Quantum Evidence 5

1.) Biophysics: Green quantum computer - Scholes G. - Discussion of relation between coherence in photosynthetic protein and neural systems does not appear convincing.

2.) Limits of quantum speedup in photosynthetic light harvesting - Hoyer, S., Sarovar, M. & Whaley, Birgitta - Argues that quantum coherence in photosynthetic protein is not a search engine, but does increase the efficiency and robustness of energy transport.

1.)

Biophysics: Green quantum computers

Gregory Scholes, Centre for Quantum Information, University of Toronto

Nature Physics, 6, 402-403, 2010

In the photosynthetic systems of plants, algae and bacteria several proteins work together to convert solar energy. The process of photosynthesis has been central to the development of life on Earth. The Fenna-Matthews-Olson (FMO) complex is important in the photosynthetic processing of green sulphur bacteria. The FMO complex wires energy between two key areas. The first of these is an antenna for capturing sunlight and transiently storing this energy in excited chlorophyll molecules. These are described in this article as 'electron waves dancing in the confines of a molecule'. The other key area is a reaction-centre protein, similar to those also found in purple bacteria. The energy of photons from the sun is carried through the light harvesting protein to the seven bacteriochlorophyll molecules that are in each FMO complex before going to the reaction centre.

Recent experiments suggested that quantum coherence causes molecules to work collectively, and thus facilitate energy transfer. Sarovar et al suggested that this system might utilise quantum entanglement, where a change in the quantum state of one particle affects the quantum states of others at a distance. Entanglement is suggested to be related to the extent that excited electrons are delocalised. They predict that this entanglement persists over most of the life of the excited state. This is also stated to be true of purple bacteria. It is suggested here that entanglement might speed up the transfer of energy to the reaction centre, and in particular avoid trap states that prevent energy reaching the reaction centre.

The discussion towards the end of the article relative to the possibility that this type of quantum processing in protein might have implications for brain processing is perhaps predictably unsatisfactory given the general unpopularity of this topic, and a suspicion that associations with quantum consciousness could be a threat to funding. The assumption here appears to be a steam age one, where the brain has to be interpreted only in terms of large scale components, as originally described in 19^th century science. The main argument against relevance to brain processing is supposed to be that the photosynthetic systems described take place on a nanometre scale of size and picosecond scale of time. However, these are exactly the sort of scales that are relevant to processing within neurons, making the argument presented here appear irrelevant.

2.)

Limits of quantum speedup in photosynthetic light harvesting

Hoyer, S., Sarovar, M. & Whaley, Birgitta

Berkeley Quantum Information and Computation Centre & Dept. of Physics

arXiv:0910.1847v2 [quant-ph]

This paper deals with the Fenna-Matthews-Olson (FMO) complex found in green sulphur bacteria. There have been suggestions that quantum coherence in light-harvesting complexes supports a speed up in information processing analogous to quantum algorithms. However, this paper tries to show that the speed up in processing lasts for only 70 femtoseconds compared to quantum coherence that persists for about 500 femtoseconds.

Recent experiments have shown that there is long-lived quantum coherence in some photosynthetic systems, and there are suggestions that this coherence plays a role in the functioning of the system. The FMO complex acts as a quantum wire transporting excitations from a large disordered antennae complex to the reaction centre. Recent experimental studies have suggested that quantum coherence in this system may assist transport along this wire, and add to the overall efficiency of the system. The FMO complex comprises three identical sub-units each comprised of seven bacteriochlorophyll molecules in a protein cage. There are dipolar couplings between the seven chromophore molecules. In these light-harvesting complexes decoherence results from interactions with the protein cage, the reaction centre and the surrounding environment. However, it has been shown that some degree of dephasing or fluctuation of electronic transition energies increases the efficiency of transport in the FMO complex.

Conclusion: The conclusion of the paper is that quantum speed up requires both long-lived quantum coherence and excitons delocalised as a wave form across the entire complex, whether photosynthetic or artificial, as in the case of a manmade quantum computer. Such completely delocalised excitons do not exist in the FMO complex, and are thought unlikely to exist in other light harvesting complexes. The transfer of energy across the FMO complexes is not viewed by the authors as a quantum search. It is considered more likely that long-lived quantum coherence contributes to the overall efficiency and robustness, as in the ability to cope with variations in external conditions. In the LH2 complex of purple bacteria quantum coherence has been shown to enhance both the speed and robustness of energy transport.