The quartic threefold is symplectically irrational

We prove that smooth quartic threefolds are symplectically irrational, i.e., cannot be related to projective space by a series of symplectic blow-ups, blow-downs, and deformations. This implies that they are algebraically irrational, recovering a classical result of Iskovskikh-Manin. Our proof involves establishing a decomposition theorem for quantum cohomology along symplectic blow-ups, following the work of Iritani.