Is the Quantum State in a Dressed Atom System Pure?

[Chenyb]

Consider a simple two level atom system,|1>,|2> (not degenerate) interacted with monochromatic laser, of which the frequency is exactly resonant with the |1> to |2> transition. The evolution start from {1>.

As far as I understand, the dressed degenerate state |1>|n> and |2>|n-1> will get splitted,(AT splitting) and Rabi oscillation occurs between |1> and |2>. My question: is the quantum state during evolution pure? What if we watch in the dressed picture? How can I get the probability of the atom lying in pure state |1>? Will performing the projection operation work? (like a = \int <1|\phi> )

 

[jfizzix]

If the initial state of the atom-plus-field is pure, and the only interaction with the atom is due to the field, and vise versa, then the state, whatever it is, will remain pure throughout the interaction.

If the initial state of just the atom is pure, and it interacts with the field, the final state of the atom itself, may be either pure or mixed, depending on the interaction.

When the state of the atom-plus-field is pure, but the state of the atom is mixed, the state of the atom-plus field is said to be in a pure entangled state.