The 2022 Nobel Prize in Physics was awarded to three scientists, Clauze of the United States, Aspe of France, and Salinger of Austria, on the grounds that "because the experiments made with entangled photons confirmed the violation of the Bell inequality and pioneered quantum inf

022 Nobel Prize in Physics was awarded to three scientists, John F. Clauser of the United States, Alain Aspect of France, and Anton Zeilinger of Austria, on the grounds that "because of experiments with entangled photons, the violation of Bell inequality was confirmed and the creation of quantum information science was created." Ordinary readers may be confused. What is this saying? But at the same time, the media widely reported that Salinger is the doctoral supervisor of the famous Chinese quantum information scientist Pan Jianwei , which everyone can understand. Then everyone will ask, how much has China contributed to quantum information? Will Chinese scientists receive the Nobel Prize for in the future? Here, I will briefly introduce it to you.

022 Nobel Prize in Physics

(https://www.nobelprize.org/prizes/physics/2022/summary/)

First of all, there is a most basic word that is "quantum". This is because there is a physics theory called quantum mechanics , which is one of the two basic theories of physics today, paralleling the theory of relativity. Modern technologies such as semiconductors, lasers, light emitting diodes, satellite navigation system , etc. are all achievements of quantum mechanics. It can even be said that all electrical appliances use quantum mechanics because conductivity must be explained by quantum mechanics.

Understanding conductivity, it is necessary to use the energy band theory derived from quantum mechanics

After quantum mechanics emerged, people called the theory in daily life, namely Newtonian mechanics also called classical mechanics. So now quantum and classics are often used as a pair of adjectives, such as quantum computer to classical computers.

The importance of quantum mechanics is that it is the essential theory that describes the microscopic world of . When we go deep into molecules, atoms, and smaller scales, we find that classical mechanics is always wrong and quantum mechanics is always right. When describing the macroscopic world, quantum mechanics will be simplified to classical mechanics, so where classical mechanics are correct, quantum mechanics is always correct. This is the relationship between the two.

The results of this Nobel Prize are ultimately solved by solving a basic worldview problem: Does a physical quantity necessarily have a definite value before measuring?

At first glance, this is not a problem at all. If you want to measure a physical quantity, of course there is a certain value waiting for you to measure! Is this still necessary to ask? This is the classic worldview. Just like a famous poem: "If you see, or don't see me, I'm there."

"The Silence of Banzaguru Baima"

(Many people think this poem was written by Tsangyang Gyatso, but it is not. It was written by contemporary female poet Tashiram Duoduo , i.e. Tan Xiaojing)

However, the worldview of quantum mechanics is not like this. Quantum mechanics clearly tells you that a physical quantity does not necessarily have a definite value before measurement, depending on the measurement settings. There is indeed a definite value for certain states and certain physical quantities. In this case, measuring this quantity will inevitably result in this value, which is the same as in the classical situation. However, in some other states, measuring this amount will not result in a definite result, but will probabilistically obtain one of several possible results. Quantum mechanics can predict what these possible outcomes are and the probability of their occurrence, but cannot predict the results of a single measurement.

For example, quantum mechanics may tell you that the probability of taking 0 and 1 in a certain property of a certain system is half. What this means is to prepare many of the same systems to measure this property, for example, if you measure 10,000 times, you will get 0 or 0 or 1 or 5,000 times. At this level, quantum mechanics predicts very accurately. But if you only do the measurement once, what will you get this time? Quantum mechanics will tell you that it is impossible to predict, and the only thing we can predict is probability.

Marius's Law : The probability of a polarized photon passing through a polarizing plate with an angle of θ to its polarization direction is cos2θ, that is, it must pass when θ = 0°, it must pass when θ = 90°, and half the probability when θ = 45°, and half the probability cannot pass when

Therefore, quantum mechanics has an amazing philosophical consequence: different results can be obtained for the same reason. In other words, there is real randomness in the world. And in the classic world, the same reason must be the same result. The randomness you see in normal times, such as tossing a coin, is actually pseudo-random because you don’t know enough about the initial conditions. This is an essential difference between the two.

