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Quantum is an important concept in modern physics. It was first proposed by German physicist M. Planck in 1900. He hypothesized that the radiant energy in blackbody radiation is discontinuous and can only be taken as an integral multiple of the basic unit of energy, so as to well explain the experimental phenomenon of blackbody radiation.

Later studies show that not only the energy shows this discontinuous separation property, but also other physical quantities such as angular momentum, spin, charge and so on. This is fundamentally different from the classical physics represented by Newtonian mechanics. The quantization phenomenon is mainly manifested in the micro physical world. The physical theory describing the micro physical world is quantum mechanics.

The word quantum comes from the Latin quantus, which means "how much" and stands for "a considerable amount of something". Since Planck put forward the concept of quantum, with the improvement of Einstein, Bohr, de Broglie, Heisenberg, Schrodinger, Dirac, born, etc., in the first half of the 20th century, a complete theory of quantum mechanics has been preliminarily established. Most physicists regard quantum mechanics as the basic theory to understand and describe nature.

If a physical quantity has the smallest indivisible basic unit, then the physical quantity is quantized, and the smallest unit is called quantum. The English name of quantum comes from the Latin quantus, which means "how many" and represents "a certain amount of substance". In physics, the concept of quantum is often used, which refers to an indivisible basic individual. For example, "quantum of light" (photon) is the basic energy unit of light with a certain frequency. The extended quantum mechanics, quantum optics and so on have become different professional research fields. Its basic concept is that all tangible properties are "quantizable". "Quantization" means that the value of its physical quantity is discrete, rather than any continuous value. For example, in an atom, the energy of an electron is quantizable. This determines the stability of atoms and emission spectra. Most physicists regard quantum mechanics as a basic theory to understand and describe nature.

Generally speaking, quantum is the smallest unit that can show the properties of a substance or physical quantity.

In classical physics, according to the theorem of energy equipartition, energy is continuously changing and can take any value. In the late 19th century, scientists found that many physical phenomena could not be explained by classical theory. At that time, German Physics focused on the research of blackbody radiation. Around 1900, M. Planck tried to solve the problem of blackbody radiation. He boldly put forward the quantum hypothesis and obtained Planck's radiation law, which has been used up to now. Planck put forward that, just as atoms are the constituent units of all matter, "quantum" is the smallest unit of energy. An object can only absorb or emit electromagnetic radiation in a quantum way. Planck first published the energy quantization value, the value of one molecular mole (mol) and the basic charge at the conference of the German Physics Society on December 14, 1900. Its numerical value is more accurate than before, and the proposed theory has successfully solved the problem of blackbody radiation, marking the birth of quantum mechanics.

In 1905, German physicist Einstein introduced the concept of quantum into the propagation process of light, put forward the concept of "photon", and put forward that light has the properties of wave and particle at the same time, that is, the "wave particle duality" of light.

In the 1920s, French physicist de Broglie proposed the concept of "matter wave", that is, all matter particles have wave particle duality; German physicist Heisenberg and others established quantum matrix mechanics; Austrian physicist Schrodinger established quantum wave dynamics. The development of quantum theory has entered the stage of quantum mechanics.

In 1928, British physicist Dirac completed the mathematical equivalence proof between matrix mechanics and wave dynamics, systematically summarized the theory of quantum mechanics, and successfully combined the two theoretical systems relativity and quantum mechanics, which opened the prelude of quantum field theory. Quantum theory is one of the two cornerstones of modern physics, which provides a theoretical basis for understanding macro phenomena from the micro level.

The proposition of quantum hypothesis has a strong impact on classical physics, promotes physics to enter the micro level, and lays the foundation of modern physics. But until now, some hypotheses about quantum mechanics still can't be fully proved, and there is still a lot to study.