# Dipole moments of two level systems in radiation

Hi I am starting to learn about perturbation theory in quantum mechanics, and hence I am learning about oscillating radiation and it's effect on two-level systems. I just want to confirm that in this context there are two types of dipoles:

The electric dipole moment operator $\vec{d} = q\vec{r}$, where $\vec{r}$ is the position operator, and then there is an induced dipole moment due to the oscillating radiation. How is the second dipole moment usuallu defined and what is the connection with the first?

In classical electrodynamics I am aware that an electric dipole moment in the presence of an oscillating electric field would oscillate as it alligns itself with the electric field. Are these two concepts of dipoles separate in quantum mechanics?

Thanks for any help.

• do you have a reference or a quotation of a source claiming there are two types of dipole moments for quantum systems? I think both situations you describe are captured by the definition for dipole moment you give. – Crimson Apr 6 '17 at 14:14
• @Crimson Thanks for your response. It might be that my understanding is incorrect, I am using "Atomic Physics by Foot". So it is the same dipole moment operator, just under radiation, it oscillates? – user101311 Apr 6 '17 at 17:52
• I don't know the book, but indeed the dipole moment oscillates (the operator does not (assuming you do not use the Heisenberg picture(if you don't know what the Heisenberg picture is, you are probably not using it))) – Crimson Apr 11 '17 at 17:53