125 research outputs found
Long-lived quantum coherence in photosynthetic complexes at physiological temperature
Photosynthetic antenna complexes capture and concentrate solar radiation by
transferring the excitation to the reaction center which stores energy from the
photon in chemical bonds. This process occurs with near-perfect quantum
efficiency. Recent experiments at cryogenic temperatures have revealed that
coherent energy transfer - a wavelike transfer mechanism - occurs in many
photosynthetic pigment-protein complexes (1-4). Using the Fenna-Matthews-Olson
antenna complex (FMO) as a model system, theoretical studies incorporating both
incoherent and coherent transfer as well as thermal dephasing predict that
environmentally assisted quantum transfer efficiency peaks near physiological
temperature; these studies further show that this process is equivalent to a
quantum random walk algorithm (5-8). This theory requires long-lived quantum
coherence at room temperature, which never has been observed in FMO. Here we
present the first evidence that quantum coherence survives in FMO at
physiological temperature for at least 300 fs, long enough to perform a
rudimentary quantum computational operation. This data proves that the
wave-like energy transfer process discovered at 77 K is directly relevant to
biological function. Microscopically, we attribute this long coherence lifetime
to correlated motions within the protein matrix encapsulating the chromophores,
and we find that the degree of protection afforded by the protein appears
constant between 77 K and 277 K. The protein shapes the energy landscape and
mediates an efficient energy transfer despite thermal fluctuations. The
persistence of quantum coherence in a dynamic, disordered system under these
conditions suggests a new biomimetic strategy for designing dedicated quantum
computational devices that can operate at high temperature.Comment: PDF files, 15 pages, 3 figures (included in the PDF file
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
Excitation energy transfer in light-harvesting system: Effect of initial state
The light-harvesting is a problem of long interest. It becomes active again
in recent years stimulated by suggestions of quantum effects in energy
transport. Recent experiments found evidence that BChla 1 and BChla 6 are the
first to be excited in the Fenna-Matthews-Olson(FMO) protein, theoretical
studies, however, are mostly restricted to consider the exciton in BChla 1
initially. In this paper, we study the energy transport in the FMO complex by
taking different initial states into account. Optimizations are performed for
the decoherence rates as to maximal transport efficiency. Dependence of the
energy transfer efficiency on the initial states is given and discussed.
Effects of fluctuations in the site energies and couplings are also examined.Comment: 6 pages, 6 figures, J Phys B accepte
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
Traffic Equations and Granular Convection
We investigate both numerically and analytically the convective instability
of granular materials by two dimensional traffic equations. In the absence of
vibrations the traffic equations assume two distinctive classes of fixed bed
solutions with either a spatially uniform or nonuniform density profile. The
former one exists only when the function V(\rho) that monitors the relaxation
of grains assumes a cut off at the closed packed density, \rho_c, with
V(\rho_c)=0, while the latter one exists for any form of V. Since there is
little difference between the uniform and nonuniform solution deep inside the
bed, the convective instability of the bulk may be studied by focusing on the
stability of the uniform solution. In the presence of vibrations, we find that
the uniform solution bifurcates into a bouncing solution, which then undergoes
a supercritical bifurcation to the convective instability. We determine the
onset of convection as a function of control parameters and confirm this
picture by solving the traffic equations numerically, which reveals bouncing
solutions, two convective rolls, and four convective rolls. Further, convective
patterns change as the aspect ratio changes: in a vertically long container,
the rolls move toward the surface, and in a horizontally long container, the
rolls move toward the walls. We compare these results with those reported
previously with a different continuum model by Hayakawa, Yue and Hong[Phys.
Rev. Lett. 75,2328, 1995]. Finally, we also present a derivation of the traffic
equations from Enskoq equation.Comment: 34 pages, 10 figure
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
Implementación de módulo de autoaprendizaje para la enseñanza de la histología del ojo y oído
La complejidad histológica de ojo y oído es un punto crítico para su aprendizaje, lo que se evidencia en los exámenes finales escritos. Para motivar su estudio y mejorar el rendimiento académico de los alumnos se implementó un Módulo de Autoaprendizaje. Se elaboró material impreso con objetivos, contenidos, metodología, cronograma de actividades y bibliografía recomendada. Presenta dos problemas (uno de ojo y otro de oído) para estimular búsqueda de información y el logro de los objetivos. Se desarrolló en 5 semanas.Facultad de Ciencias Médica
Implementación de módulo de autoaprendizaje para la enseñanza de la histología de ojo y oído
La complejidad histológica de ojo y oído es un punto crítico para su aprendizaje, lo que se evidencia en los exámenes finales escritos. Para motivar su estudio y mejorar el rendimiento académico de los alumnos se implementó un Módulo de Autoaprendizaje. Se elaboró material impreso con objetivos, contenidos, metodología, cronograma de actividades y bibliografía recomendada. Presenta dos problemas (uno de ojo y otro de oído) para estimular búsqueda de información y el logro de los objetivos. Se desarrolló en 5 semanas. El docente tuvo rol de consultor. La evaluación del aprendizaje se realizó en el examen final escrito.Facultad de Ciencias Médica
Velocity and density profiles of granular flow in channels using lattice gas automaton
We have performed two-dimensional lattice-gas-automaton simulations of
granular flow between two parallel planes. We find that the velocity profiles
have non-parabolic distributions while simultaneously the density profiles are
non-uniform. Under non-slip boundary conditions, deviation of velocity profiles
from the parabolic form of newtonian fluids is found to be characterized solely
by ratio of maximal velocity at the center to the average velocity, though the
ratio depends on the model parameters in a complex manner. We also find that
the maximal velocity () at the center is a linear function of the
driving force (g) as with non-zero in
contrast with newtonian fluids. Regarding density profiles, we observe that
densities near the boundaries are higher than those in the center. The width of
higher densities (above the average density) relative to the channel width is a
decreasing function of a variable which scales with the driving force (g),
energy dissipation parameter () and the width of the system (L) as
with exponents and . A phenomenological theory based on a scaling argument is presented to
interpret these findings.Comment: Latex, 15 figures, to appear in PR
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