Quantum compass for birds

Robins may use strange physics to migrate

A quantum effect known as entanglement may be part of the compass that birds use to sense Earth’s magnetic field, researchers report in an upcoming Physical Review Letters.

Critters from bacteria to mole rats use tiny variations in the Earth’s magnetic field to navigate, but exactly how they sense the magnetism is a mystery. One idea is that magnetic fields disrupt pairs of entangled electrons in a light-sensitive protein in the retina. In quantum entanglement, particles are linked to each other so that one always knows instantly what the other is doing, even if they get separated.

In the new research, physicists at the University of Oxford and the National University of Singapore calculated that quantum entanglement in a bird’s eye could last more than 100 microseconds — longer than the 80 microseconds achieved in physicists’ experiments at temperatures just above absolute zero, says Elisabeth Rieper, a physicist at the National University of Singapore. That would be a surprising feat for a bird warbling at room temperature, which people thought was too hot to see quantum effects.

“It may all be right, but I would personally like to be cautious about this,” says Thorsten Ritz, a biophysicist at University of California, Irvine, who is a proponent of the model but wasn’t involved in this research.

The new prediction interprets data from earlier experiments that hinted at a quantum basis for magnetic navigation in migrating birds. In 2006, researchers in Frankfurt, Germany, netted 12 European robins migrating from Scandinavia. Researchers locked the robins in a wooden room and applied small magnetic fields tuned to a frequency that would disturb entangled electrons, if the birds indeed relied on entanglement to navigate.

The magnetic field, at 150 nanoTesla, was about 300 times weaker than Earth’s magnetic field, so it wouldn’t be expected to confuse the birds in the absence of an entanglement-based navigation system. But with the magnetic field on, the birds flew randomly instead of all flying in the same direction.

To the disoriented birds, the small magnetic field may have acted like static in a radio transmission tuned to match quantum properties of electrons in the protein cryptochrome, which lines the retina. Electrons in the molecule come in pairs, each with opposite spin, like the heads and tails on a coin. When light enters the bird’s eye and hits the cryptochrome, one of the electrons is kicked out. The wayward electron wobbles under the influence of the Earth’s magnetic field, but the protein-bound electron feels both the Earth’s field and the magnetic pull from the rest of the molecule. Since quantum entanglement keeps the separated electrons linked like two sides of the same coin, they feel each other wobbling. The difference in how the two electrons wobble creates patterns on the retina that the bird can use as a compass.

If this idea is valid, then plugging the intensity of 150 nanoTesla into a complex physics analysis leads to superlong entanglement times, says Ritz.

Critics of this idea say magnetic fields could instead affect small iron molecules in the eye. But that doesn’t explain the disoriented robins either, says Ritz, because the resonance frequencies of iron are 100 to 1,000 times higher than the field that confused the birds.

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