The quantum Zeno effect states that frequent measurement of an unstable quantum system can slow — or even halt — its decay. Between measurements, a system evolves unitarily; each measurement collapses it back to the initial state with probability |⟨ψ₀|ψ(t)⟩|². For short times the survival probability goes as 1 − (t/τ_Z)², a quadratic (not exponential) short-time behavior. Very frequent projective measurements keep the system "frozen" in its initial state.