38 research outputs found
Symmetry-enhanced supertransfer of delocalized quantum states
Coherent hopping of excitation rely on quantum coherence over physically
extended states. In this work, we consider simple models to examine the effect
of symmetries of delocalized multi-excitation states on the dynamical
timescales, including hopping rates, radiative decay, and environmental
interactions. While the decoherence (pure dephasing) rate of an extended state
over N sites is comparable to that of a non-extended state, superradiance leads
to a factor of N enhancement in decay and absorption rates. In addition to
superradiance, we illustrate how the multi-excitonic states exhibit
`supertransfer' in the far-field regime: hopping from a symmetrized state over
N sites to a symmetrized state over M sites at a rate proportional to MN. We
argue that such symmetries could play an operational role in physical systems
based on the competition between symmetry-enhanced interactions and localized
inhomogeneities and environmental interactions that destroy symmetry. As an
example, we propose that supertransfer and coherent hopping play a role in
recent observations of anomolously long diffusion lengths in nano-engineered
assembly of light-harvesting complexes.Comment: 6 page
Numerical Evidence for Robustness of Environment-Assisted Quantum Transport
Recent theoretical studies show that decoherence process can enhance
transport efficiency in quantum systems. This effect is known as
environment-assisted quantum transport (ENAQT). The role of ENAQT in optimal
quantum transport is well investigated, however, it is less known how robust
ENAQT is with respect to variations in the system or its environment
characteristic. Toward answering this question, we simulated excitonic energy
transfer in Fenna-Matthews-Olson (FMO) photosynthetic complex. We found that
ENAQT is robust with respect to many relevant parameters of environmental
interactions and Frenkel-exciton Hamiltonian including reorganization energy,
bath frequency cutoff, temperature, and initial excitations, dissipation rate,
trapping rate, disorders, and dipole moments orientations. Our study suggests
that the ENAQT phenomenon can be exploited in robust design of highly efficient
quantum transport systems.Comment: arXiv admin note: substantial text overlap with arXiv:1104.481
Geometrical effects on energy transfer in disordered open quantum systems
We explore various design principles for efficient excitation energy
transport in complex quantum systems. We investigate energy transfer efficiency
in randomly disordered geometries consisting of up to 20 chromophores to
explore spatial and spectral properties of small natural/artificial
Light-Harvesting Complexes (LHC). We find significant statistical correlations
among highly efficient random structures with respect to ground state
properties, excitonic energy gaps, multichromophoric spatial connectivity, and
path strengths. These correlations can even exist beyond the optimal regime of
environment-assisted quantum transport. For random configurations embedded in
spatial dimensions of 30 A and 50 A, we observe that the transport efficiency
saturates to its maximum value if the systems contain 7 and 14 chromophores
respectively. Remarkably, these optimum values coincide with the number of
chlorophylls in (Fenna-Matthews-Olson) FMO protein complex and LHC II monomers,
respectively, suggesting a potential natural optimization with respect to
chromophoric density.Comment: 11 pages, 10 figures. Expanded from the former appendix to
arXiv:1104.481
Efficient estimation of nearly sparse many-body quantum Hamiltonians
We develop an efficient and robust approach to Hamiltonian identification for
multipartite quantum systems based on the method of compressed sensing. This
work demonstrates that with only O(s log(d)) experimental configurations,
consisting of random local preparations and measurements, one can estimate the
Hamiltonian of a d-dimensional system, provided that the Hamiltonian is nearly
s-sparse in a known basis. We numerically simulate the performance of this
algorithm for three- and four-body interactions in spin-coupled quantum dots
and atoms in optical lattices. Furthermore, we apply the algorithm to
characterize Hamiltonian fine structure and unknown system-bath interactions.Comment: 8 pages, 2 figures. Title is changed. Detailed error analysis is
added. Figures are updated with additional clarifying discussion
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
Efficient estimation of energy transfer efficiency in light-harvesting complexes
The fundamental physical mechanisms of energy transfer in photosynthetic
complexes is not yet fully understood. In particular, the degree of efficiency
or sensitivity of these systems for energy transfer is not known given their
