Temporal behavior of quantum systems and quantum zeno effect

The temporal behavior of an unstable system is analyzed quantum mechanically and compared to the exponential decay law. The general mathematical features of the quantum evolution, yielding a quadratic region at short times and a power law at long times, are briefly reviewed. The consequences of the short-time quadratic evolution are curious: By performing many measurements in rapid succession on a quantum system, in order to check whether it is still in its initial state, one can hinder its evolution. This phenomenon is known as the quantum Zeno effect and is discussed in detail. In this respect, a specific example involving neutron spin is considered. Finally, we focus our attention on some interesting features of the evolution law.