What Is Quantum Mechanics?

Quantum Mechanics is a theory of physics. It is a branch of physics that studies the laws of movement of microscopic particles in the material world. It mainly studies the basic theory of the structure and properties of atoms, molecules, condensed matter, and atomic nuclei and elementary particles. Together with relativity, it forms the theoretical basis of modern physics. Quantum mechanics is not only one of the basic theories of modern physics, but also widely used in chemistry and other modern technologies.

Quantum mechanics is a theory describing microscopic matter, and

The basic mathematical framework of quantum mechanics is based on:

From the end of the 19th century to the beginning of the 20th century, classical physics has reached a point of perfection, but it has encountered some serious difficulties in experiments. These difficulties are regarded as "a few dark clouds in the clear sky." Changed the physical world. Here are a few difficulties:

Emergence and development of theory

Quantum mechanics is a physical science that describes the structure, movement, and changes of the microscopic world of matter. It is a major leap in the development of human civilization in the 20th century. The discovery of quantum mechanics has triggered a series of epoch-making scientific discoveries and technological inventions, and has made important contributions to the progress of human society.

At the end of the 19th century, when major achievements were made in classical physics, a series of phenomena that could not be explained by classical theory were discovered one after another. Thermal radiation theorem found by German physicist Wien through the measurement of thermal radiation spectrum. German physicist

Among many modern technological equipment,

A news report entitled "Schrödinger's cat was finally rescued, and Nature research observed the first quantum transition process" swipes the screen. Title parties such as "Yale's experiment overthrows the randomness of quantum mechanics", "Einstein got it right again", and so on have appeared, as if the invincible quantum mechanics overturned the ditch overnight, many young people lamented that fatalism was back . But is this really the case? Let's check it out ^[8] .

1. What is the randomness of quantum mechanics ^[8] ?

According to the summary of the master of mathematics and mathematics, Feng Neumann, there are two basic processes in quantum mechanics, one is to evolve deterministically according to Schrödinger's equation, and the other is to randomly collapse the quantum superposition state due to measurement. Schrödinger's equation is the core equation of quantum mechanics. It is deterministic and has nothing to do with follower nature. Then the randomness of quantum mechanics comes only from the latter, that is, from measurement ^[8] .

This randomness of measurement is what makes Einstein the most incomprehensible. He used the metaphor "God won't roll the dice" to oppose the measurement of randomness, and Schrödinger also imagined measuring the superposition of life and death of a cat. It ^[8] .

But countless experiments have confirmed that to directly measure a quantum superposition state, the result is randomly on one of the eigenstates (probability is the modulus square of the coefficient of each eigenstate in the superposition state), which is the most important thing in quantum mechanics. Measurement problems. In order to solve this problem, multiple interpretations of quantum mechanics were born, of which the three main interpretations are Copenhagen interpretation, multi-world interpretation and consistent historical interpretation ^[8] .

The Copenhagen interpretation believes that measurement will cause the quantum state to collapse, that is, the quantum state is instantly destroyed, and randomly falls to an eigenstate; the multi-world interpretation feels that the Copenhagen interpretation is too mysterious, so it makes a more mysterious, thinking that every time Measurement is a division of the world. The results of all eigenstates exist, but they are completely independent (orthogonal) from each other, and they cannot interfere with each other. We are only randomly in a certain world. The consistent historical interpretation introduces a quantum decoherence process. Solve the problem from superposition state to classical probability distribution. But in choosing which classic probability, it still returns to the debate on Copenhagen interpretation and multi-world interpretation ^[8] .

From a logical point of view, the combination of multi-world interpretation and consistent historical interpretation seems to be the most perfect for explaining the measurement problem. Multiple worlds form a total superposition state, that is, the certainty of the "God perspective" is retained, and the single world is retained. Randomness of perspective. However, physics is based on experimental science. These interpretations predict the same physical results and cannot be falsified with each other, so the physical meaning is equivalent, so the academic circle still mainly adopts the Copenhagen interpretation, that is, collapse. The word) stands for measuring the randomness of a quantum state ^[8] .

