What causes stimulated emission to occur in quantum mechanics?

[Yosty22]

I have been learning some introductory quantum mechanics and stimulated Emission is giving me some problems conceptually.

I understand that photons can be absorbed to bump electrons up to higher energy levels and that electrons in the higher energy state spontaneously decay back to the lower energy state because they "want" to be in the lowest available energy state. In the texts I have used and some various wiki articles I have read, stimulated Emission is when the emitted photon is in phase with the incident EM wave and going in the same direction.

However, I am confused as to what exactly what causes this to happen. That is, what is doing the Stimulating that causes the electron to decay?

 

[vela]

The incident wave.

According to quantum mechanics, an electron in an energy eigenstate should remain in that state indefinitely, and an electronic transition occurs only because the electron interacts with an external field. In the case of stimulated emission, it's the field of the incident wave; in the case of spontaneous emission, it's the field due to vacuum fluctuations.