Department ofMec hanical Engineering ,Massac husettsInstitute Tec hnolo gy 77 A venue Cambridg e MA 02139(Dated: May 18, 2008)Ener gy transfer within photosynthetic systems can display quantum effects such as delocalized exc itations.Recently , direct evidence of long-li ved coherence has been experimentally demonstrated for the dynamics ofthe Fenna-Matthe ws-Olson (FMO) protein comple x [G. S. Engel et al., Nature 446, 782 (2007)]. Ho we ver,the rele vance of quantum dynamical processes to the exciton transfer efÞcienc y is to a large extent unkno wn.Here, we de velop a general theoretical frame w ork for studying the role of quantum interference effects in ener gytransfer dynamics of molecular arrays interacting with a thermal bath. T o this end, we generalize continuous-time quantum w alks to non-unitary and temperature-dependent dynamics in Liouville space deri ved from amicroscopic Hamiltonian. Dif ferent ph ysical effects of coherence and decoherence processes are explored via auni versal measure for the ener gy transfer efÞcienc y and its susceptibility . In parti cular ,we demonstrate that forthe FMO comple x an effecti ve interplay between free Hamiltonian and thermal suctuations in the en vironmentleads to a substantial increase in ener gy transfer efÞcienc y of about 25%.
Department ofMec hanical Engineering ,Massac husettsInstitute Tec hnolo gy 77 A venue Cambridg e MA 02139(Dated: May 18, 2008)Ener gy transfer within photosynthetic systems can display quantum effects such as delocalized exc itations.Recently , direct evidence of long-li ved coherence has been experimentally demonstrated for the dynamics ofthe Fenna-Matthe ws-Olson (FMO) protein comple x [G. S. Engel et al., Nature 446, 782 (2007)]. Ho we ver,the rele vance of quantum dynamical processes to the exciton transfer efÞcienc y is to a large extent unkno wn.Here, we de velop a general theoretical frame w ork for studying the role of quantum interference effects in ener gytransfer dynamics of molecular arrays interacting with a thermal bath. T o this end, we generalize continuous-time quantum w alks to non-unitary and temperature-dependent dynamics in Liouville space deri ved from amicroscopic Hamiltonian. Dif ferent ph ysical effects of coherence and decoherence processes are explored via auni versal measure for the ener gy transfer efÞcienc y and its susceptibility . In parti cular ,we demonstrate that forthe FMO comple x an effecti ve interplay between free Hamiltonian and thermal suctuations in the en vironmentleads to a substantial increase in ener gy transfer efÞcienc y of about 25%.
In this work, energy transfer in the photosynthetic process was studied from a quantum perspective, using the Fenna–Matthews–Olson (FMO) complex as a model. The density operator formalism and the Lindblad master equation were employed to describe the non-unitary dynamics of an open system subject to injection, extraction, and decoherence processes. A two-level system and a three-site FMO complex were analyzed, allowing the study of phenomena such as Rabi oscillations, environment-induced coherence loss, and the emergence of Environment-Assisted Quantum Transport (ENAQT). Through numerical simulations, exciton and heat currents, von Neumann entropy, global and intersite entanglement, and energy transfer efficiency were calculated. The results show that, far from being an obstacle, the interaction with the environment can enhance photosynthetic efficiency under intermediate noise regimes, thus validating the ENAQT hypothesis. This study confirms that photosynthetic systems exploit a delicate balance between coherence and decoherence to optimize energy transport.
Assessing the role of interference in natural and artificial quantum dynamical processes is a crucial task in quantum information theory. To this aim, an appropriate formalism is provided by the decoherent histories framework. While this approach has been deeply explored from different theoretical perspectives, it still lacks of a comprehensive set of tools able to concisely quantify the amount of coherence developed by a given dynamics. In this paper, we introduce and test different measures of the (average) coherence present in dissipative (Markovian) quantum evolutions, at various time scales and for different levels of environmentally induced decoherence. In order to show the effectiveness of the introduced tools, we apply them to a paradigmatic quantum process where the role of coherence is being hotly debated: exciton transport in photosynthetic complexes. To spot out the essential features that may determine the performance of the transport, we focus on a relevant trimeric subunit of the Fenna-Matthews-Olson complex and we use a simplified (Haken-Strobl) model for the system-bath interaction. Our analysis illustrates how the high efficiency of environmentally assisted transport can be traced back to a quantum recoil avoiding effect on the exciton dynamics, that preserves and sustains the benefits of the initial fast quantum delocalization of the exciton over the network. Indeed, for intermediate levels of decoherence, the bath is seen to selectively kill the negative interference between different exciton pathways, while retaining the initial positive one. The concepts and tools here developed show how the decoherent histories approach can be used to quantify the relation between coherence and efficiency in quantum dynamical processes.
