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Quantum Biology and the Puzzle of Coherence
Quantum processes shouldn’t survive in hot, wet biological systems, yet a growing body of evidence suggests they do. Now physicists think they know how.
One of the more exciting discoveries in biology in the last few years is the role that quantum effects seem to play in many living systems.
The two most famous examples are in bird navigation, where the quantum zeno effect seems to help determine the direction of the Earth’s magnetic field, and in photosynthesis, where the way energy passes across giant protein matrices seems to depend on long-lasting quantum coherence.
Despite the growing evidence in these cases, many physicists are uneasy, however. The problem is the issue of decoherence, how quickly quantum states can survive before they are overwhelmed by the hot, wet environment inside living things.
According to conventional quantum calculations, these states should decay in the blink of an eye, so fast that they should not be able to play any role in biology.
That’s led many physicists to assume something is wrong: either the measurements are faulty in some way or there is some undiscovered mechanism that prevents decoherence.
Today, Gabor Vattay and Stuart Kauffman at the University of Vermont in the U.S. and Samuli Niiranen at the Tampere Institute of Technology in Finland say it’s the latter.
These guys have worked out that in certain special circumstances, quantum systems can remain coherent over much greater timescales and distances than conventional quantum thinking gives credit for. And they argue that biology exploits this process in a way that explains the recent observations from quantum biologists.
Their discussion focuses on the weird phenomenon, even by quantum standards, of quantum chaos, in which small changes to a quantum system can have a huge influence on its evolution, just as in classical chaotic systems.
When a system changes from being merely quantum to being quantum chaotic, it passes through a kind of phase transition. The new thinking focuses on this transition.
Physicists have known for many years that when a system is finely balanced between two phases, all kinds of strange behaviour can occur. For example, water changes from a gas to a liquid to a solid at certain temperatures and pressures. These states all have well-defined properties.
However, there is a certain temperature and pressure at which all three states of water can coexist. At this so-called critical transition, the distinction between gas, liquid, and solid essentially disappears.
Kauffman and co say a similar kind of critical transition occurs as quantum systems switch to a chaotic regime. Here the distinction between chaotic behaviour and ordinary quantum behaviour disappears. And in these conditions, quantum coherence suddenly changes from the fragile, blink-and-you-miss-it regime to a much more robust long-lived phenomenon.
It is in this state, say Kauffman and co, that the observed processes of quantum biology must take place. They even demonstrate this by simulating the improved coherence of the light harvesting complexes involved in photosynthesis. “It is very likely that biological systems use this mechanism,” they say.
That’s an interesting mechanism that, if verified experimentally, could have an important impact on quantum engineering.
The critical transition that Kauffman and co talk about is also known as the the metal-to-insulator transition, which allows the transport of quantum information and energy. If that can be made to work at room temperature, as Kauffman and co suggest, all kinds of new quantum devices may be possible.
“The results may open up new possibilities to design low loss energy and information transport systems,” they say.
Worth keeping an eye on.
Ref: arxiv.org/abs/1202.6433: Quantum Biology on the Edge of Quantum Chaos