PROJECTION POSTULATE AND ATOMIC QUANTUM ZENO EFFECT

The projection postulate has been used to predict a slow-down of the t ime evolution of the state of a system under rapidly repeated measurem ents, and ultimately a freezing of the state. To test this so-called q uantum Zeno effect an experiment was performed by Itano et al. [Phys. Rev. A 41, 2295 (1990)] in which an atomic-level measurement was reali zed by means of a short laser pulse. The relevance of the results has given rise to controversies in the literature. In particular, the proj ection postulate and its applicability in this experiment have been ca st into doubt. In this paper we show analytically that for a wide rang e of parameters, such a short laser pulse acts as an effective level m easurement to which the usual projection postulate applies with high a ccuracy. The corrections to the ideal reductions and their accumulatio n over n pulses are calculated. Our conclusion is that the projection postulate is an excellent pragmatic tool for a quick and simple unders tanding of the slow-down of time evolution in experiments of this type . However, corrections have to be included, and an actual freezing doe s not seem possible because of the finite duration of measurements.