Out-of-time-order correlators (OTOCs) measure quantum chaos: C(t) = −⟨[W(t),V(0)]²⟩. In fast scramblers, C(t) ~ exp(λ_L t) with Lyapunov exponent λ_L ≤ 2πk_BT/ℏ (Maldacena-Shenker-Stanfod bound). Information spreads as a "butterfly" across the causal light cone.