Photosynthetic organisms harvest light using large antenna complexes
with many chlorophyll molecules. Because experiments have shown that
energy transport through antenna complexes—and onward to a reaction
centre—is partially coherent, it has become necessary to treat these
processes using computationally expensive techniques from the theory of
open quantum systems. This often requires integrating complicated
non-Markovian dynamics, followed by averaging over a potentially large
However, many of the quantum effects observed in photosynthetic complexes are artefacts of the ultrafast laser excitation and are not relevant in incoherent natural illumination. As a consequence, the complete description of energy transport in incoherent light is dramatically simplified. In particular, the often-dubious Markov approximation becomes exact, while the rotating-wave approximation—often unjustified but nevertheless imposed to avoid certain pathologies—becomes unnecessary. With these simplifications, computing any relevant observable is reduced to a problem of linear algebra. This allows a rapid analysis of hypothetical scenarios to determine whether natural light-harvesting architectures are already optimal or whether they could be improved.
Although some quantum effects are not important for natural light harvesting, others are nevertheless pronounced. I will provide several examples of light-harvesting complexes where the underlying coherence enhances the transport efficiency and use the techniques described above to show that although some are close to being optimal, others are not.
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