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