We review the spin radical pair mechanism which is a promising explanation ofavian navigation. This mechanism is based on the dependence of product yieldson (1) the hyperfine interaction involving electron spins and neighboringnuclear spins and (2) the intensity and orientation of the geomagnetic field.One surprising result is that even at ambient conditions quantum entanglementof electron spins can play an important role in avian magnetoreception. Thisreview describes the general scheme of chemical reactions involving radicalpairs generated from singlet and triplet precursors; the spin dynamics of theradical pairs; and the magnetic field dependence of product yields caused bythe radical pair mechanism. The main part of the review includes a descriptionof the chemical compass in birds. We review: the general properties of theavian compass; the basic scheme of the radical pair mechanism; the reactionkinetics in cryptochrome; quantum coherence and entanglement in the aviancompass; and the effects of noise. We believe that the "quantum avian compass"can play an important role in avian navigation and can also provide thefoundation for a new generation of sensitive and selective magnetic-sensingnano-devices.
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