No abstract

Charge and energy transfer in biological and synthetic organic materials are strongly influenced by the coupling of electronic states to high-frequency underdamped vibrations under dephasing noise. Non-perturbative simulations of these systems require a substantial computational effort and current methods can only be applied to large systems with severely coarse-grained environmental structures. In this work, we introduce a dissipation-assisted matrix product factorization (DAMPF) method based on a memory-efficient matrix product operator (MPO) representation of the vibronic state at finite temperature. In this approach, the correlations between environmental vibrational modes can be controlled by the MPO bond dimension, allowing for systematic interpolation between approximate and numerically exact dynamics. Crucially, by subjecting the vibrational modes to damping, we show that one can significantly reduce the bond dimension required to achieve a desired accuracy, and also consider a continuous, highly structured spectral density in a non-perturbative manner. We demonstrate that our method can simulate large vibronic systems consisting of 10-50 sites coupled with 100-1000 underdamped modes in total and for a wide range of parameter regimes. An analytical error bound is provided which allows one to monitor the accuracy of the numerical results. This formalism will facilitate the investigation of spatially extended systems with applications to quantum biology, organic photovoltaics and quantum thermodynamics. arXiv:1903.05443v2 [quant-ph]

Photosynthesis is one of the most essential biological processes in which specialized pigment-protein complexes absorb solar photons, and with a remarkably high efficiency, guide the photo-induced excitation energy toward the reaction center to subsequently trigger its conversion to chemical energy. In this work, we review the principles of optimal energy transfer in various natural and artificial light harvesting systems. We begin by presenting the guiding principles for optimizing the energy transfer efficiency in systems connected to dissipative environments, with particular attention paid to the potential role of quantum coherence in light harvesting systems. We will comment briefly on photo-protective mechanisms in natural systems that ensure optimal functionality under varying ambient conditions. For completeness, we will also present an overview of the charge separation and electron transfer pathways in reaction centers. Finally, recent theoretical and experimental progress on excitation energy transfer, charge separation, and charge transport in artificial light harvesting systems is delineated, with organic solar cells taken as prime examples.

The chlorosome antenna complex is a fascinating structure which due to its immense scale, accurate simulation of excitation energy transfer (EET) dynamics supposes a genuine computational challenge. Resonant vibronic modes have been recently identified in 2D spectra of the chlorosome which motivates our present endeavour of modelling electronic and vibrational degrees of freedom on an equal footing. Following the Dirac-Frenkel time-dependent variational principle, we exploit a general theory of polaron dynamics in two-dimensional lattices based on the Holstein molecular crystal model and investigate a single rod model of pigment aggregates. Unlike reduced formalisms, explicit integration of the degrees of freedom of both the system and the bath requires extensive computational resources. We exploit the architecture of graphic processor units (GPUs) by implementing our simulations on this platform. The simulation of dynamic properties of hundreds or even thousands of pigments is thus achievable in just a few hours. The potential investigation and design of natural or engineered two-dimensional pigment networks can thus be accommodated. Due to the lack of consensus regarding the precise arrangement of chromophores in the chlorosome, helicity and dimerization are investigated independently, extracting their contributions to both optical and EET properties. The presence of dimerization is found to slow down the delocalization process. Exciton delocalization is completed in 100 fs in a single rod aggregate whose dimensions (20 nm) fairly exceed the estimated extent of a coherent domain. Ultrafast energy relaxation in the exciton manifold occurs in 50 fs and the duration of super-diffusive transport is found to last for about 80 fs.

The measured multi-dimensional spectral response of different light harvesting complexes exhibits oscillatory features which suggest an underlying coherent energy transfer. However, making this inference rigorous is challenging due to the difficulty of isolating excited state coherences in highly congested spectra. In this work, we provide a coherent control scheme that suppresses ground state coherences, thus making rephasing spectra dominated by excited state coherences. We provide a benchmark for the scheme using a model dimeric system and numerically exact methods to analyze the spectral response. We argue that combining temporal and spectral control methods can facilitate a second generation of experiments that are tailored to extract desired information and thus significantly advance our understanding of complex open many-body structure and dynamics.The concept of excitation energy transfer between donor and acceptor molecules is essential for the elucidation of fundamental transport phenomena in interacting many-body systems [1]. Depending on the nature and strength of the system interaction, the effect of the surrounding environment and the considered timescale, the energy transfer can be well described as an incoherent process resulting in hopping kinetics [2] or it may display coherent features as a result of the formation of delocalized excitons [3,4]. Multi-dimensional spectroscopy, which applies sequential short laser pulses with controllable time separation, is particularly well suited for the characterization of energy transfer channels and the observation of coherent features of transport dynamics [5,6]. Specifically, by correlating excitation and detection frequencies as a function of the time delay, two-dimensional (2D) electronic spectra are obtained, which can exhibit cross-peaks where the two frequencies differ, indicating electronic coupling between subsystems and associated energy transfer. Varying the time delay, it is possible to monitor energy transfer paths, estimate the corresponding transfer rates and discriminate coherent from incoherent processes. The application of these techniques to the study of photosynthetic membrane pigmentproteins complexes (PPCs) [7] has shown that the spectral response contains multiple oscillatory features [8-12] whose origin and implications for the description of the system's dynamics are the subject of vigorous discussion (See [13][14][15] for recent reviews). The fact that oscillating 2D signals may not only originate from coherent motions in the excited state potential, but could also be induced by vibrational motions in the ground state has made the identification of coherent excited state features a challenging task [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. The most recent experiments [26,27] using the Fenna-Matthew-Olson (FMO) complex seem to favour a mixed origin of the observed coherences, resulting from coherent electronic-vibrational (vibronic) motions. Previous experiments analyzing charge separation in the PSII reaction ce...