Abstract Quantum effects in photosynthetic energy transport in nature, especially for the typical Fenna-Matthews-Olson (FMO) complexes, are extensively studied in quantum biology. Such energy transport processes can be investigated as open quantum systems that blend the quantum coherence and environmental noise, and have been experimentally simulated on a few quantum devices. However, the existing experiments always lack a solid quantum simulation for the FMO energy transport due to their constraints to map a variety of issues in actual FMO complexes that have rich biological meanings. Here we successfully map the full coupling profile of the seven-site FMO structure by comprehensive characterisation and precise control of the evanescent coupling of the three-dimensional waveguide array. By applying a stochastic dynamical modulation on each waveguide, we introduce the base site energy and the dephasing term in coloured noise to faithfully simulate the power spectral density of the FMO complexes. We show our photonic model well interprets the phenomena including reorganisation energy, vibrational assistance, exciton transfer and energy localisation. We further experimentally demonstrate the existence of an optimal transport efficiency at certain dephasing strength, providing a window to closely investigate environment-assisted quantum transport.
Ultrafast spectroscopy experiments show that photosynthetic systems can preserve quantum beats in the process of electronic energy transfer between pigments, even at room temperature. But what does this discovery imply for biology - and for quantum mechanics? This dissertation examines photosynthesis through the tools of quantum information science. We evaluate to what extent photosynthesis can be thought of as a type of quantum technology, and consider how we can apply experimental tools widely used for quantum technologies (state tomography and coherent control) to photosynthesis. Throughout, we use the Fenna-Matthews-Olson (FMO) complex of greens sulfur bacteria as a model photosynthetic pigment-protein complex.In Part I, we evaluate two mechanisms for the possible biological relevance of quantum coherent motion. First, we consider the extent to which dynamics in light-harvesting systems exhibit the quantum speedups characteristic of quantum algorithms and quantum walks. Second, we demonstrate a ratchet effect in electronic energy transfer enabled by partially coherent dynamics. To do so, we build a new model of energy transfer between weakly coupled light-harvesting complexes to understand how electronic coherence arises under natural conditions.In Part II, we present proposals for experimental probes of light harvesting dynamics with ultrafast spectroscopy. We show how the signal from pump-probe spectroscopy can be formally inverted to determine the excited state electronic density matrix. Finally, we examine the feasiblity of coherent control experiments on light-harvesting systems, and provide two realistic targets feasible with present-day technology.
Applications of the concepts of quantum information theory are usually related to the powerful and counter-intuitive quantum mechanical effects of superposition, interference and entanglement. In this thesis, I examine the role of coherence and entanglement in complex chemical systems. The research has focused mainly on two related projects: The first project is developing a theoretical model to explain the recent ultrafast experiments on excitonic migration in photosynthetic complexes that show long-lived coherence of the order of hundreds of femtoseconds and the second project developing the Grover algorithm for global optimization of complex systems. The first part can be divided into two sections. The first section is investigating the theoretical frame about the transfer of electronic excitation energy through the Fenna-Matthews-Olson (FMO) pigment-protein complex. The new developed modified scaled hierarchical equation of motion (HEOM) approach is employed for simulating the open quantum system. The second section is investigating the evolution of entanglement in the FMO complex based on the simulation result via scaled HEOM approach. We examine the role of multipartite entanglement in the FMO complex by direct computation of the convex roof optimization for a number of different measures, including pairwise, triplet, quadruple and quintuple sites entanglement. Our results support the hypothesis that multipartite entanglement is maximum primary along the two distinct electronic energy transfer pathways. The second part of this thesis can be separated into two sections. The first section demonstrated that a modified Grover's quantum algorithm can be applied to real problems of finding a global minimum using modest numbers of quantum bits. Calculations of the global minimum of simple test functions and Lennard-Jones clusters have been carried out on a quantum computer simulator using a modified Grover's algorithm. The second section is implementing the basic quantum logical gates upon arrays of trapped ultracold polar molecules as qubits for the quantum computer. Utilized herein is the Multi-Target Optimal Control Theory (MTOCT) as a means of manipulating the initial-to-target transition probability via external laser field. The detailed calculation is applied for the SrO molecule, an ideal candidate in proposed quantum computers using arrays of trapped ultra-cold polar molecules.
Transport phenomena are ubiquitous throughout the science, engineering and technology disciplines as it concerns energy, mass, charge and information exchange between systems.In particular, energy transport in the nanoscale regime has attracted significant attention within the physical science community due to its potential to explain complex phenomena like the electronic energy transfer in molecular crystals or the Fenna-Matthews-Olson (FMO) / light harvesting complexes in photosynthetic bacteria with long time coherences.Energy transport in these systems is highly affected by environmental noise but surprisingly not always in a detrimental way.It was recently found that situations exist where noise actually enhances the transport phenomena.Such noise can take many forms, but can be characterised in three basic behaviours: quantum, correlation in time, or space.All have been shown potential to offer an energy transport enhancement.The focus of this work is on quantum transport caused by stochastic environment with spatio-temporal correlation.We consider a multi-site nearest neighbour interaction model with pure dephasing environmental noise with spatio-temporal correlation and show how an accelerated rate for the energy transfer results especially under negative spatial correlation (anti-correlation).Spatial anti-correlation provides another control parameter to help one establish the most efficient transfer of energy and may provide new insights into the working of exciton transport in photosynthetic complexes.Further the usage of spatio-temporal correlated noise may be a beneficial resource for efficient transport in large scale quantum networks.