Coherence and photosynthesis

Gregory Engel et al

Dept. of Chemistry, University of California, Berkeley

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems

Nature, vol 446, pp. 782-786, 12 April 2007

This paper points out that photosynthetic complexes are adapted to capture light, and put its energy into long-term storage. This process has normally been described in classical terms, and quantum coherence has been to a good extent ignored in the traditional analysis. However, the possibility of quantum coherence has been predicted, and in this paper the authors describe evidence for long-lived quantum coherence being involved in energy transfer within photosynthetic systems. The wavelike process is thought to account for the efficiency of the sytem, because it allows the sampling of large areas of phase space, in order to find the most efficient path, or to transfering energy to the area in the lowest energy state.

The Engel et al experiment involved electronic spectroscopy to observe the evolution of electronic coherence. Quantum beating was found to last for 660 fs, which was much more than the 250 fs estimated for conventional models. Conventional models had assumed that quantum coherence would be rapidly destroyed, and had therfore not factored it into their models of photosynthetic systems.

By contrast, the authors conclude that long-lived quantum coherence must play an active role in photosynthetic systems. A quantum coherent system allows sampling in order to direct energy to the lowest energy state. The system is viewed as performing a quantum computation, in which it senses many states simultaneously and from these selects the correct answer. This is seen as analogous to Grover’s algorithm, allowing both the discovery of the lowest energy state and the transfer of coherence. This is more efficent than any classical search engine. Protein is seen as providing the structure in which coherence can be preserved and at the same time modulating the coherence as a result of the local dielectric environment.

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