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What is the difference between spontaneous emission and stimulated emission?

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energy levels of a luminescent centre — luminescence Figure 1: Energy levels of a luminescent centre (see text).

In physics, the primary difference between spontaneous and stimulated emission lies in how an excited atom releases energy to return to a lower energy state.

Spontaneous Emission
This is a fundamental, probabilistic process where an isolated excited atom naturally transitions to a lower energy state after a short period, emitting a photon in the process. Because these transitions occur independently for each atom in an assembly, the resulting light is incoherent, meaning the emitted waves are not in phase with one another.123

Stimulated Emission
In this process, an atom already in an excited state is triggered to release its energy by the presence of an incoming photon that has the same energy as the transition. The emitted photon is identical to the stimulating photon in every respect: it has the same energy, polarization, and direction, and it is in phase (coherent) with the incoming wave. This process is the foundational principle behind lasers, which utilize population inversion to produce intensive, monochromatic, and coherent light.423