Quantum coherence and entanglement in living matter

Posted by Olly Robinson on 3 June 2009 | 6 Comments

Tags: Physics, Consciousness

This paper by Sarovar et al at University of California, Berkeley builds on the work of G. Engel and a number of other researchers in exploring quantum coherence and quantum entanglement in photosynthetic systems. The subject is crucial to the whole question of quantum consciousness, since when the chest beating and ridicule is stripped away, the most telling argument against quantum consciousness is that quantum features in the brain would be expected to decohere much too rapidly for them to be relevant to neural processing. The existence of quantum coherence in the brain, in which quantum particles remain in superposition, is seen to be a necessary condition for quantum consciousness. The existence of entanglement in which non-local correlations arise between particles in superposition may also be involved.

Although these studies relate to photosynthetic systems in bacteria, rather than animal systems, they involve proteins that would be expected to suffer from similar decoherence trajectories to those of brain proteins. A paper by J. Cai et al (1.) is quoted as showing that quantum entanglement can be generated and destroyed by non-equilibrium effects in noisy non-equilibrium environments. The authors of the present paper ask whether this means that entanglement can be observed in the non-equilibrium environment of living matter. They quote recent ultrafast spectroscopic studies including the G. Engel paper in Nature (2.) and those published in Science by Lee et al (3.) and Collini et al (4.). These studies all demonstrate quantum coherence in non-equilibrium systems, despite a decohering type environment. The ability of the decohering environment to delete quantum coherence is the core of the best argument against quantum consciousness.

Light harvesting complexes (LHCs) are densely packed molecular structures involved in the initial stages of photosynthesis. These complexes capture light, and the resulting excitation energy is transferred to reaction centres, where chemical reactions are initiated. LHCs are particularly efficient at transporting excitation energy in disordered environments. Simulations of the dynamics of particular LHCs predict that quantum entanglement will persist over observable timescales. Entanglement here would mean that there are non-local correlations between spatially separated molecules in the LHCs.

The molecules in the light harvesting complexes, referred to as chromophores, are close enough together for considerable dipole coupling (attraction between molecules with two equal and opposite electric charges) leading to coherent interaction over observable timescales. The existence of coherence between molecules in these systems has been recognised for a decade or more. This condition is seen as the basis for entanglement. Coherence in this area, known as the site basis, is necessary and sufficient for entanglement, and any coherence in the area will lead to entanglement, and can be viewed in experiments as a signature of entanglement.

The authors base part of their study on the description of the dynamics of a molecule in a protein in a light harvesting complex. This model indicates the coupling of some pairs of molecules due to proximity and favourable dipole orientation. There are also interactions between the molecules and the rest of the protein, which involves decoherence effects. The speed with which this takes effect determines the timescale over which entanglement persists. The authors argue that the standard equation that has been applied in this case may not be valid. Two of the authors have developed an alternative equation that is argued to take better account of the dynamics of chromophores in protein-chromophore complexes.

Using this equation, the interface of the LHC with light energy leads to a rapid increase in entanglement for a short time, followed by a decay of the entanglement punctuated, however, by varying amounts of oscillation. The initial rapid increase reflects the coherent coupling of some parts of the LHC system. This entanglement decreases again as the excitation comes into contact with other parts of the protein. Some of the entanglement seen is not between immediately neighbouring molecules, but between more distant parts of the LHC. Entanglement in LHC is estimated to continue until the excitation reaches the reaction centre.

The authors view this as a remarkable conclusion, since it shows that entanglement between several particles can persist in a non-equilibrium condition, despite being in a decoherent environment. The authors stress that the predictions made in these studies are verifiable by existing spectroscopy techniques.

Studies indicate that the observed rates and robustness of excitation energy transfer are a function of inter-site coherence. Entanglement is a by-product of the coherence, and it is not clear that in itself it has a significant role in light harvesting. However, even if entanglement does not have a role in light harvesting, its existence may be significant for future technological developments. Light harvesting complexes are viewed as forming a possible basis for the design of man-made quantum devices, including quantum computers that need to utilise entanglement.

From the point of view of consciousness studies, this and other papers concerned with quantum features in proteins involved in photosynthesis look to sound the death knell for the recent orthodoxy that quantum features could not persist in biological tissues, thus leaving the road open for the possibility of quantum coherence and entanglement in the brain.

References:-

1.) J. Cai et al (2008) - Dynamic entanglement in oscillating molecules - arXiv:0809.4906

2.) G. Engel et al (2007) - Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems - Nature, 446, 782

3.) H. Lee et al (2007) - Coherence dynamics in photosynthesis: protein protection of excitonic coherence - Science, 316, 1462

4.) E. Collini et al (2007) - Coherent intrachain energy migration in a conjugated polymer at room temperature - Science, 323, 369