Quantum Biological Switch Based on Superradiance Transitions

A linear chain of connected sites with two asymmetric sinks, one attached to each end,\r\n is used as a simple model of quantum (excitonic and/or electron) transport in photosynthetic bio-c\r\nomplexes.\r\nFor a symmetric initial population in the middle of the chain, it is expected that transport\r\n is mainly directed towards the strongly coupled sink.\r\nHowever, we show that quantum effects radically change this intuitive ``classical'' mechanism,\r\nso that transport can occur through the weakly coupled sink with maximal efficiency.\r\nUsing this capability, we show how to design a quantum switch that can transfer energy\r\nor charge\r\nto the strongly or weakly coupled branch of the chain, by changing the coupling to the sinks.\r\nThe operational principles of this quantum device can be understood in terms of superradiance tra\r\nnsitions and subradiant states. This switching, being a pure quantum effect,\r\ncan be used as a witness of wave--like behaviour in molecular chains.\r\nWhen realistic data are used for the photosystem II reaction center,\r\nthis quantum biological switch is shown to retain its reliability, even at room temperature.