"...in a laser, an ensemble of excited atoms, occupying metastable energy states, is made to lose its energy in the form of a monochromatic amplification. The first few photons, emitted spontaneously, induce the emission of the next few, gradually releasing in this way a veritable avalanche of photons. The photons emitted by a laser, unlike those coming from an ordinary source of light (e.g. an incandescent lamp) are therefore not independent. Rather, they form a collective system of bosons, having well-defined phase relationships, thus giving the laser, besides its power and monochromicity, an exceptionally high degee of coherence."