Several kinds of coherence have recently been shown to affect the performance of excitonic light-harvesting systems, in some cases significantly improving their efficiency. Recently, we classified the possible mechanisms of coherent efficiency enhancements [1], showing that whether or not coherence can enhance efficiencies depends on whether the coherence characterises localised or delocalised states, and on whether exciton recombination and trapping occur locally or through non-local mechanisms. Some of these mechanisms can operate in sunlight to increase the efficiency of artificial light harvesting.

Based on this classification, I will also present a model excitonic system which would allow for the first unambiguous demonstration of coherence-enhanced efficiency [2]. Despite abundant theoretical and circumstantial evidence, no experiment has directly shown a coherence-enhanced light-harvesting process, due to the lack of a way to turn coherences on and off to create an experimental control. In our system, the magnitude of coherence can be externally controlled by varying the degree of polarisation of a light source, allowing for a direct comparison of efficiencies resulting from coherent and incoherent excited states, without having to change other, possibly confounding, characteristics of the system.

The system that we propose may be easier to implement experimentally than we originally thought. We have recently found that, surprisingly, the coherent enhancement would be strengthened—not weakened—by stronger coupling to a thermal environment, meaning that it would be less sensitive to experimental imperfections [3].

[1] Stefano Tomasi and Ivan Kassal, J. Phys. Chem. Lett. 11, 2348 (2020).

[2] Stefano Tomasi, Sima Baghbanzadeh, Saleh Rahimi-Keshari, and Ivan Kassal, Phys. Rev. A 100, 043411 (2019).

[3] Stefano Tomasi, Dominic Rouse, Erik Gauger, Brendon Lovett, and Ivan Kassal, in preparation (2020).