The sensory mechanisms that allow birds to perceive the direction of the Earth’s magnetic field for the purpose of navigation are only now beginning to be understood. One of the two leading hypotheses is founded on magnetically sensitive photochemical reactions in the retina. It is thought that transient photo-induced radical pairs in cryptochrome, a blue-light photoreceptor protein, act as the primary magnetic sensor. Experimental and theoretical support for this mechanism has been accumulating over the last few years, qualifying chemical magnetoreception for a place in the emerging field of Quantum Biology.In this proposal, we aim to determine the detailed principles of efficient chemical sensing of weak magnetic fields, to elucidate the biophysics of animal compass magnetoreception, and to explore the possibilities of magnetic sensing technologies inspired by the coherent dynamics of entangled electron spins in cryptochrome-based radical pairs.We will:(a) Establish the fundamental structural, kinetic, dynamic and magnetic properties that allow efficient chemical sensing of Earth-strength magnetic fields in cryptochromes.(b) Devise new, sensitive forms of optical spectroscopy for this purpose.(c) Design, construct and iteratively refine non-natural proteins (maquettes) as versatile model systems for testing and optimising molecular magnetoreceptors.(d) Characterise the spin dynamics and magnetic sensitivity of maquette magnetoreceptors using specialised magnetic resonance and optical spectroscopic techniques.(e) Develop efficient and accurate methods for simulating the coherent spin dynamics of realistic radical pairs in order to interpret experimental data, guide the implementation of new experiments, test concepts of magnetoreceptor function, and guide the design of efficient sensors.(f) Explore the feasibility of electronically addressable, organic semiconductor sensors inspired by radical pair magnetoreception.