CSCS Seminars -- Winter, 2006

To what extent does a spinach depend on quantum mechanics? Everybody knows that plants (and indeed many bacteria) produce food through photosynthesis -- however nobody understands the detailed dynamics of the light-harvesting process. Current thinking suggests that the remarkable efficiency of photosynthesis owes a lot to physicists' favorite phenomena: many-body excitations, hopping processes, random walks and delocalization. But given that light-harvesting is observed to occur on the picosecond timescale -- and that decoherence is expected to act on similar timescales -- could more exotic quantum phenomena such as entanglement also play a role? And if we develop similar designs for artificial nanostructure systems, could they be made to exhibit novel collective light-matter phenomena, or act as next-generation nanoscale devices?

This talk will mix fact and fiction: Fact, in the sense that there exists a reasonable yet incomplete experimental picture of photosynthetic time-scales and length-scales. And fiction in the sense that we will let our minds wander, guided by a range of plausible model Hamiltonians, in order to explore what might be possible in such light-matter nanosystems. Our calculations show that quantum entanglement can affect the photosynthetic process in several important ways, and provide an unambiguous experimental signature for detecting such entanglement. In addition, we predict that such systems could be designed to exhibit rich light-matter phase diagrams. At the end of the talk, I turn to consider whether microtubules within neurons in the brain might also exhibit, or be made to exhibit, such collective phenomena.

Why do people decide to join or leave groups? And what is the effect of the resulting group dynamics on the macroscopic behavior of the system in question? From financial markets through to insurgent warfare, the importance of understanding how the ecology of a given population evolves and acts over time, is of great importance. Depending on the situation, it may pay to seek strength in numbers -- on the other hand, such aggregation may lead to over-use of some limited resource. In this talk I will analyze several real-world situations, comparing theoretical models to empirical data. The goal is to understand when crowds might be more likely to emerge based on the characteristics of the individuals concerned, and whether these crowds then represent a potential threat to the system's stability. In addition to such endogenous crowd formation, I will look at the effect of exogenous 'kicks' from outside influences such as news, rumors, memes or antigens. The results have possible relevance to a wide variety of social, economic and biological phenomena -- in addition, they yield as a by-product an intriguing generalization of conventional many-body theory in Physics.