Quantum Zeno effect and motional narrowing in a two-level system

A model is given for the continual measurement of the inversion in a two-level system. The system is assumed to undergo oscillations between the two states. The effect of the measurement is determined by a Markovian master equation and one parameter Γ, which is a function of the measurement response bandwidth and the measuring device noise level. For rapid, accurate measurements, Γ is large. The two-level oscillation frequency provides a dynamical threshold. When Γ exceeds this threshold a two-level system prepared initially in the excited state remains so on a time scale of Γ. Thus, in the limit of Γ infinitely large, the dynamics is frozen. This is the Zeno effect. The relationship of this behavior to motional narrowing is discussed.