Quantum Biology and the Puzzle of Coherence

[Jim__]

4. Is coherence selected or a structural by-product?

That seems to be an open question. There is a little bit of discussion of this in a short
article in Nature. An excerpt:

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Quantum coherence in photosynthesis seems to be beneficial to the organisms using it. But did their ability to exploit quantum effects evolve through natural selection? Or is quantum coherence just an accidental side effect of the way certain molecules are structured? "There is a lot of speculation about the evolutionary question, and a lot of misunderstanding," says Scholes, who is far from sure about the answer. "We cannot tell if this effect in photosynthesis is selected for, nor if there is the option not to use coherence to move the electronic energy. There are no data available at all even to address the question."

He points out that it isn't obvious why selection would favour coherence. "Almost all photosynthetic organisms spend most of the day trying to moderate light-harvesting. It is rare to be light-limited. So why would there be evolutionary pressure to tweak light-harvesting efficiency?" Fleming agrees: he suspects that quantum coherence is not adaptive, but is simply "a by-product of the dense packing of chromophores required to optimize solar absorption". Scholes hopes to investigate the issue by comparing antenna proteins isolated from species of cryptophyte algae that evolved at different times.

But even if quantum coherence in biological systems is a chance effect, adds Fleming, its consequences are extraordinary, making systems insensitive to disorder in the distribution of energy. What is more, he says, it "enables 'rectifier-like' one-way energy transfer, produces the fastest [energy-transfer] rate, is temperature-insensitive and probably a few other things I haven't thought of".

These effects, in turn, suggest practical uses. Perhaps most obviously, says Scholes, a better understanding of how biological systems achieve quantum coherence in ambient conditions will "change the way we think about design of light-harvesting structures". This could allow scientists to build technology such as solar cells with improved energy-conversion efficiencies. Seth Lloyd considers this "a reasonable expectation", and is particularly hopeful that his discovery of the positive role of environmental noise will be useful for engineering photonic systems using materials such as quantum dots (nanoscale crystals) or highly branched polymers studded with light-absorbing chemical groups, which can serve as artificial antenna arrays.

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