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A Ramsey pulse is a coherent process, where the electronic states of for example an ion (or some other two-level-system) are coupled to a laser field. By tuning the laser precisely, we can coherently manipulate the states, and excite carrier rotations between $| e \rangle \leftrightarrow | g \rangle$. The system dynamics behave like coupled harmonic oscillators and is described by an interaction hamiltonian.

For 3 level lasers on the other hand, we optically pump to an excited state $| e_{1} \rangle$, there's a relaxation to a second excited state $| e_{2} \rangle$, and from there, we get stimulated emission to the ground state $| g \rangle$. Here, we use a Master equation to describe the dynamics.

What's the difference between the two? In the case of the ion, we'll have oscillations as long as the laser field is turned on. Why are there no oscillations in a 3 level laser, why do the states just happily stay in the excited state until they decide to emit a photon? Can't we actively de-excite again from $| e_{2} \rangle$ to $| g \rangle$?

My guess is that there's completely different physics going on between the processes of optical pumping and Rabi oscillations, but I'm struggling to see the difference and to understand when which applies.

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  • $\begingroup$ The difference is that Ramsey-pulsing describes oscillations of the atom-photon interaction that occurs in a 2-level system, whereas optical pumping is a technique to get actual lasing from a 3-level system (since it is not classically possible with a 2-level system). The pumping field does not hit the $|e_2\rangle$ state, so there is no "oscillations" going on between it and the ground state. $\endgroup$ – Chris Gerig Aug 8 '15 at 23:23
  • $\begingroup$ @chris-gerig: But why can't we have oscillations between the $| e_{2} \rangle$ state and $| g \rangle$ if, say, we shined a laser resonantly on the transition? Why does this work on a trapped ion while it doesn't work on a three level system used to build a laser? $\endgroup$ – poppie Aug 9 '15 at 16:30
  • $\begingroup$ This optical pumping, by definition, only hits the $|e_1\rangle$ state. If you want to "shine a laser" elsewhere, you are no longer describing the same phenomenon. I can pick up a soccer ball and start bouncing it, but I'm no longer playing soccer. $\endgroup$ – Chris Gerig Aug 9 '15 at 17:08
  • $\begingroup$ @chris-gerig: Okay so how about this: In our 3 level system, we could excite the $|g\rangle \leftrightarrow|e_{1}\rangle$ transition, and do coherent manipulation of the state. However, we have two problems: First, we have quick decay from $|e_{1}\rangle$ to $|e_{2}\rangle$, so before we can do any coherent manipulation, the state actually decays (non-coherently). Second, our three state system that we want to use to build a laser consists of a solid state material which has a lot more than just three pure states (as is the case for ions). Does this make more sense? $\endgroup$ – poppie Aug 19 '15 at 11:34

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