is shown to be consistent with
the sampling of random states through the generation of random unitary
matrices
Optimal number of pigments in photosynthetic complexes
We study excitation energy transfer in a simple model of photosynthetic
complex. The model, described by Lindblad equation, consists of pigments
interacting via dipole-dipole interaction. Overlapping of pigments induces an
on-site energy disorder, providing a mechanism for blocking the excitation
transfer. Based on the average efficiency as well as robustness of random
configurations of pigments, we calculate the optimal number of pigments that
should be enclosed in a pigment-protein complex of a given size. The results
suggest that a large fraction of pigment configurations are efficient as well
as robust if the number of pigments is properly chosen. We compare optimal
results of the model to the structure of pigment-protein complexes as found in
nature, finding good agreement.Comment: 20 pages, 7 figures; v2.: new appendix, published versio
Constructive control of quantum systems using factorization of unitary operators
We demonstrate how structured decompositions of unitary operators can be
employed to derive control schemes for finite-level quantum systems that
require only sequences of simple control pulses such as square wave pulses with
finite rise and decay times or Gaussian wavepackets. To illustrate the
technique it is applied to find control schemes to achieve population transfers
for pure-state systems, complete inversions of the ensemble populations for
mixed-state systems, create arbitrary superposition states and optimize the
ensemble average of dynamic observables.Comment: 28 pages, IoP LaTeX, principal author has moved to Cambridge
University ([email protected]
Optimization search effort over the control landscapes for open quantum systems with Kraus-map evolution
A quantum control landscape is defined as the expectation value of a target
observable as a function of the control variables. In this work
control landscapes for open quantum systems governed by Kraus map evolution are
analyzed. Kraus maps are used as the controls transforming an initial density
matrix into a final density matrix to maximize the expectation
value of the observable . The absence of suboptimal local maxima for
the relevant control landscapes is numerically illustrated. The dependence of
the optimization search effort is analyzed in terms of the dimension of the
system , the initial state , and the target observable
. It is found that if the number of nonzero eigenvalues in remains constant, the search effort does not exhibit any significant
dependence on . If has no zero eigenvalues, then the
computational complexity and the required search effort rise with . The
dimension of the top manifold (i.e., the set of Kraus operators that maximizes
the objective) is found to positively correlate with the optimization search
efficiency. Under the assumption of full controllability, incoherent control
modelled by Kraus maps is found to be more efficient in reaching the same value
of the objective than coherent control modelled by unitary maps. Numerical
simulations are also performed for control landscapes with linear constraints
on the available Kraus maps, and suboptimal maxima are not revealed for these
landscapes.Comment: 29 pages, 8 figure
Coherent Optimal Control of Multiphoton Molecular Excitation
We give a framework for molecular multiphoton excitation process induced by
an optimally designed electric field. The molecule is initially prepared in a
coherent superposition state of two of its eigenfunctions. The relative phase
of the two superposed eigenfunctions has been shown to control the optimally
designed electric field which triggers the multiphoton excitation in the
molecule. This brings forth flexibility in desiging the optimal field in the
laboratory by suitably tuning the molecular phase and hence by choosing the
most favorable interfering routes that the system follows to reach the target.
We follow the quantum fluid dynamical formulation for desiging the electric
field with application to HBr molecule.Comment: 5 figure
- …