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Biophysics

Bird’s eye view of Earth’s magnetism

09 Feb 2011 James Dacey

Clearly, birds have the right idea by migrating to warmer climes when the winter starts to bite at high latitudes, but scientists are starting to suspect that our feathered friends may be even smarter than we first thought. University of Oxford physicist Erik Gauger talks to Physics World reporter James Dacey about how birds may possess tiny internal compasses that allow them to navigate using the Earth’s magnetic field. Gauger discusses the mechanism of the avian compass and why it could be mimicked to develop quantum computers.

What evidence is there that birds use the Earth’s magnetic field in navigation?
The evidence that birds use the Earth’s magnetic field for navigation spans more speculative anecdotal reports of disruptions to birds’ migrations caused by magnetic disturbances, as well as very reliable behavioural studies of birds in artificially controlled magnetic environments.

Why might they need this ability for migration?
To avoid a harsh winter, for instance, Scandinavian and Russian robins migrate in a southerly direction in autumn, returning back to northern latitudes in springtime. Birds are known to use visual clues for navigation, for example celestial bodies, but also landmarks such as mountains, rivers, and possibly even motorways. Nonetheless, having additional information about the magnetic heading would clearly help migratory species on their long journey through less familiar territory.

But can they tell north from south?
Interestingly, experiments showed that European robins only perceive the inclination angle of the magnetic field lines with Earth’s surface and not the polarity of the field. This means that they cannot per se distinguish north from south. However, in practical terms the knowledge of the inclination angle – which is the angle the magnetic field lines make with the horizontal plane – provides enough information to tell north from south in the northern hemisphere.

How do birds detect the field?

These robins may be able to ‘see’ the magnetic field

Magnetoreception works differently for different species of bird. For example, homing pigeons most likely have a compass based on ferromagnetic substances placed in the upper part of their beaks. In contrast, the evidence points to a light-activated compass situated in the bird’s eye for other species such as the European robin. Among other clues, the apparent location of the compass in the bird’s eye has led to the suggestion that these robins may be able to “see” the magnetic field.

Do we know how these compasses work – what is going on in the bird’s biology?
The central idea is known as the “radical pair” mechanism, which involves chemical compass molecules located on the bird’s retina in some sort of ordered pattern. The energy of an incident photon can excite such a compass molecule from its ground state to an excited state where two of its electrons are sent into a special quantum state called a “singlet state”.

How does this state interact with the Earth’s magnetic field?
The orientation of the compass molecule with Earth’s static field can change the correlation existing between the two electron spins, in turn leading to different chemical reactions in the bird’s retina. Having many such molecules in an ordered pattern on the retina can constitute the compass signal.

So you’re saying that quantum mechanics may be at the heart of this system?
You could say it is quantum mechanical simply because it involves electron spins, which are quantum properties. By that measure, most chemical processes would be “quantum”. The really interesting question is whether quantum mechanics plays a role at the much deeper level involving quantum coherence and the associated phenomena of quantum superposition and entanglement.

And this is something that your research group is interested in?

It seems that the bird’s compass can sustain its singlet states for at least 100 µs if not much longer

Yes, our recent analysis of the European robin, published in Physical Review Letters, shows that in order to reconcile the radical pair model with recent data from experiments, quantum coherence must in fact exist in the compass – though it is not clear how this helps the compass to work. Normally, coherent quantum states decay very rapidly, and for technological applications, such as quantum information systems, the challenge is to hold on to them for as long as possible. But it seems that the bird’s compass can sustain its singlet states for at least 100 µs if not much longer. This may sound like a short time, but the best comparable artificial molecules only achieve 80 µs at room temperature in ideal laboratory conditions.

So what can you conclude from your study?
Our paper raises two questions to be addressed. First, why are the birds using these quantum states and how are they benefiting from them? Second, how can they sustain quantum coherence for such a remarkably long time? If we can find an answer to the latter question, this may help researchers to mimic the way birds harness these quantum states over extended periods of time and help in the development of practical quantum technologies.

How did you become interested in this field of research?
My research lies in quantum information technology, which normally has nothing to do with living creatures. My colleagues and I became interested in the avian compass after reading about the latest experimental results from Roswitha and Wolfgang Wiltschko, researchers in Frankfurt. We were intrigued after a simple back-of-the-envelope calculation made us realize that quantum coherence must be sustained for a very long time in the molecular structure of the radical pair, so we performed more sophisticated calculations, and found that our initial estimate was indeed correct. Regardless of whether or not this discovery will eventually prove useful for quantum information applications, the avian compass is a fascinating subject, and working on this study has simply been great fun.

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