• the_wizard
In summary, the conversation discusses the differences between population lifetime and dipole lifetime in the context of atomic transitions. The population lifetime refers to the time atoms stay in a certain state before transitioning, while the dipole lifetime refers to the time it takes for an electric dipole moment to change orientation. In a two-level system, the dipole lifetime is much shorter than the population lifetime. An example is seen in the behavior of electrons in a hydrogen atom, where the dipole lifetime is determined by how quickly the electron's spin changes, which is faster than the transition between states.

#### the_wizard

Reading a book about atomic transitions i really don't understand physically what the book means when it talks about differences between the population lifetime and the dipole lifetime that appear in the coerence part of the density matrix of a 2 or a 3 level system.
I imagine that the population lifetime refers to how many atoms of the system pass from the upper state to the lower one, but i can't visualize the meaning of the dipole lifetime.
How can be possible that the lifetime of the population is much longer than the dipole lifetime? Can anyone give me an example of what happens in systems like these?

The population lifetime is the expected amount of time that a certain number of atoms remain in a particular state before transitioning to another state. The dipole lifetime, on the other hand, refers to the amount of time that it takes for an electric dipole moment to change its orientation. In a two-level system, the dipole lifetime is much shorter than the population lifetime because the dipole moment can flip much more quickly than the atoms can transition between states.An example of this difference can be seen in the behavior of electrons in a hydrogen atom. Here, the population lifetime is determined by how long the electrons remain in an excited state before transitioning to a lower energy state. Meanwhile, the dipole lifetime is determined by how quickly the electron's spin changes as it moves from one state to the next. This spin change is much faster than the transition between states, meaning the dipole lifetime is much shorter than the population lifetime.