1,500 research outputs found
Quasi-Bell inequalities from symmetrized products of noncommuting qubit observables
Noncommuting observables cannot be simultaneously measured, however, under
local hidden variable models, they must simultaneously hold premeasurement
values, implying the existence of a joint probability distribution. We study
the joint distributions of noncommuting observables on qubits, with possible
criteria of positivity and the Fr\'echet bounds limiting the joint
probabilities, concluding that the latter may be negative. We use
symmetrization, justified heuristically and then more carefully via the Moyal
characteristic function, to find the quantum operator corresponding to the
product of noncommuting observables. This is then used to construct Quasi-Bell
inequalities, Bell inequalities containing products of noncommuting
observables, on two qubits. These inequalities place limits on local hidden
variable models that define joint probabilities for noncommuting observables.
We find Quasi-Bell inequalities have a quantum to classical violation as high
as , higher than conventional Bell inequalities. The result
demonstrates the theoretical importance of noncommutativity in the nonlocality
of quantum mechanics, and provides an insightful generalization of Bell
inequalities.Comment: 17 page
Influences of quantum mechanically mixed electronic and vibrational pigment states in 2D electronic spectra of photosynthetic systems: Strong electronic coupling cases
In 2D electronic spectroscopy studies, long-lived quantum beats have recently
been observed in photosynthetic systems, and it has been suggested that the
beats are produced by quantum mechanically mixed electronic and vibrational
states. Concerning the electronic-vibrational quantum mixtures, the impact of
protein-induced fluctuations was examined by calculating the 2D electronic
spectra of a weakly coupled dimer with vibrational modes in the resonant
condition [J. Chem. Phys. 142, 212403 (2015)]. This analysis demonstrated that
quantum mixtures of the vibronic resonance are rather robust under the
influence of the fluctuations at cryogenic temperatures, whereas the mixtures
are eradicated by the fluctuations at physiological temperatures. However, this
conclusion cannot be generalized because the magnitude of the coupling inducing
the quantum mixtures is proportional to the inter-pigment coupling. In this
study, we explore the impact of the fluctuations on electronic-vibrational
quantum mixtures in a strongly coupled dimer. with an off-resonant vibrational
mode. Toward this end, we calculate electronic energy transfer (EET) dynamics
and 2D electronic spectra of a dimer that corresponds to the most strongly
coupled bacteriochlorophyll molecules in the Fenna-Matthews-Olson complex in a
numerically accurate manner. The quantum mixtures are found to be robust under
the exposure of protein-induced fluctuations at cryogenic temperatures,
irrespective of the resonance. At 300 K, however, the quantum mixing is
disturbed more strongly by the fluctuations, and therefore, the beats in the 2D
spectra become obscure even in a strongly coupled dimer with a resonant
vibrational mode. Further, the overall behaviors of the EET dynamics are
demonstrated to be dominated by the environment and coupling between the 0-0
vibronic transitions as long as the Huang-Rhys factor of the vibrational mode
is small.Comment: 20 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1505.0528
Energy-dependent quenching adjusts the excitation diffusion length to regulate photosynthetic light harvesting
An important determinant of crop yields is the regulation of photosystem II
(PSII) light harvesting by energy-dependent quenching (qE). However, the
molecular details of excitation quenching have not been quantitatively
connected to the PSII yield, which only emerges on the 100 nm scale of the
grana membrane and determines flux to downstream metabolism. Here, we
incorporate excitation dissipation by qE into a pigment-scale model of
excitation transfer and trapping for a 200 nm x 200 nm patch of the grana
membrane. We demonstrate that single molecule measurements of qE are consistent
with a weak-quenching regime. Consequently, excitation transport can be
rigorously coarse-grained to a 2D random walk with an excitation diffusion
length determined by the extent of quenching. A diffusion-corrected lake model
substantially improves the PSII yield determined from variable chlorophyll
fluorescence measurements and offers an improved model of PSII for
photosynthetic metabolism.Comment: 19 pages, 4 figures, 3 supplementary figure
Models and measurements of energy-dependent quenching.
