Describing the phase of an electromagnetic field mode or harmonic oscillator has been an obstacle since the early days of modern quantum theory. The quantum phase operator was even more problematic with the invention of the maser and laser in the 1950s and 1960s. This problem was not solved until the Pegg-Barnett formalism was developed in the 1980
Precursors. The Phase Operator. Mathematical Elaborations. Phase Dynamics and Uncertainties. Theory of Phase Measurement. Experimental Demonstrations. Time.