Fausto Borgonovi - Università Cattolica Brescia #
A quantum biological switch #
A linear chain of connected electron sites with two independent sinks, one attached 
to each end, is considered as a simple model for a quantum electron transfer
 in photosynthetic bio-complexes. Under the condition of a symmetric initial 
population  in the middle of the chain,  it is expected that the efficiency of the 
electron
transfer from the middle of the chain to the sinks
 is dominated by the sink  with the strongest coupling.
However,  we show that quantum effects can  modify this intuitive ``classical'' 
mechanism
so that electron transfer can occur through the weaker coupled sink with   maximal 
efficiency.
Using this capability we show how to make a quantum switch  that can transfer an 
electron
to the left or right branch of the chain, by changing the coupling to  the sinks.
The operational principles of this  quantum device are given in term of  
superradiance  transitions
  and  subradiant states. When the realistic data are taken from the photosystem II 
reaction center,
this quantum biological switch is shown to retain  its reliability, even at room 
temperature.