Akihito Ishizaki

Unified treatment of quantum coherent and incoherent hopping dynamics in electronic energy transfer: reduced hierarchy equation approach.

A new quantum dynamic equation for excitation energy transfer is developed which can describe quantum coherent wavelike motion and incoherent hopping in a unified manner. The developed equation reduces to the conventional Redfield theory and Forster theory in their respective limits of validity. In the regime of coherent wavelike motion, the equation predicts several times longer lifetime of electronic coherence between chromophores than does the conventional Redfield equation. Furthermore, we show quantum coherent motion can be observed even when reorganization energy is large in comparison to intersite electronic coupling (the Forster incoherent regime). In the region of small reorganization energy, slow fluctuation sustains longer-lived coherent oscillation, whereas the Markov approximation in the Redfield framework causes infinitely fast fluctuation and then collapses the quantum coherence. In the region of large reorganization energy, sluggish dissipation of reorganization energy increases the time electronic excitation stays above an energy barrier separating chromophores and thus prolongs delocalization over the chromophores.

On the adequacy of the Redfield equation and related approaches to the study of quantum dynamics in electronic energy transfer

The observation of long-lived electronic coherence in photosynthetic excitation energy transfer (EET) by Engel et al. [Nature (London) 446, 782 (2007)] raises questions about the role of the protein environment in protecting this coherence and the significance of the quantum coherence in light harvesting efficiency. In this paper we explore the applicability of the Redfield equation in its full form, in the secular approximation and with neglect of the imaginary part of the relaxation terms for the study of these phenomena. We find that none of the methods can give a reliable picture of the role of the environment in photosynthetic EET. In particular the popular secular approximation (or the corresponding Lindblad equation) produces anomalous behavior in the incoherent transfer region leading to overestimation of the contribution of environment-assisted transfer. The full Redfield expression on the other hand produces environment-independent dynamics in the large reorganization energy region. A companion paper presents an improved approach, which corrects these deficiencies [A. Ishizaki and G. R. Fleming, J. Chem. Phys. 130, 234111 (2009)].

Quantum superpositions in photosynthetic light harvesting: delocalization and entanglement

We explore quantum entanglement among the chlorophyll mol- ecules in light-harvesting complex II, which is the most abundant photosynthetic antenna complex in plants containing over 50% of the worlds chlorophyll molecules. Our results demonstrate that there exists robust quantum entangle- ment under physiological conditions for the case of a single elementary excitation. However, this nonvanishing entanglement is not unexpected because entanglement in the single-excitation manifold is conceptually the same as quantum delocalized states, which are the spectroscopically detectable energy eigenstatesofthe system. Wediscuss theimpact ofthe surroundingenvironments and correlated fluctuations in electronic energies of different pigments upon quantum delocalization and quantum entanglement. It is demonstrated that investigations with tools quantifying the entanglement can provide us with more detailed information on the nature of quantum delocalization, in particular the so-called dynamic localization, which is difficult for a traditional treatment to capture.

Iterative path-integral algorithm versus cumulant time-nonlocal master equation approach for dissipative biomolecular exciton transport

We determine the real-time quantum dynamics of a biomolecular donor–acceptor system in order to describe excitonic energy transfer in the presence of slow environmental Gaussian fluctuations. For this, we compare two different approaches. On the one hand, we use the numerically exact iterative quasi-adiabatic propagator path-integral scheme that incorporates all non-Markovian contributions. On the other, we apply the second-order cumulant time-nonlocal quantum master equation that includes non-Markovian effects. We show that both approaches yield coinciding results in the relevant crossover regime from weak to strong electronic couplings, displaying coherent as well as incoherent transitions.