Investigation of birds' quantum skills prompts new magnetoreception theory

The suggestion last year that European robins' magnetic sense involves a long-lived quantum state has prompted CQT researchers and their collaborators to propose a new mechanism to explain the avian ability. The work is published in the March issue of Biophysical Journal.

Exactly how birds such as the European robin detect magnetic fields — a trick that may help them navigate by Earth's magnetic field — is poorly understood. A popular model holds that the mechanism is part of the visual system, involving an unidentified magnetoreceptor molecule that is excited by light to form a 'radical pair'. The external magnetic field would determine what proportion of the excited molecules end up in a 'spin-triplet' versus a 'spin-singlet' state.

In the new work, the researchers consider what happens to these radicals in the final sensing step. Typically the radical pair model is thought to conclude in chemistry: for example, the triplet state may decay in a different way to the singlet state, with detection of the field via the resulting chemical products.

What if the last bit is physics, instead? CQT Research Fellow Erik Gauger and CQT Visiting Research Associate Professor Simon Benjamin and their colleagues suggest that the birds' eyes may see the electric fields generated by the triplet states. These states have an electric dipole moment (the singlet state has none). The cumulative electric field from magnetoreceptors patterned across the bird's retina could be strong enough to directly disrupt the light-sensing components of the eye, the researchers suggest. "The bird would literally be able to see themagnetic field as a superimposed feature in its normal visual image," the authors write. The collaboration involved researchers from University College London, the University of Oxford and Heriot Watt University, all in the UK.

Various numerical calculations back up the idea. For one, the researchers show that field strengths could exceed those known to influence bacterial light-sensing molecules. They also propose various experiments to test the idea further.

This diversion into quantum biology for Erik, currently a visitor in the University of Oxford's Department of Materials, and Simon, who is permanent faculty there, was motivated by research they contributed to last year published in Physical Review Letters. This was analysis of experimental data on the disruption of European robins' magnetic sense by external fields, which pointed to the quantum spin state being remarkably long-lived by the standards of lab-based quantum experiments.

Quantum longevitiy is interesting to quantum physicists seeking to study and manipulate quantum states in the lab — but the result also piqued the physicists' interest for its biological implications. A long-lived triplet state would be counter-productive if the bird hoped to detect the state by its decay products.

"With the new model, however, a longer coherence time directly entails a stronger signal, providing a clear evolutionary pathway for nature to select in favour of longer and longer coherence times," explains Erik.

The physicists must now hope that the biologists, and their birds, can provide more data to settle the question.

The reference for the new work is "A New Type of Radical-Pair-Based Model for Magnetoreception" Biophysical Journal 102, 961 (2012); arXiv:1003.2628. See also a popular account of the work on the Oxford University Science Blog. The paper is dedicated to co-author A. Marshall Stoneham, who passed away in 2011.