The most typical system that expresses this difference is " entangled state ", which is a state of two or more particles. Many people may have heard of this concept and heard of the metaphor: There are two balls, one black and one white, and send two balls to two people respectively. If you open the box and find that you are receiving a black ball, then you immediately know that the other person is receiving a white ball. Many people may think that this is quantum entanglement.

Quantum entanglement

But in fact, this metaphor similar to opening a blind box misses a point. In this metaphor, whether the ball is black or white has long been determined, which is a classic worldview. But in quantum entanglement, the measurement results are uncertain! The Nobel Prize Committee drew a cartoon for this to express the difference between the two worldviews. In quantum entanglement, the ball is neither black nor white before measurement, and it can be considered gray. It is not until the moment you measure that the gray ball suddenly becomes a black ball or a white ball, and the probability of both is half. At the same time, you will know the color of the other ball, and your measurements will determine the color of the two balls at the same time. This is quantum entanglement, it is much more wonderful than opening a blind box.

Example of "black ball and white ball" used in the Nobel Committee introduction

(https://www.nobelprize.org/prizes/physics/2022/press-release/)

Up to this point, you may find that although these are interesting, they cannot be verified experimentally. Regardless of whether the ball has a certain color before measurement, isn’t it certain after measurement? What difference can this tell?

In fact, quantum entanglement was proposed by Einstein and his two assistants, Boris Podolsky and Nathan Rosen, in 1934, with the goal of refuting quantum mechanics because they thought it was ridiculous. Another great scientist, Bohr (Niels Henrik David Bohr), had a long debate with Einstein for this. But in the eyes of most scientists, this is just a philosophical debate between space and therefore does not pay attention to it.

Bol and Einstein

However, in the 1960s, Northern Ireland scientist John Stewart Bell pointed out a shocking thing: this difference is actually testable! He proposed an inequality, adding and decreasing the probability of various possible measurement results. As long as the quantities to be measured have a certain value before the measurement, the results of this combination of these probabilities must have an upper limit, which is the Bell’s inequality. But there are certain quantum entangled states, making the combination of these probabilities exceed this upper limit. Therefore, people can do experiments to test the Bell inequality. If you find that this inequality is not true, it means that the classic worldview is wrong, and the quantum worldview is correct.

Bel and Bell inequality

The three winners this time have done the work of testing Bell inequality. In 1972, Crowze and Stuart Freedman did such an experiment for the first time, and found that quantum mechanics was correct in violation of the Bell inequality. But their experiments are still relatively rough. People can say that if nature is determined to cheat, there are loopholes to be exploited.For example, their two detectors are relatively close, only 3 meters away (https://news.berkeley.edu/2022/10/04/physics-nobel-recognizes-berkeley-experiment-on-spooky-action-at-a-distance). In principle, it is feasible if someone cheats at the speed of light between these two detectors. Then Aspe and others improved the experiment in 1982 and plugged some loopholes. Since then, Salinger and others have plugged more loopholes and further confirmed that the quantum mechanics is correct.

Friedman and his and Crowze test the experimental device for Bell's inequality

By the way, Friedman is my colleague and the doctoral supervisor of Professor Lu Zhengtian , the dean of the Youth Class School of the University of Science and Technology. He had hope of winning the Nobel Prize, but died unexpectedly in 2012 and failed to wait until today. In fact, he was born in 1944, two years younger than Clauze. When the two worked together, Friedman was a doctoral student and Crowze was a postdoctoral student. In the past, many people and I only knew that Aspe did the experiment to test Bell's inequality in the early 1980s, but I don't know that Friedman and Crowze did it in the 1970s. They were the earliest ones. This was the information Lu Zhengtian told me last year. Unexpectedly, a year later, the Nobel Prize was awarded to their jobs. It’s time to feel relieved that Friedman knows it under the spring!

