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How to describe quantum effects in steady-state light harvesting


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 ensemble.

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.