School Colloquia Series - Dr Ivan Kassal - How Photosynthetic Organisms Use Coherence
Dr Ivan Kassal
Senior Lecturer & Westpac Research Fellow, School of Chemistry, University of Sydney
Photosynthetic organisms harvest light using large arrays of organic molecules, which also transfer the collected energy over relatively large distances before it is used to drive chemical reactions. This energy transfer was thought to be incoherent, governed by classical rate equations, until recent spectroscopic studies suggested that it could be wavelike. This observation raised the possibility that evolution selected quantum effects because they improve transport efficiency and that we could engineer similar coherence in artificial light-harvesting devices. To make some of the first precise claims about coherence in photosynthesis, we accelerated simulations of energy transport in photosynthetic complexes by orders of magnitude by exploiting the incoherent nature of the incident light. Although we showed that some observed aspects of coherence are evolutionary accidents that cannot affect efficiency, we also identified coherent effects that can enhance light harvesting even in sunlight. In particular, we found the the most statistically significant coherence-related enhancement of photosynthetic energy transport so far, showing that purple bacteria exploit a coherent, collective process called supertransfer, using a symmetric arrangement of chlorophyll molecules whose efficiency exceeds random orientations by over five standard deviations.