Light propagating in a restricted volume of space has its energy and momentum quantized according to an integer number of particles known as photons.
The first major development leading to that understanding was the correct modeling of the blackbody radiation spectrum by Max Planck in 1899 under the hypothesis of light being emitted in discrete units of energy.
The photoelectric effect was further evidence of this quantization as explained by Albert Einstein in a 1905 paper, a discovery for which he was to be awarded the Nobel Prize in 1921.
As laser science needed good theoretical foundations, and also because research into these soon proved very fruitful, interest in quantum optics rose.
Following the work of Dirac in quantum field theory, John R. Klauder, George Sudarshan, Roy J. Glauber, and Leonard Mandel applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the statistics of light (see degree of coherence).
Applications for solid state research (e.g. Raman spectroscopy) were found, and mechanical forces of light on matter were studied.
However, the actual invention of the maser (and laser) many years later was dependent on a method to produce a population inversion.
The use of statistical mechanics is fundamental to the concepts of quantum optics: light is described in terms of field operators for creation and annihilation of photons—i.e.
Solid state physics regularly takes quantum mechanics into account, and is usually concerned with electrons.
The term also encompassed the basic processes of laser operation, which is today studied as a topic in quantum optics.