麻豆传媒

ONE response to the question 鈥渄oes life use quantum effects?鈥� comes in the form of another question: 鈥渨hy wouldn鈥檛 it?鈥�. All life has evolved to make use of the world we happen to find ourselves in, so why should the magic of quantum effects remain off limits? After all, phenomena such as the telepathic connections implied by entanglement or 鈥渜uantum tunnelling鈥�, in which quantum objects pass effortlessly through energy barriers that on the face of it they shouldn鈥檛 be able to surmount, look like useful survival tools.

The counterargument is that, as any biologist will tell you, living organisms are wet, warm and very, very noisy: their molecules jiggle and their fluids flow, creating an environment where the phenomenon of decoherence would overpower any quantum effects. In recent years, though, we have been able to map out the delicate connections between atoms and molecules inside cells 鈥� and found some tantalising hints that life might indeed exploit quantum weirdness.

Take one of the most important innovations in the history of life: photosynthesis, the process by which plants and some bacteria convert sunlight to chemical energy. The reaction starts with photons of light exciting electrons in chlorophyll molecules to generate quasiparticles 鈥� packets of energy that move around as if they are particles 鈥� called excitons. These are shuttled around until they find 鈥渞eaction centres鈥� where their energy can be captured and stored. But excitons lose energy as they go, so researchers wondered if they might be able to use quantum effects to simultaneously try out all routes and take only the most efficient one.

Subtle effects

Sure enough, this phenomenon of quantum coherence has been observed in and at physiological temperatures. But just because a quantum effect is detected in a living thing doesn鈥檛 mean it offers an evolutionary advantage. Indeed, the importance of coherence in photosynthesis is 鈥渕ore subtle than originally thought鈥�, says at Princeton University, who led some of the initial experiments. What we need, he says, is a less ambiguous example.

That might come from migratory bird species, whose extraordinary navigational prowess is key to their survival. One explanation for how they might do it points to quantum-entangled particles in proteins called cryptochromes, found in some species鈥� eyes. The idea is that during flight, tiny changes in Earth鈥檚 magnetic field are registered by these entangled states and relayed to a bird鈥檚 brain.

There is some evidence for this: cryptochrome sensitivity is known to increase when birds are migrating, and these proteins are conspicuously absent in chickens, which barely fly and so wouldn鈥檛 need this ability. In June, at the University of Oldenburg in Germany and her colleagues demonstrated that cryptochromes in the eyes of European robins are magnetically sensitive. That is highly suggestive, but the experiments were done on proteins suspended in liquid in test tubes, and it is possible they respond differently inside the eyes of the birds. For the moment, then, we are still searching for a clear-cut example of quantum mechanics offering plants or animals an evolutionary upper hand.

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