A linear chain of connected electron sites with two asymmetric sinks, one attached to each end, is used as a simple model of quantum electron transfer in photosynthetic bio-complexes. For a symmetric initial population in the middle of the chain, it is expected that electron transfer is mainly directed towards the strongest coupled sink. However, we show that quantum effects radically change 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 design 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 can be understood in terms of superradiance transitions and subradiant states. This switching, being a pure quantum effect, can be used as a witness of wave-like behaviour of excitations 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. PACS numbers: 05.60.Gg, 03.65.Yz, 72.15.Rn 1 arXiv:1307.1557v1 [quant-ph] 5 Jul 2013 I.
INTRODUCTION.Understanding how biological systems transfer and store energy is a basic energy science challenge that can lead the design of new bio-nanotechnological devices [1][2][3][4][5].Recent experiments on photosynthesis by several groups [6][7][8][9][10][11][12][13][14], have demonstrated the striking role of quantum coherence in the form of long lasting oscillations of the population of excitonic states in light harvesting complexes (LHC), at room temperature. Even though there is no general consensus on the role quantum coherence plays in the electron transfer (ET) efficiency ( 99%) [15][16][17][18][19][20], there is no doubt that the models for exciton transport in the LHCs and primary charge separation in the reaction centers (RCs) should utilize quantum coherent effects.The photosystem II (PSII) RC of many bacteria, plants and algae, where the primary charge separation occurs, is arranged in two symmetric branches, even if only one of them is active for the ET. Different mechanisms which could be responsible for the asymmetry in the ET in the PSII RCs, and the related experiments, are discussed in [21][22][23][24][25][26][27][28][29][30] (see also references therein).Here we do not address the question why only one branch is active, but we use the PSII RC as a prototype for an artificial biological switch, able to drive the ET to the left or the right symmetric branch, by controlling the couplings to the sinks.Primary charge separation in the RC can be modeled starting from a donor (a dimer, called the special pair where the excitation starts) and then including the ET through different protein subunits, (bacterio)chlorophylls and (bacterio)pheophytins, generally called chromophores. This transfer occurs in a very short time (a few picoseconds). On the other hand, the effective ET to the quinone occurs over a much lon...
