Quantum Mechanics - Time-Dependent Perturbation Theory : Adiabatic and Sudden Approximations

In discussing the time-dependent perturbation theory,we have dealt with phenomena where the perturbation V(t) is small,but we have paid no attention to the rate of change of the perturbation. In this video we want to discuss approximation methods treating phenomena where V(t) is not only small but also switched on either adiabatically (slowly) or suddenly (rapidly). Adiabatic approximation has been briefly discussed firstly without combining it with perturbation theory. Secondly it has been explained by applying time dependent perturbation theory. It’s found that no transition occurs when the perturbation is turned on and off adiabatically. In the similar manner Sudden approximation has been explained firstly without using the perturbation theory and secondly by the use of this. We observe that the Sudden approximation is applicable only for transitions between discrete states.