There’s a compass in my eyes (Image: Kim Taylor/NsaturePL)

“BIRD brain” is usually an insult, but that may have to change. A light-activated compass at the back of some birds’ eyes may preserve electrons in delicate quantum states for longer than the best artificial systems.

Migrating birds navigate by sensing Earth’s magnetic field, but the exact mechanisms at work are unclear. Pigeons are thought to rely on bits of magnetite in their beaks. Others, like the European robin (pictured), may rely on light-triggered chemical changes that depend on the bird’s orientation relative to Earth’s magnetic field.

A process called the radical pair (RP) mechanism is believed to be behind the latter method. In this mechanism, light excites two electrons on one molecule and shunts one of them onto a second molecule. Although the two electrons are separated, their spins are linked through quantum entanglement.


The electrons eventually relax, destroying this quantum state. Before this happens, however, Earth’s magnetic field can alter the relative alignment of the electrons’ spins, which in turn alters the chemical properties of the molecules involved. A bird could then use the concentrations of chemicals at different points on its eye to deduce its orientation.

Intrigued by the idea that, if the RP mechanism is correct, a delicate quantum state can survive a busy place like the back of an eye, Erik Gauger of the University of Oxford and colleagues set out to find out how long the electrons remain entangled.

They turned to results from recent experiments on European robins, in which the captured birds were exposed to flip-flopping magnetic fields of different strengths during their migration season. The tests revealed that a magnetic field of 15 nanoTesla, less than one-thousandth the strength of Earth’s magnetic field, was enough to interfere with a bird’s sense of direction (Biophysical Journal, DOI: 10.1016/j.bpj.2008.11.072).

These oscillating magnetic fields will only disrupt the birds’ magnetic compass while the electrons remain entangled. As a weaker magnetic field takes longer to alter an electron’s spin, the team calculated that for such tiny fields to have such a strong impact on the birds’ compasses the electrons must remain entangled for at least 100 microseconds. Their work will appear in Physical Review Letters.

The longest-lived electrons in an artificial quantum system – a cage of carbon atoms with a nitrogen atom at its centre – survived for just 80 microseconds at comparable temperatures, the team points out. “Nature has, for whatever reason, been able to protect quantum coherence better than we can do with molecules that have been specially designed,” says team member Simon Benjamin of the Centre for Quantum Technologies in Singapore.

Thorsten Ritz of the University of California, Irvine, who helped perform the robin experiments, cautions that the RP mechanism has yet to be confirmed. But he is excited by the prospect of long-lived quantum states. “Maybe we can learn from nature how to mimic this,” he says.