Quantum Coherence in Photosynthesis

Photosynthesis is a fundamental biological process that provides the energy for almost all life on Earth.  Although a wide variety of organisms perform photosynthesis in different ways, the initial stages of the process are essentially the same in all species.  In these stages photons are absorbed by special light-harvesting  pigments and the captured light energy is transported to molecular reaction centers where it is used to initiate the chemistry of photosynthesis.  It is well established that the efficiency at low light intensity for these early stages is very close to 100% i.e. an absorbed photon always reaches the reaction center and initiates a charge separation event [1].

Recently,  experiments using two-dimensional fourier transform electronic spectroscopy have revealed the presence of long-lasting quantum coherence in the excitation energy transport (EET) of light-harvesting molecules taken from bacteria and marine algae [2,3,4].  This fascinating observation suggest that the high efficiency of energy capture and tranport in these molecules might be due to quantum mechanical effects. Understanding how this coherence is both harnessed and protected in the noisy, high temperature environment of a living organism could thus be immensely useful in the future design of efficient solar cells, as well as the creation of room temperature quantum devices.

Since the first discovery of long-lasting coherences in the Fenna-Matthews-Olson (FMO) complex [2], the possible roles of quantum coherence in photosynthesis have begun to be investigated in great detail -see [5] for a recent review.  Our group is actively engaged in the study of photosynthetic EET in a variety of  light-harvesting structures,  with a particular focus on the general mechanisms and strategies by which quantum mechanics and decoherence can be combined to drive efficient Noise-Assisted Transport (NAT) in light-harvesting molecules.

Although very little is often known about the environments of light-harvesting molecules, it is widely accepted that photosynthetic EET dynamics cannot be treated with standard approaches to open quantum dynamics [5].  To deal with we have used a variety of  non-perturbative and non-markovian approaches such as effective environment theories and stochastic schrodinger equations. Recently, we have created a powerful and completely general new method of simulating dissipative quantum dynamics using the time-adaptive density matrix renormalisation group (t-DMRG) method. Further details can be found here.

We are also interested in the possibilities of making artifical light harvesting systems in which we could demonstrate the key concepts of NAT and non-markovian open-system dyanmics, as well as exploring new effects.  By moving to artifical systems in which we have greater control over the preparation and parameters of the system and environment, we might be able to disentangle, isolate and assess the role of many different effects that are invariably convoluted in current experiments on biological light-harvesting molecules.

References:  (our papers are here):

[1] R. E. Blankenship, Molecular Mechanisms of Photosynthesis, World Scientific, London, (2002).

[2] G. S. Engel et al. Nature 446, 782-786 (2007)

[3] E. Collini et al. Nature 463, 644-648 (2010)

[4] Panitchayangkoon et al. PNAS : 1005484107 v1-5 (2010)

[5] Ishizaki et al.  Phys. Chem. Chem. Phys. 12, 7319-7337, (2010)