  A discussion of the quantum Zeno effect and paradox is given. The quantum
Zeno paradox claims that a continuously observed system, prepared in a state
which is not an eigenstate of the Hamiltonian operator, never decays. To
recover the classical behavior of unstable systems we consider a two-level
system interacting with a Bose field, respectively prepared in the excited
state and in the Poincare invariant vacuum state. Using time-dependent
perturbation theory, we evaluate for a finite time interval the probability of
spontaneous decay of the two-level system. Using the standard argument to
obtain the quantum Zeno paradox, we consider N measurements where N goes to
infinity and we obtain that the non-decay probability law is a pure
exponential, therefore recovering the classical behavior.

Quantum Physics