Shulou(Shulou.com)11/24 Report--

Niels Bohr, the head of the Copenhagen School of Quantum Mechanics, has said many times: "if quantum mechanics has not shocked you deeply, then you have not understood it."

Because Niels Bohr is far from daily life, many quantum phenomena are incredible: quantum transitions of electrons, uncertain measurement relations, electronic multiplets, Schrodinger cats, Wheeler cats, and so on. These strange phenomena are not only unknown to ordinary people, but sometimes even great scientists, such as Einstein and Erwin Schrodinger, one of the founders of quantum mechanics, have been "entangled". Not all of this is over, but there is another phenomenon that can be called "stranger in strangeness", which is "quantum entanglement", which Einstein was bitter about before his death because he did not accept it.

The word "entanglement" seems to imply "getting into some kind of predicament" or "trouble". The phenomenon of "quantum entanglement" has caused "big trouble" to the scientific community. In 1935, Schrodinger introduced quantum entanglement into physics for the first time in an article in the Annual report of the Cambridge philosophical Society. His behavior caused a lot of trouble. first of all, Einstein used this stubble to find trouble with quantum mechanics again. Originally, "quantum entanglement" (quantum entanglement) is an inevitable corollary in quantum mechanics, but Einstein thought it was a "ghost" breaking into physics, directly challenging his principle of "locality" and shaking his theory of relativity.

It is pointed out that the "locality" that Einstein did his best to defend in the period of EPR means that if any two things that are not in the same place want to communicate with each other by signal, the fastest information transmission can not exceed the speed of light. In other words, the two places cannot contact each other "in an open space and in an instant". For example, when two people are talking, the sound is transmitted through sound waves, and the transmission process of sound waves takes time. For example, when watching TV, picture information is transmitted through light waves. Even if light travels fastest, it takes time. Everything must abide by this "locality" constraint.

Most people will think that Einstein is right, and there is no doubt about the principle of "locality". It is the belief in "locality" that Einstein put "light travels at a limited speed" into his principle of relativity. If he shakes "locality", it is tantamount to shaking the foundation of the "building of relativity". In addition, Einstein firmly believed that everything must be restricted by causality, that is, the cause of the occurrence of things comes first, and the consequences follow. But in 1978 his colleague John Wheeler subverted causality completely through a hypothetical "delayed selection experiment". This hypothetical experiment has been verified by a number of experiments, and the "delayed selection experiment" proves that the singularity of quantum phenomena is boundless.

Although Einstein broke through Newton's classical view of time and space, and although he was the general ancestor of quantum theory, many subsequent developments in quantum mechanics inevitably clashed with his classical ideas. It was for conceptual reasons that Einstein got into the habit of laughing at Bohr. He challenged Bohr twice at the Solvay Physics conferences in 1927 and 1930, but both ended in his miscalculation.

After two defeats, Einstein knew that the opponent in Copenhagen was not easy to mess with, so he changed his strategy and stopped casually asking Bohr any nightmarish questions. He gathered two collaborators, Boris Podolski and Nathan Rosen, to write a paper that openly questioned "quantum entanglement". On May 15, 1935, this paper was published in the physical Review, entitled "is the description of physical reality in quantum mechanics complete?" The thesis, which comes from three authors, begins with the first letter of their surname, and becomes the famous EPR query in the history of quantum mechanics, also known as EPR paradox. One of the main targets of the query is "quantum entangled state".

For more than a decade after the Fifth Solvay Congress, Einstein has made Bohr feel uneasy. EPR's paper is like a heavy shell, forcing Bohr and his colleagues of the Copenhagen School to the point where there is no way to escape. EPR has a strong philosophical meaning, coupled with the topics involved are very strange and difficult, it is difficult for ordinary people to understand. But Bohr knew that the paper was aimed at "quantum entanglement".

EPR proposed an incredibly hypothetical experiment. They imagine that dividing a particle into "two halves", such as a photon splitting into a pair of positron and positron electrons, because they are not in motion at birth, the two electrons will always be in momentum canceling each other. The system cannot inexplicably generate extra momentum. When one electron flies out in one direction, another electron flies out in the opposite direction; when one electron spins clockwise, the other spins counterclockwise. This is how they are in an "entangled state".

This example tells us a very interesting thing. If you know the information of one particle, you can know the information of another particle immediately. For example, once you determine the position of one particle, you can immediately know the position of another particle; once you measure the momentum of one particle, you can immediately know the momentum of another particle, no matter how far away they are, even a few light-years away.

