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The stimulated emission of light was a discovery by Einstein around 1916. All we need is an atom, which possesses electrons just like any other atom in our world and a photon, that is: a quantum of light.
1) Energy levels of atoms and electrons
2) Absorbing energy in the form of photons
3) Stimulated and spontaneous emission
4) Photons with same phase, frequency, polarization, direction
5) Parts of a LASER device (reflectors, gain medium)
6) Chain reaction for coherent and monochromatic light
7) Population inversion (ground state vs. excited state)