The junior class college of the University of Science and Technology of China, the tallest on the left is Dean Professor Lu Zhengtian (https://sgy.ustc.edu.cn/administration)

Up to this point, many people may think that these achievements are only very important at the world perspective, but they seem to have nothing to do with real life. However, in fact, this change in the worldview will bring about a lot of real technology. This is the quantum information mentioned in the reason for the Nobel Prize award. In fact, the quantum technology reported by the media now refers to quantum information. It is a new discipline that emerged in the grafting of quantum mechanics and information science since the 1980s, and is expected to achieve results that cannot be achieved by many traditional information technologies. For example, teleportation.

Yes, teleportation is now a real technology, and its professional name is quantum teleportation.

Telection technique in "Star Trek "

However, we need to explain immediately that we cannot transmit a person yet, what can transmit is a particle. Specifically, it uses a pair of entangled particles 1 and 2 to transmit the unknown state of particle No. 3 to particle No. 2, and the initial state of particle No. 3 changes.

Quantum stealth transmission principle

It’s like there is a car on one side and a pile of car parts on the other side. The experimental result is that the state of the car is transmitted over and the car parts are assembled into a car, but it turns out that the car has disintegrated. So this is a teleportation of the state rather than a copy, and you will never get two identical cars. These principles are introduced in detail in my popular science book "A Brief Introduction to Quantum Information", and it is said that the most science fiction-rich among all quantum information technologies is probably quantum teleportation.

"A Brief Introduction to Quantum Information"

Theoretical plan for quantum teleportation was proposed in 1993. In 1997, Salinger's research group realized it for the first time, among which Pan Jianwei was the second author. In the introduction materials of the Nobel Prize website, many new achievements in quantum information were also mentioned, such as the "Mozi" satellite realizes quantum confidential communication between heaven and earth (spanning 4,600 kilometers, China's quantum communication is leading the world. How long will it take for Europe and the United States to catch up? | Yuan Lanfeng ), and the entanglement pair sent by "Mozi" realizes quantum stealth transmission between two ground stations 1,200 kilometers apart (the experiment that scientists all over the world want to do, Chinese scientists succeed first | Science and Technology Yuan Ren). These are the achievements of Chinese scientists.

quantum information is one of the most vigorous developments in all scientific fields. It is divided into three parts: quantum communication, quantum computing and quantum precision measurement. It is expected to achieve unconditional and secure confidential transmission, unprecedented computing power, unprecedented detection accuracy, etc.Therefore, quantum information is also called the second quantum revolution, which is contrary to the first quantum revolution that brought semiconductors, lasers and other technologies. Currently, China is at the forefront of the world in quantum communication, and is ranked first in quantum computing with the United States. There is still a big gap in quantum precision measurement, but it has made rapid progress.

Three branches of quantum information

Therefore, many Chinese scientists have contributed to the results of this Nobel Prize. If we achieve greater results in the future, such as building a global quantum communication network, or creating a quantum computer with practical value, or using quantum precision measurement technology to discover dark matter , it is entirely possible to win the Nobel Prize. Of course, the value of these achievements is far more than the Nobel Prize. Maybe, you will make a key contribution.

■ Extended reading

"Yuan Lanfeng: China's quantum mechanics later development depends on the future" (https://www.guancha.cn/YuanLanFeng/2022_10_09_661224.shtml)

spans 4,600 kilometers, China's quantum communications are leading the world. How long will it take for Europe and the United States to catch up? | Yuan Lanfeng

The experiment that scientists around the world want to do, Chinese scientists succeeded first | Science and Technology Yuanren

Nobel Prize in Physics: Why quantum information? | Interview

022 Nobel Prize in Physics Correct Solution: Principles and Experiments of Quantum Entanglement and Bell Inequality | Han Feng

■ Introduction to the author

Yuan Lanfeng

Deputy Director of the Science Communication Research Center of the Chinese Academy of Sciences

Deputy Director of the Department of Science and Technology Communication of the University of Science and Technology

Associate Researcher of the National Research Center of Hefei Microscale Material Science, University of Science and Technology of China

President of the Society of Science and Technology of China

President of the Society of Science and Technology of China

Voice of the Storm

Science · Patriotic · Value