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How Coherence Can Impart Function

The possible connections between coherence and biological organisms have been explored extensively over the past several decades. Evidence suggests that natural light-harvesting organisms have evolved to take advantage of quantum coherence in order to boost their effectiveness in harnessing sunlight. Like members of a crowd singing a common tune, coherence on the quantum level implies synchronization. Only this time, the interference is occurring between molecules, and the effect relies not on sound waves, but on the energy that the molecules capture from the sun. Members of the CSQC are striving to use nature as inspiration for designing the next generation of high-efficiency solar cells.

Envision the light-harvesting proteins in a leaf as people at a dance. At the beginning of the party, the people who first step out onto the floor are the starting source of energy for the celebration. This is analogous to the capture, or absorption, of sunlight by the plant proteins. Ideally, this initial energy motivates every dancer at the party to hit the dance floor immediately! However, this is rarely the case as the party goers have a diverse range of interests. Instead, the energy at the party takes time to spread among the attendees, resulting in a gradual population of the dance floor. This dynamic and directional flow of dancing is quite similar to how energy is transported throughout the pigments in a leaf. The goal in our dancefloor example is to get all the party goers to join in on the fun quickly. For plants, as well as for synthetic light harvesting technologies, the objective is to funnel energy from the sun through the network of molecules in a leaf or solar cell to a specific harvesting location as rapidly and efficiently as possible.

You may find yourself wondering “well what does coherence and synchronization on the microscopic scale of molecules mean?” Well let’s again consider the dancefloor analogy and delve deeper into what other aspects of the celebration may influence how quickly people join the dance. One immediate question to consider is: Do the attendees know each other? The effect of coherence on the speed at which the dance floor is populated must depend on this consideration. For example, if the party attendees have never met one another prior to the event, chances are that it will take a long time to get everyone dancing. Conversely, if all the attendees are already great friends with one another, they likely would feel comfortable hitting the dance floor almost immediately. Therefore Coherence in this example can therefore be thought of as the interpersonal relationships and connections that the party goers share with each other. Zooming in on the network of light-harvesting pigments in a leaf or a solar cell, coherence can arise in a similar fashion on the microscale from relationships between individual molecules! Of course the essence of the relationships is quite different for these two scenarios, but the general idea is shared.

Researchers in the CSQC are fascinated by the notion that the fine details of coherence between neighboring molecules can affect their collective behavior, such as in harvesting light and transporting energy. Chemists have developed countless strategies over hundreds of years figuring out how to manipulate materials at the smallest scales. The CSQC aims to leverage our knowledge of molecular design in order to elucidate and control coherence in synthetic systems. A party planner that wants to see the dancefloor populated rapidly may choose to invite people that are well acquainted with each other, or may provide opportunities for people to meet each other at the party, such as at shared dinner tables. These aspects of designing a dance party that benefits from coherence are precisely what the CSQC scientists are striving to harness for building coherent molecular assemblies.

Contributors: Kaydren Orcutt, Jonathan Schultz, Reshmi Dani