2. What did Yale s thesis say? ^[8]

This Nature paper from Yale University first paves up a knowledge of quantum mechanics, that is, the quantum transition is a deterministic process in which the quantum superposition state evolves completely according to the Schrödinger equation, that is, the probability amplitude on the ground state | G> is continuously transferred to the excitation according to the Schrödinger equation. State | E>, and then continuously transferred back to form an oscillation (frequency is called Rabbi frequency), which belongs to the first type of process summarized by Von Neumann.

This paper measures such a deterministic quantum transition, so it is no surprise to get deterministic results. The selling point of this article is how to prevent this measurement from destroying the original superposition state, or how to prevent quantum transitions from stopping due to sudden measurements. This is not a mysterious technology, but a "weak measurement" method currently widely used in the field of quantum information.

This experiment uses a three-level system artificially constructed by a superconducting circuit. The signal-to-noise ratio is much worse than the actual atomic energy level ^[8] .

The weak measurement technique used in the experiment is to divide the number of particles in the original ground state | G> (this experiment uses superconducting current), and let it form a superposition state with | D>, while | G> the rest The number of particles continues to be superimposed with | B>. These two superimposed states are (almost) independent and (almost) do not affect each other. For example, by controlling the intensity of the two transitional rabies frequencies with light (microwave), the probability amplitude can be close to 1 when | B> is close to 1, and | D> is also close to 1. At this time, when the superposition state of | G> and | B> is measured, it will be found that the number of particles collapses on | B>. At this time, although the superposed states of | G> and | D> have not collapsed, we can know that the probability amplitudes are all above | D>, and then measure the superposed states of | G> and | D>. As a result, the number of particles collapses in | D> Up. So measuring the superposed state of | G> and | B> itself is a measurement that causes random collapse, but this measurement does not cause the superposed state to collapse for the superposed state of | G> and | D> (only very weak Changes), and at the same time can monitor the evolution of the superposition of | G> and | D>, which has become a weak measurement of the superposition of | G> and | D> ^[8] .

If this three-level system has only one particle, then when the number of particles collapsed on | B> is 1, the number of particles collapsed on | D> and | G> is zero. But this three-level system is artificially prepared with superconducting current, which is equivalent to having many electrons available. After some electrons collapse on | B>, there are still some electrons in the superposition of | D> and | G>. Therefore, the multi-particle system also guarantees that this weak measurement experiment can be performed. This is very similar to the cold atom experiment, that is, a large number of atoms have the same energy level system, and the probability of the superposition state can be reflected in the relative number of atoms ^[8] .

3. God still rolls the dice ^[8]

To summarize in a sentence, in this nature paper, experimental techniques are used to weakly measure a deterministic process, and to actively avoid the measurement of this process that can lead to random results. Everything conforms to the predictions of quantum mechanics, and the measurement of quantum mechanics is random. Sex has no effect. So Einstein didn't turn over, and God still rolled the dice ^[8] .

This nature paper just once again verified the correctness of quantum mechanics. Why did it cause such a big misunderstanding? I have to talk about it here. This has nothing to do with the wrong targets that the authors set in the abstract and introduction. It is estimated to make big news. They found the instantaneous idea of quantum transition proposed by Bohr in 1913 as a target, but this idea was put forward as early as 1925 by Heisenberg equation and Schrodinger equation in 1926 (that is, the formal establishment of quantum mechanics ) Was later denied, and they also stated clearly in the paper that experiments actually verified Schrödinger's view that transitions are continuously determined evolution. Poor's move was probably to create an effect that was opposed to Einstein, to continue the century debate, and to get more attention. But on the question of quantum transition, it was Bohr's earliest idea that was wrong. Heisenberg and Schrödinger were right. It didn't matter what Einstein did.

The author of this article's English report is Phillip Ball. Although he has written a lot of excellent scientific news, this time I probably encountered a blind spot of knowledge. Go and accompany Bohr to give instantaneous jumps (don't know the Heisenberg equation and Schrödinger equation are essentially equivalent?). Then the Chinese media translated it again, and the other media released it freely, and it became the "scene of accidents" in scientific communication. ^[8]

Since quantum technology is aimed at the second information revolution, its future applications will determine its value, and it should not be contaminated by the popularity of publishing top journals. Even if this is favored for a while, it will soon be buried in history ^[8] .