Quantum fragility may help guide birds: magnetic field's effect on retinal chemistry could aid navigation.

Harnessing the weirdness of the quantum world is difficult--fragile quantum properties quickly degrade under typical conditions. But such fragility could help migrating birds find their way, scientists report in the June New Journal of Physics. Some scientists think birds navigate with quantum-mechanical compasses, and the new study suggests quantum fragility would enhance birds' sense of direction.

Molecules known as cryptochromes, found within avian retinas, may be behind birds' uncanny navigational skills (SN: 5/9/09, p. 26). When light hits cryptochromes, they undergo chemical reactions that may be influenced by the direction of Earth's magnetic field, providing a signal of the bird's orientation.

"At first sight, you wouldn't expect any chemical reaction to be affected by a magnetic field as weak as the Earth's," says study coauthor Peter Hore, a chemist at the University of Oxford. Quantum properties can strengthen a cryptochrome's magnetic sensitivity, but their effect sticks around only for fractions of a second. Any chemical reactions that could signal the bird would have to happen fast enough to skirt a quick breakdown.

But Hore and colleagues' simulations of cryptochromes show that a little bit of quantum deterioration can actually enhance the strength of the magnetic field's effect on the chemical reactions.

According to scientists' theories, light striking a cryptochrome produces a pair of radicals--molecules with a singleton electron. These unpartnered electrons feel the tug of magnetic fields, thanks to a quantum property known as spin, which makes them act a bit like tiny bar magnets. But those magnets are not enough to serve as a compass--instead, the electrons' magnetic sensitivity is the result of a strange quantum dance.

The two radicals' electrons can spin in either the same or opposite directions. But rather than choosing one option, the electrons pick both at once, a condition called quantum superposition. Quantum mechanics can describe only the odds that the electrons would be found in each configuration if forced to choose. As time passes, these probabilities oscillate up and down in a pattern swayed by Earth's magnetic field. These oscillations then affect the rate of further chemical reactions--the details of which are not well understood--which signal to the bird which direction it's facing.

As the electrons interact with their environment, their coordinated oscillations dissipate, weakening their magnetic sensitivity. Hore and colleagues show that some loss of quantumness may be useful. "Not only does it not hurt the compass signal, it can make it stronger," says physicist Erik Gauger of Heriot-Watt University in Edinburgh.

That's because the magnetic field's direction also determines how quickly electrons lose their coordination, further enhancing the difference in the chemical reaction rates based on the bird's direction in the magnetic field. So the magnetic field would do double duty: affect chemical reaction rates by altering the oscillating states of the electrons and determine when they break off their oscillation.

It's still not certain that birds navigate with cryptochromes at all, says computational biophysicist Klaus Schulten of the University of Illinois at Urbana-Champaign. More work is needed to understand how they function.