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Reverse engineering a quantum compass

Physicists propose a way to probe birds' magnetic-sensing organs

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2:09pm, April 30, 2010
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If the weird rules of atomic physics do help birds find their way around the globe — as some scientists suspect — a new study has identified ways of finding out how.

The study is among the first to propose a direct test of how quantum entanglement, an effect that inexorably links two electrons in a way that Einstein called “spooky,” could change the behavior of whole animals.

“This paper has really made a contribution by suggesting an experimental test,” comments Thorsten Ritz, a physicist at the University of California, Irvine, who was not involved in the new work.

The new paper, which will appear in an upcoming issue of Physical Review Letters, builds on a growing body of evidence that the bizarre rules of quantum mechanics, which govern particles like atoms and electrons, can be exploited by living organisms to solve problems like photosynthesis and navigation (SN: 5/9/09, p.26).

“For a long time people thought this couldn’t be, because nature is just too messy,” Ritz says. But work over the past decade by Ritz and others suggests that the mechanism birds use to “see” Earth’s magnetic field and navigate accordingly could rely on quantum effects.

Ritz and colleagues previously identified a molecule called cryptochrome, which is found in the nerves of avian eyes, as a candidate for controlling birds’ ability to sense magnetic fields.

Electrons in cryptochrome usually come in pairs, each with an opposite spin like a planet revolving on its axis. But when light strikes the molecule, it can carry one of the electrons away. The presence of a magnetic field can then make each spinning electron wobble like a plate balanced on a stick. When the wayward electron returns to its original molecule, any change it has picked up in its spin sparks a chemical signal that some scientists believe allows birds to see magnetic fields as a pattern of colors.

How this chemical signal gets triggered may depend whether or not the spins of cryptochrome electrons are intimately entwined in a quantum state called entanglement. When two electrons’ spins are entangled, one always knows what the other is doing, spinwise, even if they’re far apart.

Physicist Hans Briegel and colleagues at the University of Innsbruck in Austria wondered if having entangled electrons in cryptochrome could make birds more sensitive to magnetic fields. The researchers calculated how strongly a pair of electrons with entangled spins should respond to a magnetic field, compared with an unentangled pair.

The results showed that entanglement can be helpful, but maybe not for birds. Briegel and colleagues ran the numbers for cryptochrome and for another molecule, pyrene, a well-studied molecule with little biological significance. For pyrene, they found that entanglement between the electrons’ spins made the particles much more sensitive to magnetic fields. But for cryptochrome, entanglement made no difference.

“This first indication says no,” Briegel says.

A disappointment, perhaps. But spinning off the theoretical work on cryptochrome, the researchers have come up with a way to test this effect in real molecules. Zapping the electron pair with short, energetic bursts of microwaves could reveal whether the electrons are entangled or not. The same calculations used for cryptochrome suggest that if electrons’ spins are entangled, their sensitivity to magnetic field shouldn’t change. If the spins are not entangled, their sensitivity should drop, Briegel says.

This technique could be used to test many different molecules. Finding that different molecules can use entanglement differently highlights the complexity of the role quantum mechanics may play in biology, Briegel says. “This is one example where you would think upon first seeing this, well, that is a proper quantum feature,” he says. “But it requires a closer look. It’s nontrivial because, as we see in our study, it depends on the molecule.”

Briegel imagines jolting individual molecules in a lab, but Ritz hopes to find a way to test quantum entanglement effects in live birds. He’s already doing the legwork to see if it’s possible, he says.

“I’m sufficiently interested in these suggestions to see whether they can be realized, to work on designing experiments,” Ritz says. “There’s a lot of promise there.”

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