Quantum fragility may help birds navigate

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 believe birds navigate with sensitive quantum-mechanical compasses, and the new study suggests that quantum fragility enhances birds’ sense of direction.

Molecules known as cryptochromes, found within avian retinas, may be behind birds’ uncanny navigational skills (SN Online: 1/7/11). 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 tiny fractions of a second. Any chemical reactions that could signal the bird would have to happen fast enough to skirt this breakdown.

But Hore and colleagues’ new simulations of the inner workings 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 lonely singleton electron. These unpartnered electrons feel the tug of magnetic fields, thanks to a quantum property known as spin, which makes them behave a bit like tiny bar magnets. But those minuscule magnets are not enough to serve as a compass on their own — instead, the electrons’ magnetic sensitivity is the result of a strange quantum dance.

The two radicals’ electrons can spin either in the same direction or opposite directions. But rather than choosing one of these two options, the electrons pick both at once — a condition known as a 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 that is swayed by Earth’s magnetic field. These oscillations in turn 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.

These chemical reactions must happen quickly. As the electrons interact with their environment, their coordinated oscillations dissipate, weakening their magnetic sensitivity. But Hore and colleagues show that this isn’t the complete picture — some loss of quantumness can help birds navigate. “Not only does it not hurt the compass signal, it can make it stronger,” says physicist Erik Gauger of Heriot-Watt University in Edinburgh, who was not involved with the research.

That’s because the direction of the magnetic field 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 does double duty: It affects chemical reaction rates by altering the oscillating states of the electrons and by determining when they break off their oscillation.

Although similar types of sensitivity-boosting effects have been suggested before, they weren’t based on a cryptochrome model, says Gauger.

It’s still not certain that birds navigate with cryptochromes at all, says Klaus Schulten, a computational biophysicist at the University of Illinois at Urbana-Champaign. More research is needed to understand the details of how the cryptochromes might function. “There, this paper is very valuable,” he says. “It’s an interesting idea that’s worth pursuing.”