One boson in a state can stimulate or induce another boson into the same state, causing a quantum event (eg. an atomic transition).

"A splendid light has dawned on me about the absorption and emission of radiation..."

What Einstein had realized is that light shined on an atom which is in an excited state can induce the atom to make a downward transition (emitting a photon) if the incoming light's frequency matches the atomic transition energy. The incoming photon is a boson, and for this reason it stimulates the emission of a second photon in the same state, inducing an atomic transition. (Otherwise the "spontaneous emission" would happen randomly.)

• Thus, in stimulated emission we have an example of "quantum causality."

• This process combined with reflection can yield many photons in the same state: coherent light. Stimulated emission underlies the laser.

From the point of view of quantum field theory, even random quantum quantum events such as so called 'spontaneous emission' are really 'stimulated' by the vacuum's zero point energy.

"When you come right down to it, there is really no such thing as truly spontaneous emission; its all stimulated emission. The only distinction to be made is whether the field that does the stimulating is one that you put there or one that God put there..."

"From the perspective of quantum electrodynamics, spontaneous emission is a type of stimulated emission induced by fluctuations in the electromagnetic field of the vacuum. Confining an excited atom or molecule to a sufficiently small enclosure significantly modifies the fluctuations of the vacuum, and therefore also the spontaneous decay rate of a quantum state; see D. Kleppner, Inhibited Spontaneous Emission, Phys. Rev. Lett. 47 (1981) 233."