Figure 4
Effective model of the cryptochrome radical pair mechanism. The first radical pair is created in the state \(\rho _1\) by an excitation of the precursor molecule \(\rho _5\). Electron hopping gives rise to other pairs, \(\rho _2\) and \(\rho _3\). Their spin dynamics, coherent evolution between the singlet (S) and triplet (T) states, depends on the external magnetic field and they can recombine to the precursor only if they are in the singlet state. This selective recombination is represented by the singlet projectors \(Q^S\) in front of the back-transfer rates, \(k_b\), and modifies the yield of the signalling state \(\rho _4\), perceived by the animal. Variants of the model with solid transitions have been studied previously, see e.g.43,48,49. We also consider the model with additional dashed arrows, which close the dynamics of the whole system.
