Quantum Non-Demolition & Ramsey Zones

[Ylle]

A little question I'm not quite sure of.
When you try to do a Quantum Non-Demolition measurement, you have an atom which first goes into a Ramsey zone, and then into a cavity which shoots some photons at it. After that it comes out into another Ramsey zone again. And for the atom in a Ramsey zone the atoms state gets transformed by something like |+> -> (1/sqrt(2))(|-> + |+>) if I remember correctly. But does it also gets transformed when it goes into the cavity and gets time dependent, or does it only get the time dependent factor multiplied ?

 

[Evalesco]

When the atom enters the cavity, it does not get transformed into a new state. Instead, it experiences a time-dependent phase shift as the photons interact with it. This phase shift is what causes the atom to leave the cavity in a different state than it was in before entering the cavity. The exact nature of this phase shift depends on the details of the experiment, such as the type of cavity and the type of atom being used.