Quantum Zeno Effect Underpinning the Radical-Ion-Pair Mechanism of Avian Magnetoreception

The intricate biochemical processes underlying avian magnetoreception, the sensory ability of migratory birds to navigate using earth's magnetic field, have been narrowed down to spin-dependent recombination of radical-ion pairs to be found in avian species' retinal proteins. The avian magnetic field detection is governed by the interplay between magnetic interactions of the radicals' unpaired electrons and the radicals' recombination dynamics. Critical to this mechanism is the long lifetime of the radical-pair's spin coherence, so that the weak geomagnetic field will have a chance to signal its presence. It is here shown that a fundamental quantum phenomenon, the quantum Zeno effect, is at the basis of the radical-ion-pair magnetoreception mechanism. The quantum Zeno effect naturally leads to long spin coherence lifetimes, without any constraints on the system's physical parameters, ensuring the robustness of this sensory mechanism. Basic experimental observations regarding avian magnetic sensitivity are seamlessly derived. These include the magnetic sensitivity functional window and the heading error of oriented bird ensembles, which so far evaded theoretical justification. The findings presented here could be highly relevant to similar mechanisms at work in photosynthetic reactions. They also trigger fundamental questions about the evolutionary mechanisms that enabled avian species to make optimal use of quantum measurement laws.