non-perturbative and non-Markovian interactions with proteins backbone and
surrounding photonic and phononic environments. One major problem in studying
light-harvesting complexes has been the lack of an efficient method for
simulation of their dynamics in biological environments. To this end, here we
revisit the second-order time-convolution (TC2) master equation and examine its
reliability beyond extreme Markovian and perturbative limits. In particular, we
present a derivation of TC2 without making the usual weak system-bath coupling
assumption. Using this equation, we explore the long time behaviour of exciton
dynamics of Fenna-Matthews-Olson (FMO) protein complex. Moreover, we introduce
a constructive error analysis to estimate the accuracy of TC2 equation in
calculating energy transfer efficiency, exhibiting reliable performance for
environments with weak and intermediate memory and strength. Furthermore, we
numerically show that energy transfer efficiency is optimal and robust for the
FMO protein complex of green sulphur bacteria with respect to variations in
reorganization energy and bath correlation time-scales.Comment: 16 pages, 9 figures, modified version, updated appendices and
reference lis
Polynomial-time quantum algorithm for the simulation of chemical dynamics
The computational cost of exact methods for quantum simulation using
classical computers grows exponentially with system size. As a consequence,
these techniques can only be applied to small systems. By contrast, we
demonstrate that quantum computers could exactly simulate chemical reactions in
polynomial time. Our algorithm uses the split-operator approach and explicitly
simulates all electron-nuclear and inter-electronic interactions in quadratic
time. Surprisingly, this treatment is not only more accurate than the
Born-Oppenheimer approximation, but faster and more efficient as well, for all
reactions with more than about four atoms. This is the case even though the
entire electronic wavefunction is propagated on a grid with appropriately short
timesteps. Although the preparation and measurement of arbitrary states on a
quantum computer is inefficient, here we demonstrate how to prepare states of
chemical interest efficiently. We also show how to efficiently obtain
chemically relevant observables, such as state-to-state transition
probabilities and thermal reaction rates. Quantum computers using these
techniques could outperform current classical computers with one hundred
qubits.Comment: 9 pages, 3 figures. Updated version as appears in PNA
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
Environment-Assisted Quantum Transport
Transport phenomena at the nanoscale are of interest due to the presence of
both quantum and classical behavior. In this work, we demonstrate that quantum
transport efficiency can be enhanced by a dynamical interplay of the system
Hamiltonian with pure dephasing induced by a fluctuating environment. This is
in contrast to fully coherent hopping that leads to localization in disordered
systems, and to highly incoherent transfer that is eventually suppressed by the
quantum Zeno effect. We study these phenomena in the Fenna-Matthews-Olson
protein complex as a prototype for larger photosynthetic energy transfer
systems. We also show that disordered binary tree structures exhibit enhanced
transport in the presence of dephasing.Comment: 7 pages, 3 figures, improved presentation, to appear in New Journal
of Physic
Environment-Assisted Quantum Walks in Photosynthetic Energy Transfer
Energy transfer within photosynthetic systems can display quantum effects
such as delocalized excitonic transport. Recently, direct evidence of
long-lived coherence has been experimentally demonstrated for the dynamics of
the Fenna-Matthews-Olson (FMO) protein complex [Engel et al., Nature 446, 782
(2007)]. However, the relevance of quantum dynamical processes to the exciton
transfer efficiency is to a large extent unknown. Here, we develop a
theoretical framework for studying the role of quantum interference effects in
energy transfer dynamics of molecular arrays interacting with a thermal bath
within the Lindblad formalism. To this end, we generalize continuous-time
quantum walks to non-unitary and temperature-dependent dynamics in Liouville
space derived from a microscopic Hamiltonian. Different physical effects of
coherence and decoherence processes are explored via a universal measure for
the energy transfer efficiency and its susceptibility. In particular, we
demonstrate that for the FMO complex an effective interplay between free
Hamiltonian and thermal fluctuations in the environment leads to a substantial
increase in energy transfer efficiency from about 70% to 99%.Comment: 9 pages, 3 figures, improved presentation, updated references, to
appear in Journal of Chemical Physic