73,249 research outputs found
Coherence and Decoherence in Biological Systems: Principles of Noise Assisted Transport and the Origin of Long-lived Coherences
The quantum dynamics of transport networks in the presence of noisy
environments have recently received renewed attention with the discovery of
long-lived coherences in different photosynthetic complexes. This experimental
evidence has raised two fundamental questions: Firstly, what are the mechanisms
supporting long-lived coherences and secondly, how can we assess the possible
functional role that the interplay of noise and quantum coherence might play in
the seemingly optimal operation of biological systems under natural conditions?
Here we review recent results, illuminate them at the hand of two paradigmatic
systems, the Fenna-Matthew-Olson (FMO) complex and the light harvesting complex
LHII, and present new progress on both questions. In particular we introduce
the concept of the phonon antennae and discuss the possible microscopic origin
or long-lived electronic coherences.Comment: Paper delivered at the Royal Society Discussion Meeting
"Quantum-coherent energy transfer: implications for biology and new energy
technologies", 27 - 28 April 2011 at The Kavli Royal Society International
Centre, Buckinghamshire, UK. Accepted for publication in Philosophical
Transactions of the Royal Society
Control of quantum phenomena: Past, present, and future
Quantum control is concerned with active manipulation of physical and
chemical processes on the atomic and molecular scale. This work presents a
perspective of progress in the field of control over quantum phenomena, tracing
the evolution of theoretical concepts and experimental methods from early
developments to the most recent advances. The current experimental successes
would be impossible without the development of intense femtosecond laser
sources and pulse shapers. The two most critical theoretical insights were (1)
realizing that ultrafast atomic and molecular dynamics can be controlled via
manipulation of quantum interferences and (2) understanding that optimally
shaped ultrafast laser pulses are the most effective means for producing the
desired quantum interference patterns in the controlled system. Finally, these
theoretical and experimental advances were brought together by the crucial
concept of adaptive feedback control, which is a laboratory procedure employing
measurement-driven, closed-loop optimization to identify the best shapes of
femtosecond laser control pulses for steering quantum dynamics towards the
desired objective. Optimization in adaptive feedback control experiments is
guided by a learning algorithm, with stochastic methods proving to be
especially effective. Adaptive feedback control of quantum phenomena has found
numerous applications in many areas of the physical and chemical sciences, and
this paper reviews the extensive experiments. Other subjects discussed include
quantum optimal control theory, quantum control landscapes, the role of
theoretical control designs in experimental realizations, and real-time quantum
feedback control. The paper concludes with a prospective of open research
directions that are likely to attract significant attention in the future.Comment: Review article, final version (significantly updated), 76 pages,
accepted for publication in New J. Phys. (Focus issue: Quantum control
Noise-induced synchronization and anti-resonance in excitable systems; Implications for information processing in Parkinson's Disease and Deep Brain Stimulation
We study the statistical physics of a surprising phenomenon arising in large
networks of excitable elements in response to noise: while at low noise,
solutions remain in the vicinity of the resting state and large-noise solutions
show asynchronous activity, the network displays orderly, perfectly
synchronized periodic responses at intermediate level of noise. We show that
this phenomenon is fundamentally stochastic and collective in nature. Indeed,
for noise and coupling within specific ranges, an asymmetry in the transition
rates between a resting and an excited regime progressively builds up, leading
to an increase in the fraction of excited neurons eventually triggering a chain
reaction associated with a macroscopic synchronized excursion and a collective
return to rest where this process starts afresh, thus yielding the observed
periodic synchronized oscillations. We further uncover a novel anti-resonance
phenomenon: noise-induced synchronized oscillations disappear when the system
is driven by periodic stimulation with frequency within a specific range. In
that anti-resonance regime, the system is optimal for measures of information
capacity. This observation provides a new hypothesis accounting for the
efficiency of Deep Brain Stimulation therapies in Parkinson's disease, a
neurodegenerative disease characterized by an increased synchronization of
brain motor circuits. We further discuss the universality of these phenomena in
the class of stochastic networks of excitable elements with confining coupling,
and illustrate this universality by analyzing various classical models of
neuronal networks. Altogether, these results uncover some universal mechanisms
supporting a regularizing impact of noise in excitable systems, reveal a novel
anti-resonance phenomenon in these systems, and propose a new hypothesis for
the efficiency of high-frequency stimulation in Parkinson's disease
Self-induced switchings between multiple space-time patterns on complex networks of excitable units
We report on self-induced switchings between multiple distinct space--time
patterns in the dynamics of a spatially extended excitable system. These
switchings between low-amplitude oscillations, nonlinear waves, and extreme
events strongly resemble a random process, although the system is
deterministic. We show that a chaotic saddle -- which contains all the patterns
as well as channel-like structures that mediate the transitions between them --
is the backbone of such a pattern switching dynamics. Our analyses indicate
that essential ingredients for the observed phenomena are that the system
behaves like an inhomogeneous oscillatory medium that is capable of
self-generating spatially localized excitations and that is dominated by
short-range connections but also features long-range connections. With our
findings, we present an alternative to the well-known ways to obtain
self-induced pattern switching, namely noise-induced attractor hopping,
heteroclinic orbits, and adaptation to an external signal. This alternative way
can be expected to improve our understanding of pattern switchings in spatially
extended natural dynamical systems like the brain and the heart
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