Quantum Biological Switch Based on Superradiance Transitions

A linear chain of connected sites with two asymmetric sinks, one attached to each end, is used as a simple model of quantum (excitonic and/or electron) transport in photosynthetic bio-c omplexes. For a symmetric initial population in the middle of the chain, it is expected that transport is mainly directed towards the strongly coupled sink. However, we show that quantum effects radically change this intuitive ``classical'' mechanism, so that transport can occur through the weakly coupled sink with maximal efficiency. Using this capability, we show how to design a quantum switch that can transfer energy or charge to the strongly or weakly coupled branch of the chain, by changing the coupling to the sinks. The operational principles of this quantum device can be understood in terms of superradiance tra nsitions and subradiant states. This switching, being a pure quantum effect, can be used as a witness of wave--like behaviour in molecular chains. When realistic data are used for the photosystem II reaction center, this quantum biological switch is shown to retain its reliability, even at room temperature.