Frequent measurement of an unstable quantum system slows its decay.
For a state |ψ⟩ evolving under H, after time δt the survival probability is
P = |⟨ψ|e^{−iHδt}|ψ⟩|² ≈ 1 − (ΔH)²(δt)²
(quadratic for short times). After n measurements in total time T: τ = T/n → 0,
so P^n → 1. The system is "frozen."
Contrast with the Anti-Zeno effect: measurement at resonance frequencies of the
environment can accelerate decay. The Zeno/anti-Zeno crossover depends on
the spectral density of the bath at the measurement frequency.
Demonstrated experimentally: beryllium ions (Itano et al. 1990), cold atoms, NV centers.