MATERIAL WITNESS news & views he quantum efficiency of to suggestions that the observed photosynthesis has long ‘coherence’ — that is, beats between Tbeen admired by researchers the spectroscopic signals from pump– developing solar energy-harvesting probe experiments — could be better technologies. Romero et al. argued explained as vibrational effects, not recently that, if we can understand how as the superposition and interference photosystems in plants and bacteria of excitons . All parties now seem to are so efficient in converting absorbed agree that vibrations are involved. In photons into charge separation, we one picture, they act to couple and might be able to use the same ‘design delocalize excitonic ground states, Philip Ball principles’ in materials and devices for thereby assisting the transfer of 1 7,8 solar energy generation . energy . Romero et al. suggest that That aspiration seems laudable and this kind of tuning of vibrations to well motivated. The problem is that facilitate energy transfer may be the these principles of energy transfer trick worth emulating in synthetic calling photosynthesis a manifestation in photosystems have been hotly light-harvesting systems . of ‘quantum biology’, rather than contended over the past decade, and There’s another view, however: seeing it as an example of long there is still no consensus about them. that any vibrational coupling giving understood electronic-vibrational The debate began in 2007 rise to coherences and beating is of coupling and energy transfer, is up when Fleming and co-workers far too small an amplitude to have for debate. reported experiments on bacterial any significance for photosynthesis . These two scenarios proposed photosystems at cryogenic In this picture, the mechanism of for energy transfer in photosystems temperatures that seemed to imply photosynthetic energy transfer is (coherent and incoherent) suggest some role for what they described just what it had long been assumed very different ‘design principles’ 2,3 as quantum coherence . As it was to be: an incoherent hopping of to emulate in solar technologies. popularly explained, excitons created energy from site to site in the But — whisper it — wouldn’t it be a from the absorption of photons by photosystem, happening basically at rather splendid outcome if, regardless chlorophyll pigments evolve in a random but given directionality by of what nature does, both proved coherent manner, as a superposition an overall downhill energy gradient to be effective? ❐ of electronic quantum states that is and some guidance by variations in sufficiently long-lived to coordinate the local polarizability within the Published online: 21 February 2018 the transfer of energy to the reaction molecular complex. https://doi.org/10.1038/s41563-018-0032-6 centre where charge separation of ions The arguments continue. All the References creates an electrochemical gradient. same, it appears that the early 1. Romero, E., Novoderezhkin, V. I. & van Grondelle, R. It was suggested that this process was comparisons with quantum computing Nature 543, 355–365 (2017). 2. Engel, G. S. et al. Nature 446, 782–786 (2007). akin to a quantum computation, in and entanglement-mediated 3. Lee, H., Cheng, Y. C. & Fleming, G. R. Science 316, which all possible paths for energy quantum coherence are not the 1462–1465 (2007). transfer were simultaneously explored most useful way to frame the issue. 4. Collini, E. et al. Nature 463, 644–647 (2010). 5. Panitchayangkoon, G. et al. Proc. Natl Acad. Sci USA and the optimal one selected. The Engels et al. say that the language 107, 12766–12770 (2010). picture of quantum coherence was of coherence in energy and electron 6. Tiwari, V., Peters, W. K. & Jonas, D. M. Proc. Natl Acad. supported by subsequent experiments transport is much broader, connecting Sci. USA 110, 1203–1208 (2013). 4,5 7. Romero, E. et al. Nat. Phys. 10, 676–682 (2014). at ambient temperatures . to the familiar valence-bond picture 8. Maiuri, M. et al. Nat. Chem. 10, 177–183 (2018). How a warm, wet cell could sustain of orbital resonance and delocalization 9. Duan, H.-G. et al. Proc. Natl Acad. Sci. USA 114, long-lived quantum coherence seemed in aromatic and conjugated 8493–8498 (2017). puzzling. But later experiments led molecules . Whether this justifies 10. Scholes, G. D. et al. Nature 543, 647–656 (2017). Na TurE Ma TErials | VOL 17 | MARCH 2018 | 210–220 | www.nature.com/naturematerials © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Nature Materials – Springer Journals
Published: Feb 21, 2018
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