10 research outputs found
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
Exciton Dynamics in Photosynthetic Complexes: Excitation by Coherent and Incoherent Light
In this paper we consider dynamics of a molecular system subjected to
external pumping by a light source. Within a completely quantum mechanical
treatment, we derive a general formula, which enables to asses effects of
different light properties on the photo-induced dynamics of a molecular system.
We show that once the properties of light are known in terms of certain
two-point correlation function, the only information needed to reconstruct the
system dynamics is the reduced evolution superoperator. The later quantity is
in principle accessible through ultrafast non-linear spectroscopy. Considering
a direct excitation of a small molecular antenna by incoherent light we find
that excitation of coherences is possible due to overlap of homogeneous line
shapes associated with different excitonic states. In Markov and secular
approximations, the amount of coherence is significant only under fast
relaxation, and both the populations and coherences between exciton states
become static at long time. We also study the case when the excitation of a
photosynthetic complex is mediated by a mesoscopic system. We find that such
case can be treated by the same formalism with a special correlation function
characterizing ultrafast fluctuations of the mesoscopic system. We discuss
bacterial chlorosom as an example of such a mesoscopic mediator and propose
that the properties of energy transferring chromophore-protein complexes might
be specially tuned for the fluctuation properties of their associated antennae.Comment: 12 page
Distribution of entanglement in light-harvesting complexes and their quantum efficiency
Recent evidence of electronic coherence during energy transfer in
photosynthetic antenna complexes has reinvigorated the discussion of whether
coherence and/or entanglement has any practical functionality for these
molecular systems. Here we investigate quantitative relationships between the
quantum yield of a light-harvesting complex and the distribution of
entanglement among its components. Our study focusses on the entanglement yield
or average entanglement surviving a time scale comparable to the average
excitation trapping time. As a prototype system we consider the
Fenna-Matthews-Olson (FMO) protein of green sulphur bacteria and show that
there is an inverse relationship between the quantum efficiency and the average
entanglement between distant donor sites. Our results suggest that longlasting
electronic coherence among distant donors might help modulation of the
lightharvesting function.Comment: Version accepted for publication in NJ
Motional effects on the efficiency of excitation transfer
Energy transfer plays a vital role in many natural and technological
processes. In this work, we study the effects of mechanical motion on the
excitation transfer through a chain of interacting molecules with application
to biological scenarios of transfer processes. Our investigation demonstrates
that, for various types of mechanical oscillations, the transfer efficiency is
significantly enhanced over that of comparable static configurations. This
enhancement is a genuine quantum signature, and requires the collaborative
interplay between the quantum-coherent evolution of the excitation and the
mechanical motion of the molecules; it has no analogue in the classical
incoherent energy transfer. This effect may not only occur naturally, but it
could be exploited in artificially designed systems to optimize transport
processes. As an application, we discuss a simple and hence robust control
technique.Comment: 25 pages, 11 figures; completely revised; version accepted for
publicatio
Non-Markovian stochastic description of quantum transport in photosynthetic systems
We analyze several aspects of the transport dynamics in the LH1-RC core of
purple bacteria, which consists basically in a ring of antenna molecules that
transport the energy into a target molecule, the reaction center, placed in the
center of the ring. We show that the periodicity of the system plays an
important role to explain the relevance of the initial state in the transport
efficiency. This picture is modified, and the transport enhanced for any
initial state, when considering that molecules have different energies, and
when including their interaction with the environment. We study this last
situation by using stochastic Schr{\"o}dinger equations, both for Markovian and
non-Markovian type of interactions.Comment: 21 pages, 5 figure
Vibrational excitons in ionophores: Experimental probes for quantum coherence-assisted ion transport and selectivity in ion channels
Despite a large body of work, the exact molecular details underlying
ion-selectivity and transport in the potassium channel have not been fully laid
to rest. One major reason has been the lack of experimental methods that can
probe these mechanisms dynamically on their biologically relevant time scales.
Recently it was suggested that quantum coherence and its interplay with thermal
vibration might be involved in mediating ion-selectivity and transport. In this
work we present an experimental strategy for using time resolved infrared
spectroscopy to investigate these effects. We show the feasibility by
demonstrating the IR absorption and Raman spectroscopic signatures of potassium
binding model molecules that mimic the transient interactions of potassium with
binding sites of the selectivity filter during ion conduction. In addition to
guide our experiments on the real system we have performed molecular
dynamic-based simulations of the FTIR and 2DIR spectra of the entire KcsA
complex, which is the largest complex for which such modeling has been
performed. We found that by combing isotope labeling with 2D IR spectroscopy,
the signatures of potassium interaction with individual binding sites would be
experimentally observable and identified specific labeling combinations that
would maximize our expected experimental signatures
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
Dephasing-induced diffusive transport in anisotropic Heisenberg model
We study transport properties of anisotropic Heisenberg model in a disordered
magnetic field experiencing dephasing due to external degrees of freedom. In
the absence of dephasing the model can display, depending on parameter values,
the whole range of possible transport regimes: ideal ballistic conduction,
diffusive, or ideal insulating behavior. We show that the presence of dephasing
induces normal diffusive transport in a wide range of parameters. We also
analyze the dependence of spin conductivity on the dephasing strength. In
addition, by analyzing the decay of spin-spin correlation function we discover
a presence of long-range order for finite chain sizes. All our results for a
one-dimensional spin chain at infinite temperature can be equivalently
rephrased for strongly-interacting disordered spinless fermions.Comment: 15 pages, 9 PS figure
Noise-assisted energy transfer in quantum networks and light-harvesting complexes
'This is an author-created, un-copyedited version of an article accepted for publication in New Journal of Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher authenticated version is available online at: http://dx.doi.org/10.1088/1367-2630/12/6/065002 .'We provide physically intuitive mechanisms for the effect of noise on excitation energy transfer (EET) in networks. Using these mechanisms of dephasing-assisted transport (DAT) in a hybrid basis of both excitons and sites, we develop a detailed picture of how noise enables energy transfer with efficiencies well above 90% across the Fenna–Matthew–Olson (FMO) complex, a type of light-harvesting molecule. We demonstrate explicitly how noise alters the pathways of energy transfer across the complex, suppressing ineffective pathways and facilitating direct ones to the reaction centre. We explain that the fundamental mechanisms underpinning DAT are expected to be robust with respect to the considered noise model but show that the specific details of the exciton–phonon coupling, which remain largely unknown in these type of complexes, and in particular the impact of non-Markovian effects, result in variations of dynamical features that should be amenable to experimental verification with current or planned technology. A detailed understanding of DAT in natural compounds could open up a new paradigm of 'noise-engineering' by which EET can be optimized in artificial light-harvesting structures.Peer reviewe
Exact matrix product solutions in the Heisenberg picture of an open quantum spin chain
10.1088/1367-2630/12/2/025005New Journal of Physics12