Quantum coherence: myth or fact?
It has recently been argued that the inability to measure the absolute phase of an electromagnetic field prohibits the representation of a laser's output as a quantum optical coherent state. This argument has generally been considered technically correct but conceptually disturbing. Indeed, it would seem to place in question the very concept of the coherent state. Here we show that this argument fails to take into account a fundamental principle that not only re-admits the coherent state as legitimate, but formalizes a fundamental concept about model building in general, and in quantum mechanics in particular.