In physics, quantization (in British English quantisation) is the systematic transition procedure from a classical understanding of physical phenomena to a newer understanding known as quantum mechanics. It is a procedure for constructing quantum mechanics from classical mechanics. A generalization involving infinite degrees of freedom is field quantization, as in the "quantization of the electromagnetic field", referring to photons as field "quanta" (for instance as light quanta). This procedure is basic to theories of atomic physics, chemistry, particle physics, nuclear physics, condensed matter physics, and quantum optics. Historical overview In 1901, when Max Planck was developing the distribution function of statistical mechanics to solve ultraviolet catastrophe problem, he realized that the properties of blackbody radiation can be explained by the assumption that the amount of energy must be in countable fundamental units, i.e. amount of energy is not continuous but discrete. That is, a minimum unit of energy exists and the following relationship holds for the frequency . Here, is called Planck's constant and is a unique constant representing the amount of the quantum mechanical effect. It means a fundamental change of mathematical model of physical quantities. In 1905, Albert Einstein published a paper "On a heuristic viewpoint concerning the emission and transformation of light" explaining photoelectric effect based on quantized electromagnetic waves. The energy quantum referred to in this paper was later called "photon". In July 1913, Niels Bohr used quantization to describe the spectrum of a hydrogen atom in his paper "'On the Constitution of Atoms and Molecules. The preceding theories have been successful, but they are very phenomenological theories. However, the French mathematician Henri Poincaré first gave a systematic and rigorous definition of what quantization is in his 1912 paper "Sur la théorie des quanta". The term "quantum physics" was first used in Johnston's Planck's Universe in Light of Modern Physics. (1931). Canonical quantization Canonical quantization develops quantum mechanics from classical mechanics. One introduces a commutation relation among canonical coordinates. Technically, one converts coordinates to operators, through combinations of creation and annihilation operators. The operators act on quantum states of the theory. The lowest energy state is called the vacuum state. Quantization schemes Even within the setting of canonical quantization, there is difficulty associated to quantizing arbitrary observables on the classical phase space. This is the ordering ambiguity: Classically, the position and momentum variables x and p commute, but their quantum mechanical operator counterparts do not. Various