Your garden is full of quantum computers

April 24, 2007

It hasn’t made a big splash in the media, but the revelation about photosynthesis in Nature two weeks ago might be the sleeper science story of the year. (You’ll need a subscription or an academic account to follow the link.)

Photosynthesis has always posed a conundrum. It’s unreasonably efficient. While materials scientists struggle to get solar cells up to 30% efficiency, green plants everywhere chug happily along, converting photons to bound chemical energy with effiiciencies topping 95%.

How on earth do they manage it? That solar cell just converts a photon’s energy to charge, in a single step, and then drains off the charge. But in the light-eating organism, the photon excites an electron in one atom, and the excitation goes through a long cascade of other atoms in a complex molecule like chlorophyll, presumably losing energy all the way, until it finally creates a high-energy bond in a carbohydrate at the other end.

In a world run according to classical physics, not much energy could trickle through that whole process. But direct measurements have now indicated that what passes through the photosynthesizing molecule isn’t a series of distinct particles. It appears to be a single quantum wave, which doesn’t lose its coherence.

Let me unpack that just a bit more. In the two-slit experiment, the textbook example of a quantum process, an electron passes through a shield with two openings to land on a target plane. And what we learned in the ’20s and ’30s is that the electron will act like a wave which passes through both slits at once. The peaks and troughs of the wave passing through one slit will intefere with those of the wave passing through the other slit. At some points on the target plane the two parts of the wave will reinforce each other – the electron will be more likely to show up at those places – and at some they’ll cancel each other out, so the electron can’t show up there at all. Until the rest of the world interacts somehow with the electron, forcing the wave to collapse into a particle, it will retain this wavy character. The wtave state is said to be “coherent”, until such time as a collapse makes it decohere.

What Nature tells us is, that the excited electron at one end of the photosynthetic complex remains coherent, taking all possible paths through the molecule to the other end. And it appears that the complex is so cunningly arranged, that the inefficient, energy-losing paths cancel each other out, while the efficient paths enhance one another. As a result, hardly any energy is lost. It’s a process analogous to the “try all possible answers” method by which quantum computers are expected to filter out all but the right answer to a difficult factorization problem.

Such sustained coherence isn’t supposed to be possible very far from absolute zero. Thermal disturbances ordinarily force decoherence. But it seems that evolution, that clever artificer, has found some way to fend it off.

What does all this signify?

Weird as it is, quantum mechanics really does undergird the seemingly solid physical world. Over the years, we’ve grown used to quantum effects, whether we know it or not, since transistors – and with them our whole panorama of blinking, beeping, mousing, clicking, vlogging consumer electronics world – would be so much dead silicon in a classical Newtonian world.

Every so often some maverick will come along (Roger Penrose being the most credentialed) to suggest that something about our mental lives, from free will to consciousness itself, rests in some vaguely defined fashion on quantum strangeness. And those mavericks are generally laughed out of court, with very little hearing. Brains, neurons, proteins, are so big, and quanta are so small!

Now, the likes of Frank Capra may not deserve much hearing. But the bald assertion that quantum effects can’t figure in to the workings of the brain, because neurons, and even neural synapses, are several orders of magnitude larger than elementary particles, never really made sense. Geiger counters are several orders of magnitude larger still, but their macroscopic behavior will differ, depending on how the Schroedinger wave cookie crumbles.

Thanks to this article, the notions that free will, or consciousness itself, might be quantum-generated effects within the brain, have instantly become orders of magnitude more respectable.

In amore practical terms, the new result raises the faint possibility that plants and microbes may eventually teach us how to triple the efficiency of our solar systems. Why faint? Precise calculation of the quantum states of something as simple as a lithium atom push the limits of today’s supercomputers. To model the green sulphur bacterium’s “Fenna-Matthews-Olsen antenna complex” , its chlorophyll cradled by the attendant chromophores that maintain its subtle balances, would push the limits of Douglas Adams’ Deep Thought.

Some enterprising bioengineer may find an ingenious workaround to avoid brute force calculation. But unless she does, chlorophyll will keep most of its quantum secrets until long after we humans have either solved our CO2 problems by other means, or brought our own quantum computer technology into its full maturity , or descended into barbarism.


One comment

  1. […] 3rd, 2007 by therevr A fascinating bit of science from our fellow blogger Tinsel Wing, reporting on a recent article in Nature…. Photosynthesis […]

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