Zeno effect preventing Rabi transitions onto an unstable energy level

We consider a driven 2-level system with one level showing spontaneous deca y to an otherwise uncoupled third level. Rabi transitions to the unstable l evel are strongly damped. This simple configuration can be used to demonstr ate and to explore the quantum Zeno effect leading to a freezing of the sys tem to the initial level. A comparison with repeated projection measurement s is given. A treatment within a phenomenological theory of continuous meas urements is sketched. The system visualizes the important role of null meas urements (negative result measurements) and may serve as a goad example for a truly continuous measurement. (C) 1999 Published by Elsevier Science B.V . All rights reserved.