How cryptochrome radical pairs stay coherent long enough to sense Earth's field
In plain words
Migratory birds are thought to sense the direction of Earth's magnetic field through pairs of molecules with one unpaired electron each (radical pairs) in an eye protein called cryptochrome. Calculations say the electron spins lose their correlation too fast for so weak a field to matter, so either the calculations miss something or the receptor works differently.
Precise statement
For the light-induced radical pair in avian cryptochrome 4 (flavin radical FAD.- with a tryptophan or another partner radical) in a field $B \sim 0.5\,\mathrm{G}$ at $T \sim 313\,\mathrm{K}$, compute the spin relaxation and decoherence time $T_2$ including hyperfine modulation by protein motion and spin-rotation, and the resulting dependence of the singlet yield on field direction. Simulated $T_2$ for the flavin-tryptophan pair is of order $0.1\,\text{microsecond}$ (approximately, model dependent), while a usable heading signal at $0.5\,\mathrm{G}$ is estimated to need $T_2$ of order $1\,\text{microsecond}$ or longer (approximately). An answer is either a computed and measured $T_2$ and anisotropic yield for the identified pair large enough to support orientation, or the identification of a different radical pair or mechanism that achieves it.
What would settle it
Time-resolved spin measurements on purified and in-retina avian cryptochrome 4 at Earth-strength fields showing a heading-dependent product yield, matched by a quantitative spin-dynamics simulation of the same pair.
Status in the literature
Unverified note
A 2024 Nature Communications theory paper showed that a tightly bound FAD-superoxide pair can respond to Earth-strength fields if recombination is strongly asymmetric (a quantum Zeno effect); no in-vivo test exists and the flavin-tryptophan coherence gap is unresolved.