Frequent measurements on an unstable quantum state inhibit its decay. In the limit of continuous measurement, the quantum Zeno effect freezes evolution. The anti-Zeno effect can also occur at intermediate rates.
Misra & Sudarshan (1977): for a system with survival probability P(t)≈1−Γt² (short times), repeated measurements at intervals τ give P_survival(t/τ steps) ≈ (1−Γτ²)^(t/τ). As τ→0, P→1 (Zeno). Anti-Zeno: for structured environments, measurement can accelerate decay.