In the race to own the golden goose that is quantum information
processing, quantum physicists are scrabbling to find ways of storing
and manipulating quantum information. That turns out to be
hard. Quantum information is fragile stuff: sneeze and you lose it.
But while researchers have been puzzling over this
problem for a few years now, nature has had 4.5 billion years to work on
it using the tools of natural selection. Various scientists have
pointed out that photosynthesis and bird navigation must rely on
quantum effects (we’ve looked at them on this blog here
and here). So it’s just possible that the solution to this problem of handling
quantum information has been staring us in the face.
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Now Vladko Vedral at the University of Oxford and a few pals have
calculated just how good nature could be at this game. The answer is
very good: it looks as if nature has worked out how to preserve
entanglement at body temperature over time scales that
physicists can only dream about.
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The system that Vedral and co have studied is a model that describes how birds
navigate using the earth’s magnetic field. The most recent thinking
is that birds have molecules at the back of their eyes that
are sensitive to both photons and the orientation of the earth’s
magnetic field. When one of these molecules absorbs a photon, an
electron pair is split, and one of these electrons is transferred to
another part of the molecule. These electrons then form a “radical
pair” that are entangled.
In the absence of a magnetic field, this pair would recombine to
form the original molecular state. But the earth’s magnetic field can
flip the spin of one of these electrons, allowing them to
recombine in a different way and leaving the molecule in an alternative
chemical state that the bird can sense. The result is that the bird
“sees” the earth’s magnetic field as it flies.
This raises an interesting question: how long does this entangled
state last?
Vedral and co have done the numbers and say that it lasts for at least
100 microseconds. That’s an extraordinary figure. The best that humans
have measured is 80 microseconds for so-called electron spin
relaxation in C60 buckyballs.
Curiously, entanglement is not being put to work in
magnetoreception; it is simply a by-product of the process. It also
seems to play a nonspeaking role in photosynthesis too, as we saw
here.
Given that nature seems to have created the conditions in which
entanglement thrives, the big question now is whether there are any
natural systems that exploit it.
