concept

Radical pair

Two molecules, each carrying one unpaired electron, whose quantum spins remain linked after being created together. The spin pair flickers back and forth between two states, and the remarkable thing is that the rate of that flicker is sensitive to the direction of the surrounding magnetic field. This is the mechanism thought to underlie the magnetic sense in birds: a quantum-mechanical effect subtle enough that a living creature can read a planet's faint magnetic field. Being subtle, it is also fragile - even weak radio-frequency noise can break the quantum coherence and leave a bird disoriented.