Luis Aina, Alfredo2023-06-202023-06-202003-06-301050-294710.1103/PhysRevA.67.062113https://hdl.handle.net/20.500.14352/51521©2003 The American Physical Society.We examine the appearance of Zeno and anti-Zeno effects when the unitary evolution of a two-level system is observed. We demonstrate that, contrary to previous claims, the anti-Zeno effect can occur in two-level systems so that the observation can enhance the transition probability. Moreover, for nonresonant couplings we show that the Zeno and anti-Zeno effects always appear sequentially in the evolution of the same system: there is Zeno effect in the short-time regime and anti-Zeno effect in the long-time regime.engjournal articlehttp://dx.doi.org/10.1103/PhysRevA.67.062113http://journals.aps.org/open access535Intermittent atomic fluorescenceQuantum zenoSingle-atomDown-conversionDark periodsParadoxJumpsDynamicsCollapseLightÓptica (Física)2209.19 Óptica Física