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Quantum control of energy flow in light harvesting


Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter1,2. This approach has been experimentally implemented to control a variety of processes3,4,5,6,7,8,9, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor–acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.

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Figure 1: Properties of the LH2 antenna complex of R. acidophila, a, Geometrical arrangement of the carotenoid (‘Car’) and bchl molecules (data taken from the Protein Data Bank; identification code 1kzu).
Figure 2: Optical set-up.
Figure 3: Optimization results.
Figure 4: Transient absorption kinetics.
Figure 5: Evidence of coherence in the control mechanism for the light-harvesting function.


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We acknowledge support and encouragement from K.-L. Kompa and V. Sundström. We thank A. Gardiner for help with sample preparation, T. Hornung for discussions and M. Mathys for help with experiments.

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Correspondence to Marcus Motzkus.

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Herek, J., Wohlleben, W., Cogdell, R. et al. Quantum control of energy flow in light harvesting. Nature 417, 533–535 (2002).

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