K. Birgitta Whaley, Mohan Sarovar & Akihito Ishizaki, University of California Berkeley
arXiv: 1012.4059vl [quant-ph] 18 December 2010
Keywords: K. Birgitta Whaley, entanglement in the FMO complex, quantum coherence in plants
This paper discusses recent studies of photosynthetic light harvesting complexes. The studies are seen as having established the existence of quantum entanglement in biologically functional systems that are not in thermal equilibrium. However, this does not necessarily mean that entanglement has a biological function. The authors point out that the modern discussion of entanglement has moved from simple arrangements of particles to entanglement in larger scale systems.
Measurements of excitonic energy transport in photosynthetic light harvesting complexes show evidence of quantum coherence in these systems. A particular focus of research has been the Fenna-Matthew-Olson (FMO) complex in green sulphur bacteria. The FMO serves to transport electronic energy from the light harvesting antenna to the photosynthetic reaction centre. Coherence is present here at up to 300K. In 2009, quantum coherence was also detected in the light harvesting antenna of green plants (1. Calhoun et al). Studies have also demonstrated quantum coherence within the photosynthetic reaction centre. There is thus now a growing body of evidence for quantum coherence in connection with energy transports in plants and bacteria.
The electronic excitations in the chromophores are coupled to the vibrational modes of the surrounding protein scaffolding. One study (2. Scholak et al, 2010) shows a correlation between the extent of entanglement and the efficiency of energy transport. That study went on to claim that efficient transport requires entanglement, although the authors of the present paper query such a definite assertion.
The FMO complex is described as acting as a ‘quantum wire’ to transmit electronic excitation from the light harvesting antenna to the reaction centre. The authors draw attention to the relationship between electronic excitations in the chromophores and those in the surrounding protein. A previous study by one of the authors (3. Sarovar et al, 2010) shows that for structures such as the FMO coherence and entanglement are necessary and sufficient for one another.
The pigment-protein dynamics generates entanglement across the entire FMO complex in only 100 femtoseconds, but followed by oscillations that damp out over several hundred femtoseconds with a subsequent longer contribution continuing beyond that for up to about five picoseconds. This more persistent entanglement can be at between a third and a half of the initial value and 15% of the maximum possible value. Long-lived entanglement takes place between four or five of the existing seven chromophores. The most extended entanglement is between chromophores one and three, and these are also two of the most widely separated chromophores. Studies also show that this entanglement is quite resistant to temperature increase, with only a 25% reduction when the temperature rises from 77K to 300K. Overall studies indicate long-lived entanglement of as much as five picoseconds between numbers of excitations on spatially separated pigment molecules. This is described here as long-lived coherence because energy transfer through the FMO complex is on a time span of a few picoseconds meaning that the up to five picoseconds of entanglement seen between the chromophores represents a functional timescale. However, the authors do not consider this by itself to be a conclusive argument for entanglement being functional in the FMO.
This paper also looks at light harvesting complex II (LHCII), which is also shown to have long-lived electronic coherence. LHCII is the most common light harvesting complex in plants. The system comprises three subunits each of which contains eight chlorophyll ‘a’ molecules and six chlorophyll ‘b’ molecules. A study by two of the authors (4. Ishizaki & Fleming, 2010) indicates that only one out of chlorophyll molecules would be initially excited by photons, and this molecule would then become entangled with other chlorophyll molecules. Entanglement decreases at first, but then persists at a significant proportion of the maximum possible value. This is also an important feature of the FMO complex. In both these complexes entanglement is seen to be generated by the passage of electronic excitation through the light harvesting complexes, and to be distributed over a number of chromophores. Entanglement persists over a longer time and is more resistant to temperature increase than might have been previously expected. A functional biological role is suggested by the persistence of entanglement over the same timescale as the energy transfer within the light harvesting complexes.Tags: Birgitta Whaley, Ishizaki, quantum entanglement, Sarovar Posted by