Quantum Secrets of Photosynthesis Revealed

In an old randform post about solar cells I was writing a bit about the computer modelling of solar cells. In particular I mentioned that it seems that the involved models use mainly a theory which was to a great part developped by Shockley and Queisser in the 50/60s.




Among others -due to the assumptions made- the theory proposes a limit for the efficiency of solar cells which is called the socalled Shockley-Queisser limit (for details see e.g. this survey by Solanki and Beaucarne from IIT Bombay and Imec Leuven, Belgium respectively). However it is possible to loosen the assumptions and a lot of experiments have been done in this direction (see also e.g. the paper by Solanki and Beaucarne).

An experiment which seems to indicate that the efficiency of solar cells may be increased by a lot (coming close to Carnot efficiency) had been done sort of recently by Researchers of Berkeley Lab’s Physical Biosciences Division in particular members of the Fleming group by Prof. Fleming (not to confuse with Alexander Fleming). The results had been published in Nature. Unfortunately I can’t afford a Nature subscription and likewise it makes no sense to link to it, since most readers of this blog will probably neither have a Nature subscription.

Luckily here is another report about their results:

“Quantum Secrets of Photosynthesis Revealed” (there is also a german report!)

Some excerpts:

Through photosynthesis, green plants and cyanobacteria are able to transfer sunlight energy to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. Speed is the key – the transfer of the solar energy takes place almost instantaneously so little energy is wasted as heat. How photosynthesis achieves this near instantaneous energy transfer is a long-standing mystery that may have finally been solved.

and

“We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis,” said Graham Fleming, the principal investigator for the study. “This wavelike characteristic can explain the extreme efficiency of the energy transfer because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one.”

The experiments are done with very fast lasers:

“….Fleming and his research group have developed a technique called two-dimensional electronic spectroscopy that enables them to follow the flow of light-induced excitation energy through molecular complexes with femtosecond temporal resolution. The technique involves sequentially flashing a sample with femtosecond pulses of light from three laser beams. A fourth beam is used as a local oscillator to amplify and detect the resulting spectroscopic signals as the excitation energy from the laser lights is transferred from one molecule to the next. (The excitation energy changes the way each molecule absorbs and emits light.)”

The key to the efficiency seems to be a stable coherence (which looks at the moment to be unlikely to happen in brains and not too much more likely to happen in quantum computers in a commercial context (as it seems big branding campaigns are not enough (?))):

….the duration of the quantum beating signals was unexpected because the general scientific assumption had been that the electronic coherences responsible for such oscillations are rapidly destroyed. For this reason, the transfer of electronic coherence between excitons during relaxation has usually been ignored,” …

…“By demonstrating that the energy transfer process does involve electronic coherence and that this coherence is much stronger than we would ever have expected, we have shown that the process can be much more efficient than the classical view could explain. However, we still don’t know to what degree photosynthesis benefits from these quantum effects.”

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