Understanding the idea of coherence

[acme37]

I'm having some trouble understanding the idea of coherence.

Consider the phrase: "a coherent superposition of states."

Here I understand the coherence to refer to the states being coupled to each other and capable of interfering, due to a perturbing field.

How does this relate to coherent light, which I understand to be light that is monochromatic and phase-correlated? What happens if you shine light that is monochromatic, but not coherent, on the atomic system?

 

[acme37]

I think I basically get it.

"In general, we say that atomic coherence exists when the density matrix has off-diagonal elements" - Quantum Optics, Scully & Zubairy

A coherence exists when an interference can occur

 

[acme37]

In case anybody is interested, here is why I was asking...

For our QM final we had to pick a topic and do a writeup/presentation. I was intrigued by http://en.wikipedia.org/wiki/Electromagnetically_induced_transparency" and ended up writing generally about interference in three-level atomic systems. I've attached my writeup - I actually haven't turned it in yet, so if you have any comments, please do share.

Only lame thing is I use a probability amplitude treatment and couldn't get nice closed forms for the absorption and refractive index curves near resonance, which are the useful result here. I could have done the density matrix, but probably not in the 15 minutes I have to present. Solving for the susceptibility gets a little ugly

cheers,
Kevin