Quantum optics is the study, and application of the quantum interactions of light (photons) with matter is an active and expanding field of experiment and theory. The study of individual photons is crucial to understanding the behavior of electromagnetic waves as a whole.
The word “quantum” refers to the smallest amount of any physical entity that can interact with another entity. Quantum physics, therefore, deals with the smallest particles; these are incredibly tiny sub-atomic particles which behave in unique ways.
The word “optics,” in physics, refers to the study of light. Photons are the smallest particles of light (though it is essential to know that photons can behave as both particles and waves).