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A quantum is the smallest unit of discrete variation, and this is the precise definition of it.
This video is an excerpt from the live broadcast of Professor Yuan Lanfeng on the theme of "Principles and Technologies of Quantum Information" on September 11, 2021
(Why has quantum, which was proposed a hundred years ago, suddenly become a hot topic |? Technology Yuan Ren)
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So the most basic question is, what exactly is quantum? Because it's called "son", right? So there are many people who immediately feel that the quantum should be a particle, you usually know that there are electrons, protons, neutrons, neutrinos, etc. These are all particles, its first reaction is to say what is a quantum, it is compared with these electrons, protons, neutrons, neutrinos, whether it is big or small, the most basic answer is that the question itself is wrong, this quantum is not this meaning at all, the quantum is not a particle, what is the correct understanding of the quantum?
What is discrete change? The most typical performance is like the figure on the left, that is, going up the steps, the characteristics of the steps are that you can go up one step, two steps and three steps, but you can't go up half a step, right? If you go up half a step, you'll fall and you'll be in the hospital, right? And then it's the same when we count the number of people, you can count one person, two people, three people, but there can't be half a person, right? This is called discrete variation.
So for this discrete change, the lattice with the least change, the smallest unit is a quantum, so for the upper step, a step is a quantum, and for the number of people, a person is a quantum.
So when we mention the word quantum, we must say that it is a quantum for something, for example, a photon is a quantum of light, that is to say, we usually see a beam of light, it is composed of many quantum of light, the amount of this light is a photon, and then the electron was first found in the cathode rays, for example, if we look at the following figure, at the end of the 19th century, Joseph John Thomson he first discovered electrons. At that time, it was in the form of cathode rays, and such a beam of radiation was emitted from the cathode, and you could use a magnet to deflect it, which showed that it was negatively charged.
Now we know that cathode rays are actually electron beams, which are composed of electrons, you can have 1 electron, 2 electrons, but you can't have half an electron, so the electron is the electron of the cathode ray, the quantum of the cathode ray, so everyone remembers that whenever we talk about the quantum, it must be a quantum for something, and there is no particle specifically called quantum, so you can't ask whether the quantum is big or small compared to electrons, protons, neutrons, and the problem itself is wrong.
Then in the microscopic world, there is a large class of phenomena, it is quantized, that is, it is discrete change, that is, discontinuous change, such as the energy level of the hydrogen atom, this hydrogen atom is the smallest atom in the world, it has only one electron, and then this one electron, the energy it can appear is discrete, For example, the lowest energy is -13.6eV, the unit in the figure is eV, what does eV mean, it is electron-volt, that is, electron volt, it is a unit of energy, the lowest, is now the standard 1 -13.6eV, the previous section, the second is -3.4, the negative 3.4 how did it come from? Is -13.6 divided by 4, is the square divided by 2, the previous is the square of it divided by 3, and the last is the square divided by 4, that is, the nth energy, that is, -13.6eV divided by the square of n, it is not equally spaced, but it is indeed discrete, that is, you can take 1 to any value in this, n, but you will not have an energy in the middle, let's say you will not appear -11eV, right? So for this discontinuous change, we call it quantization.
So this microscopic world, one of its essential characteristics, that is, there are many physical quantities that are quantized, so there is a whole big basic physics discipline called quantum mechanics.