Guerreschi, G., Cai, J., Popescu, S. & Briegel, H., Universities of Innsbruck, Ulm and Bristol
arXiv: 1111.2126v1 [quant-ph] 9 Nov 2011
The authors’ model studies the cyclic regeneration of quantum entanglement in hot systems. This looks to open the road to modelling or even experimental simulation that would constitute a possible test for/falsification of non-trivial quantum states in proteins such as those found in neurons.
The paper refers to a simple mechanism by which a molecule forced out of thermal equilibrium by oscillations, can sustain quantum entanglement. This type of entanglement can survive intense noise, but cannot survive if the oscillation ceases. This is argued to be the basis for non-trivial quantum entanglement in biological matter.
The authors remark that this reverses the previous orthodoxy, which held that quantum effects could not exist in biological systems because of the amount of noise in these systems. They note that research in photosynthetic organisms have undermined this case in recent years. The existence of entanglement in a system is seen as greatly increasing information processing capacity, and this underlies the potential of quantum computing. It is pointed out that the previous orthodoxy was based on the assumption of thermal equilibrium, whereas biological systems are open and driven systems far from thermal equilibrium. Such systems are suggested to be capable of quantum error correction that could sustain longer-lived quantum entanglement in biological systems.
In a 2010 paper in Phys Rev E (1.) the authors presented a mechanism by which a molecule subjected to non-thermal equilibrium oscillations could sustain entanglement between two states. This could be maintained despite a level of environmental noise that would not allow entanglement to persist in the absence of non-equilibrium oscillations. Protein molecules, which undergo conformational changes are suggested as the sort of environment in which quantum entanglement of the type found in this model could arise.
In the first section of their paper, the authors look at the possibility of entanglement generated by molecular motion. A biomolecule undergoing conformational change can lead to an interaction between different sites of the molecule. The conformational changes of the molecule can force localised spins to come close or move apart. With the molecular configuration oscillating in a periodic way, cyclic regeneration of entanglement can be sustained over long periods of time, despite noise that would make static entanglement impossible. With thermal equilibrium, entanglement becomes impossible above a certain temperature. The authors, however, ask what happens when molecular motion is involved, and seek to demonstrate that entanglement can keep recurring in an oscillating molecule despite a hot environment.
The authors consider a simple process, with spins that are far apart and with an interaction that is weaker than the surrounding field. In this state, there will be no entanglement. When the spins approach one another entanglement can appear transiently on time scales shorter than that required for thermalisation. The molecule is seen as being kicked out of thermal equilibrium. The generation of entanglement depends on the rate of thermalisation not being too fast. The sustained recurrence of entanglement requires a persistent supply of free energy that can be produced by the conformational changes of the protein. In the author’s model the background field predominates when the spins of the particles are widely separated, but when they are close together their interaction predominates. The authors assume that two spins start far apart and are in a state of thermal equilibrium. The spins oscillate, move closer together, are driven out of thermal equilibrium, and entanglement is generated. Environmental noise here drives a persistent and cyclic generation of new entanglement. The periodic oscillations are seen to keep molecules far away from thermal equilibrium, with the continuous change in the shape of the molecule preventing thermalisation.
The authors emphasise the constructive role played by thermalisation. In a hot thermal bath the first oscillation of the molecule is lost more quickly than in a cooler environment. However, the pumping of energy is seen to provide a reset mechanism. In discussing biological systems, the authors consider that chemical interactions would serve to keep the system out of equilibrium. But in gaps between chemical activity, equilibrium could return, and entanglement would therefore be transient.
In summary, the authors say that they have demonstrated that entanglement can recur even in a hot noisy environment. In biological systems this can be related to changes in the conformation of macromolecules. The authors say that this modelling is a route by which to search for the signatures of entanglement in biomolecular systems. They also think that existing technology could provide an experimental simulation of their model. This could possibly amount to a test for/falsification of the hypothesis that non-trivial quantum states act within proteins, and thus test related theories of consciousness.