Stochastic Path Integral Formalism and Applications to Coherent Energy Transfer

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Jianshu Cao from the Massachusetts Institute of Technology is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry. Cao and coworkers develop new theoretical and computational methods to analyze coherent energy transfer in photosynthetic systems. Photosynthesis is the biological process that converts solar energy into biomass. The first step of this process involves the capture of photons (particles of light) by arrays of light-harvesting complexes. These photons excite electrons in the complexes. This excitation energy is transferred to the reaction center for subsequent conversion to chemical energy. The energy transfer process in photosynthesis exhibits remarkable efficiency. There is extensive basic research to understand the basis of this efficiency. This research provides useful insight into the optimal design of artificial light-harvesting devices. In the past, these design efforts attempted to mimic photosynthesis based on the idea that the excitation energy is transferred from pigment molecules to the reaction center by hopping down the molecular energy ladder, one molecule at a time. However, recent experimental evidence reveals a crucial role for quantum coherence and for cooperative effects. These new findings raise fundamental questions that inspire the proposed theoretical and computational developments. While addressing these questions, Prof. Cao devotes significant efforts to training students and postdoctoral associates in his group, compiling new course materials on MIT OpenCourseWare for public access, and further developing an open website for the distribution of simulation codes. Arguably the most subtle quantum mechanical effect, quantum coherence has no simple classical analog but lies at the root of diffraction, interference, delocalization, and quantum entanglement. Evidently, nature takes advantage of subtle coherence effects in the optimal design of photosynthetic systems to effectively convert photons into chemical bonds. It is, however, unclear how the coherence effects can facilitate the remarkably efficient and robust energy transfer in photosynthesis and how quantum coherence can be properly defined, measured, and controlled. To address these questions, Jianshu Cao develops an active research program by integrating polaron theory, stochastic simulations, and experimental collaborations. Specifically, the research plan consists of three components: (i) theoretical and conceptual issues centered on quantum coherence; (ii) a unified stochastic formalism for simulating coherent exciton dynamics; and (iii) applications to light-harvesting structures in natural photosynthetic systems and in artificial energy devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.