Energy-dependent quenching (qE) in photosystem II (PSII) is a pH-dependent response that enables plants to regulate light harvesting in response to rapid fluctuations in light intensity. In this review, we aim to provide a physical picture for understanding the interplay between the triggering of qE by a pH gradient across the thylakoid membrane and subsequent changes in PSII. We discuss how these changes alter the energy transfer network of chlorophyll in the grana membrane and allow it to switch between an unquenched and quenched state. Within this conceptual framework, we describe the biochemical and spectroscopic measurements and models that have been used to understand the mechanism of qE in plants with a focus on measurements of samples that perform qE in response to light. In addition, we address the outstanding questions and challenges in the field. One of the current challenges in gaining a full understanding of qE is the difficulty in simultaneously measuring both the photophysical mechanism of quenching and the physiological state of the thylakoid membrane. We suggest that new experimental and modeling efforts that can monitor the many processes that occur on multiple timescales and length scales will be important for elucidating the quantitative details of the mechanism of qE
Coherent exciton dynamics in the presence of underdamped vibrations
Recent ultrafast optical experiments show that excitons in large biological
light-harvesting complexes are coupled to molecular vibration modes. These
high-frequency vibrations will not only affect the optical response, but also
drive the exciton transport. Here, using a model dimer system, the frequency of
the underdamped vibration is shown to have a strong effect on the exciton
dynamics such that quantum coherent oscillations in the system can be present
even in the case of strong noise. Two mechanisms are identified to be
responsible for the enhanced transport efficiency: critical damping due to the
tunable effective strength of the coupling to the bath, and resonance coupling
where the vibrational frequency coincides with the energy gap in the system.
The interplay of these two mechanisms determines parameters responsible for the
most efficient transport, and these optimal control parameters are comparable
to those in realistic light-harvesting complexes. Interestingly, oscillations
in the excitonic coherence at resonance are suppressed in comparison to the
case of an off-resonant vibration
Insights into photosynthetic energy transfer gained from free-energy structure: Coherent transport, incoherent hopping, and vibrational assistance revisited
Giant strides in ultrashort laser pulse technology have enabled real-time
observation of dynamical processes in complex molecular systems. Specifically,
the discovery of oscillatory transients in the two-dimensional electronic
spectra of photosynthetic systems stimulated a number of theoretical
investigations exploring possible physical mechanisms of the remarkable quantum
efficiency of light harvesting processes. However, the theories employed have
reached a high degree of sophistication and have become complex, making it
difficult to gain insights into microscopic processes and biologically
significant questions. In this work, we revisit the elementary aspects of
environment-induced fluctuations in the involved electronic energies and
present a simple way to understand energy flow with the intuitive picture of
relaxation in a funnel-type free-energy landscape. The presented free-energy
description of energy transfer reveals that typical photosynthetic systems
operate in an almost barrierless regime. The approach also provides insights
into the distinction between coherent and incoherent energy transfer and
criteria by which the necessity of the vibrational assistance is considered
Recommended from our members
Vibronic mixing enables ultrafast energy flow in light-harvesting complex II.
Since the discovery of quantum beats in the two-dimensional electronic spectra of photosynthetic pigment-protein complexes over a decade ago, the origin and mechanistic function of these beats in photosynthetic light-harvesting has been extensively debated. The current consensus is that these long-lived oscillatory features likely result from electronic-vibrational mixing, however, it remains uncertain if such mixing significantly influences energy transport. Here, we examine the interplay between the electronic and nuclear degrees of freedom (DoF) during the excitation energy transfer (EET) dynamics of light-harvesting complex II (LHCII) with two-dimensional electronic-vibrational spectroscopy. Particularly, we show the involvement of the nuclear DoF during EET through the participation of higher-lying vibronic chlorophyll states and assign observed oscillatory features to specific EET pathways, demonstrating a significant step in mapping evolution from energy to physical space. These frequencies correspond to known vibrational modes of chlorophyll, suggesting that electronic-vibrational mixing facilitates rapid EET over moderately size energy gaps
Competition between Energy and Phase Relaxation in Electronic Curve Cross- ing Processes
We present results from simulations of vibrational energy and phase relaxation and electronic curve crossing using a multilevel formulation of Redfield theory, which demonstrate the shortcomings of the optical Bloch approximation and the importance of coherence transfer processes in the relaxation dynamics of multilevel systems. Specifically, we show that for a harmonic well, energy relaxation can occur with retention of vibrational phase, and that for sufficiently strong electronic coupling, the product of an electronic curve crossing process can be formed vibrationally coherent even when no coherence is present in the initially excited state
Dipolar solvation dynamics
The dynamics of solvation of newly created dipoles is discussed. Developments of standard continuum models to include non-Debye dielectric response and saturation effects are described. Equilibrium and non-equilibrium molecular-dynamics simulations of ST2 model water are described. The simulations predict non-exponential solvation dynamics as a result of the radial dependence of the solvent response. Experimental data on time-resolved fluorescence Stokes shifts for a number of probe molecules in a variety of polar solvents are discussed in the context of the theoretical results
- …