On the face of it, there seems to be nothing else here, but if you dig deep, there will be something strange. It should be noted here that, unlike the small balls seen in ordinary life, microscopic particles involve the "superposition of multiplets", that is, before we measure them, they are always in the superposition of various possible "possible states". Unless we measure it, it will immediately "collapse" to "a certain state", such as a certain value of momentum. The key question now is, when we measure the momentum of the first particle, how does the second particle "know" how its momentum should "collapse"? What made it jump to this state? Is it possible that the information it gets is transmitted "in an open space and in real time"? In this challenging EPR challenge, Einstein raised the question of "locality". It makes people either accept Einstein's view that there is a defect in quantum mechanics in Copenhagen, where quantum mechanics will open a gap and fall into crisis, or Einstein's "locality" will be disintegrated, which destroys the foundation of relativity.

In order to save the defeat, Schrodinger proposed a workaround. He believes that "entanglement" occurs only over very short distances, so that the time of light from one point to another is completely negligible compared with the measurement time that causes the particles to "collapse". However, his assumption still did not save the game. On the other hand, as for the Copenhagen side, there was no defense against EPR, and even Bohr himself finally had to admit that his refutation to Einstein's EPR theory was weak.

Neither side of the debate expected that within a few years, the quantum telemetry experiment had completely defeated Einstein, the struggle between the first and second generations of quantum mechanics came to an end, and the task of development in this field had fallen on the shoulders of the third generation.

Since the 1950s, physics has entered the "silver age" of development. Wheeler's "delayed selection experiment" and Bell's "inequality theorem" are equivalent to two "golden keys", which open the door to the experimental study of quantum mechanics. Since then, taking the "superposition state" and "entangled state" as the center, the new and most wonderful parts of the theoretical and experimental research of quantum mechanics have been raised. The third generation masters of quantum mechanics, such as Wheeler, Neumann, Feynman, Bohm, Bell, Aspe, Krausse and so on, have appeared one after another. their research results not only deepen and expand the interpretation of quantum mechanics theory in theory, but also broaden the scope of the discussion of quantum theory from the micro field to the large-scale universe, but also promote this research to the level of philosophy, from the discussion of "quantum reality" to "physical reality". As a result, it goes deep into people's understanding of a series of fundamental philosophical issues, such as "what the world is" and "what can be said to the world".

In the experiment, the study of quantum mechanics is pushed from the original "software" scale, that is, mathematical deduction, thinking, speculation and dialogue research, to the "hardware" scale of large-scale experimental research and technical application. The development after the event has proved that quantum mechanics is a precious gift given to the 21st century in the 20th century.

John John Bell was born in Belfast, Northern Ireland on July 28, 1928. Young Bell is very talented. he likes reading and knows almost everything like an encyclopedia. His classmates jokingly call him a "professor". Because of their poor family, other brothers and sisters dropped out of school early to make a living, but Bell was the longest educated. In the 1950s, Bell mastered quantum mechanics and particle physics through self-study, but in order to lighten the burden on his family, after graduating from Queen's University in Belfast, he had to find a low-paying job at the British Atomic Energy Research Institute.

The year 1960 marked a turning point in Bell's life, when he and his wife were hired by the European Center for Nuclear Research (CERN) in Geneva. Here, Bell works in particle physics, doing his share of work during the day, sometimes working late, returning home, and spending time on his favorite quantum theory in the dead of night. especially the theoretical study of quantum "multiplet" and "entangled state" that he is infatuated with. He did not expect that it was this hobby that made him famous.

Bell often takes advantage of his annual leave to visit Stanford University, the University of Illinois and the University of Brandys with questions in his study. In 1964, he published a paper entitled "the study of EPR paradox" (On the Einstein Podolsky Rosen paradox) in the journal Physics. In this paper, Bell first assumes that Einstein's "locality" principle holds, and then deduces an important result from the conditions proposed by EPR, which is the famous Bell inequality.

Bell inequality is equivalent to giving a criterion. If any experiment proves that this inequality is true, it means that Einstein's "locality" is valid. In this paper, Bell also proposed a hypothetical experiment to prove the entanglement phenomenon by the polarization of photons. Bell established a realistic research direction for the mysterious phenomena of quantum entanglement, which jumped into the forefront of physics and became a key figure in the study of quantum entanglement.

Since Bell, many physicists have tried to prove the Bell inequality by using various experiments, but all the experimental results completed so far violate the Bell inequality, so they all prove that the "locality" repeatedly emphasized by Einstein does not exist. these experiments promote the great success in the study of quantum entangled states, among which the work of Alan Aspe is worth mentioning. His achievements are not only the best of many studies, but also reveal more profound problems and have a more far-reaching impact.

