The Jaynes-Cummings model describes a two-level atom in a single-mode cavity. Atom-photon coupling g causes vacuum Rabi splitting: two polariton branches separated by 2g. With a coherent state (Poissonian photon distribution), the atomic inversion undergoes collapse and revival: collapses at t ~ 1/g, revives at t_rev ~ 2π√n̄/g — a quantum interference effect with no classical analogue. κ adds cavity decay.