# Why is spontaneous emission neglected in standard discussions of the two-level system?

When discussing two-level systems, spontaneous emission is often neglected `until later'. However, when discussed later, the two-level system is no longer discussed. For example, see Straten and Metcalf.

Why in simple descriptions of the two-level system can spontaneous emission be safely ignored? I think it is simply assumed that the Rabi frequency is far greater than the spontaneous emission rate? Or, to put another way, the lifetime of the excited state is very long lived.

• I'm a bit confused here: every textbook I had in high school and college did not ignore spontaneous emission. The spont.emission coefficient is critical in calculating the available time for a population inversion to contribute to stimulated emission. Moving on to 4-level systems, the need for extremely "fast" spontaneous emission is critical. Nov 22, 2021 at 18:16
• @CarlWitthoft I didn't say they ignored spontaneous emission completely, just when formulating the two level system. Nov 22, 2021 at 18:18
• In physics it is often helpful to simplify the model to such a degree that we are able to describe it, and therefore understand it. Hence, all models are wrong, but some are useful. Nov 22, 2021 at 21:05
• The link takes you to the preface of the book. Can you provide a better link or give page numbers. Nov 23, 2021 at 7:20

What I mean by this is that there are experimental regimes where we can have what you say, either very high Rabi frequencies or very long lifetimes, which are both conceptual statements of the mathematical condition $$\Omega \gg \gamma$$. In those conditions the predictions from ignoring spontaneous emission match observation really well.
There's also a deeper issue here: you'll never get spontaneous emission out of any theory that treats the EM field classically but the atom quantum-ly. Instead you have to quantize the EM field as well; and then the combined atom+field system is, in a real sense, infinitely far from a two-level system, since the states are now $$|g,n+1\rangle$$ and $$|e,n\rangle$$ for any $$n \geq 0$$ (and also $$|g,0\rangle$$), where the second label indicates the number of photons in the relevant mode (which I incidentally assumed there was only one of).