Experimental observations of partially coherent energy transfer in photosynthetic complexes raised the possibility that quantum coherence—previously thought to be too fragile—could affect biological function in vivo or in artificial light-harvesting devices. We showed that quantum effects observed in coherent laser experiments only sometimes affect light-harvesting efficiency in incoherent sunlight.11, 14 Although many of the observed effects are indeed artifacts of the coherent laser excitation and play no role in light harvesting, we also identified two coherent mechanisms that can survive in sunlight and be used to improve artificial solar energy conversion.11
Despite the advances, coherence-enhanced light harvesting has not been directly observed experimentally. The main obstacle has been the difficulty in isolating the effect of coherence in the presence of confounding variables. To enable direct experimental verification, we recently showed that coherent enhancements of light harvesting can be made controllable.26 Our scheme for turning coherence on and off (while keeping everything else constant) using external optical control would make it possible to directly compare efficiencies with and without coherence. And surprisingly, the coherent enhancements of efficiencies are stronger in noisy systems, meaning they could be observed in a wide range of materials.32
Our recent review article29 summarises our past work and goes beyond it by classifying all of the possible mechanisms of coherent efficiency enhancements. Our classification allowed us to predict a previously unreported coherent enhancement mechanism and to clarify which mechanisms would be most useful and most easily engineered into light-harvesting devices.
At the moment, we are working on the first conclusive demonstration of a coherence-enhanced light harvester—stay tuned or come join us!
Coherence-enhanced light harvesting has not been directly observed experimentally, despite theoretical evidence that coherence can significantly enhance light-harvesting performance. The main experimental obstacle has been the difficulty in isolating the effect of coherence in the presence of confounding variables. Recent proposals for externally controlling coherence by manipulating the light’s degree of polarization showed that coherent efficiency enhancements would be possible, but they were restricted to light-harvesting systems weakly coupled to their environment. Here, we show that increases in system–bath coupling strength can amplify coherent efficiency enhancements, rather than suppress them. This result dramatically broadens the range of systems that could be used to conclusively demonstrate coherence-enhanced light harvesting or to engineer coherent effects into artificial light-harvesting devices.
Several kinds of coherence have recently been shown to affect the performance of light-harvesting systems, in some cases significantly improving their efficiency. Here, we classify the possible mechanisms of coherent efficiency enhancements, based on the types of coherence that can characterize a light-harvesting system and the types of processes these coherences can affect. We show that enhancements are possible only when coherences and dissipative effects are best described in different bases of states. Our classification allows us to predict a previously unreported coherent enhancement mechanism, where coherence between delocalized eigenstates can be used to localize excitons away from dissipation, thus reducing the rate of recombination and increasing efficiency.
Spectroscopic experiments have identified long-lived coherences in several light-harvesting systems, suggesting that coherent effects may be relevant to their performance. However, there is limited experimental evidence of coherence enhancing light-harvesting efficiency, largely due to the difficulty of turning coherences on and off to create an experimental control. Here we show that coherence can indeed enhance light harvesting and that this effect can be controlled. We construct a model system in which initial coherence can be controlled using the incident light and which is significantly more efficient under coherent, rather than incoherent, excitation. Our proposal would allow for an unambiguous demonstration of light harvesting enhanced by intermolecular coherence, as well as demonstrate the potential for coherent control of excitonic energy transfer.
It has been argued that excitonic energy transport in photosynthetic complexes is efficient because of a balance between coherent evolution and decoherence, a phenomenon called environment-assisted quantum transport (ENAQT). Studies of ENAQT have usually assumed that the excitation is initially localized on a particular chromophore, and that it is transferred to a reaction center through a similarly localized trap. However, these assumptions are not physically accurate. We show that more realistic models of excitation and trapping can lead to very different predictions about the importance of ENAQT. In particular, although ENAQT is a robust effect if one assumes a localized trap, its effect can be negligible if the trapping is more accurately modeled as Forster transfer to a reaction center. Our results call into question the suggested role of ENAQT in the photosynthetic process of green sulfur bacteria and highlight the subtleties associated with drawing lessons for designing biomimetic light-harvesting complexes.
Recent observations of coherence in photosynthetic complexes have led to the question of whether quantum effects can occur in vivo, not under femtosecond laser pulses but in incoherent sunlight and at steady state, and, if so, whether the coherence explains the high exciton transfer efficiency. We distinguish several types of coherence and show that although some photosynthetic pathways are partially coherent processes, photosynthesis in nature proceeds through stationary states. This distinction allows us to rule out several mechanisms of transport enhancement in sunlight. In particular, although they are crucial for understanding exciton transport, neither wavelike motion nor microscopic coherence, on their own, enhance the efficiency. By contrast, two partially coherent mechanisms—ENAQT and supertransfer—can enhance transport even in sunlight and thus constitute motifs for the optimisation of artificial sunlight harvesting. Finally, we clarify the importance of ultrafast spectroscopy in understanding incoherent processes.