Alan Aspe in Europe, Aspe is recognized by the physics circles as the most in-depth understanding of Bell theory. Not only did he successfully realize Wheeler's delayed selection experiment, but his research group also made the most outstanding contribution to the experimental study of quantum entangled states.

Aspe carefully studied every step of EPR's hypothetical experiment. He believes that if an experiment can be made to prevent the "real-time signal" connection between the two entangled particles, but the experimental results show that the two entangled particles are still connected, it proves that Einstein's "locality" is no longer valid. Aspe was 27 years old when he thought about it and was a graduate student at Orsay at the University of Paris. He decided to write his doctoral thesis on the basis of this experiment. In order to realize this dream, he personally went to Geneva to find Bell. Bell asked him, "do you have a permanent job now?" When Dambert learned that the young man didn't even get his degree, he couldn't help worrying about him. This experiment is not only extremely difficult, but also requires great sensitivity, and the design idea of the experiment can not have any defects. He said to Aspe, "you must be a very courageous young man!" After saying this, Bell swallowed back what he was going to say, and he wanted to warn Aspe that it was bound to ruin his future.

The experiment designed by Aspe is indeed very beautiful, but it is extremely difficult and risky to implement. In order to have a long enough experimental passage, he and his accomplices chose to conduct it in the basement of the University of Paris. It took much longer than expected to design and clean up the experimental site to prepare the equipment. After selecting lasers, computers, polarizers and light signal sensitive switches, they debugged again and again, and did not start formal experiments until the beginning of 1982, and finally achieved success on June 22, 1982.

The experimental device, as shown in the picture, Aspe uses a calcium light source S. After the calcium atom is excited to a very high energy state, when it returns to a low energy state for a single transition, a pair of optically moving photons are released at the same time, and the two photons show polarization-dependent entangled states. At the same time, they try to make the polarization direction of the two polarizer P also show some correlation, if the two photons really show polarization correlation, when the photons traveling in one direction pass through the polarizer, the photons in the other direction must also pass through the other polarizer at the same time, that is to say, the two photons either pass through or fail at all. In order to prevent the signal connection between the two photons, it is necessary to increase the distance between the two polarizers P, which is the key point of the experiment. In the underground laboratory of the University of Paris, they increased the distance between the polarizer P and the light source S to 6 meters. At the moment the two photons reached the polarizer respectively, the fastest light signal needed at least 40 nanoseconds, or 1/4000000000 seconds. However, when the distance between them is more than 12 meters, even if they use the light signal, it is impossible to contact within this time. In order to control the incidence of a single photon, another switch has to be installed to allow only one photon to enter at a time interval.

In order to verify Einstein's challenge to the "non-locality" of the theory of quantum mechanics, they increased the distance between the two polarizers to 13 meters. In addition, they used a quantum switch controlled by an acousto-optic modulator. The polarizer can be moved in or out quickly, and the time interval happens to be the 13 meters of light transmission, that is, 43 nanoseconds. Aspe designed the quantum switching time to be between 6.7 and 13.3 nanoseconds, eliminating the possibility that the two photons are related to each other before entering the switch.

Then, in order to further eliminate the systematic error of the experiment, they improved it and adopted the photon dual-channel scheme, so that the photon first passed through a gate K, and then entered the polarizer, which could change the direction of the photon and guide it in two different directions. Finally, the measurement data of the four channels are collected into the monitor D for rechecking. The results of the experiment were published in the December 20 issue of the physical Review KuaiBao.

The ingenuity of the experimental design idea and the excellent cooperation of the equipment in the Aspe experimental group were admired by all the people in the same industry. Finally, his results confirm with very high accuracy that the correlation between the states of two entangled particles does not take time, and the law of quantum mechanics is "non-local", which is amazing. In 1983, at Aspe's doctoral thesis defense meeting, Bell personally came to inspect Aspe's experiment and praised his achievement, calling it "the best thesis at the level of the Nobel Prize in Physics".

This success of Aspe's research group is of special significance. Not only does it unquestionably confirm the "non-locality" of quantum mechanics, but the experiment itself is also groundbreaking, making the University of Paris the center of the experimental field of quantum mechanics for more than a decade. Years later, when Aspe recalled the series of experiments they had done in the 1980s, he said proudly: "at that time, what we did meant by these things was not known. But our work has attracted people's attention, which is something we are proud of."

Inspired by the success of the Aspe experiment, entangled state experiments around the world have sprung up since the 1980s, which lays a solid foundation for the basic research of quantum mechanics.

Source: 365th days in the History of Science author: Wei Fengwen Wu Yi some of the pictures are copyrighted by the original author. Editor: Zhang Runxin this article comes from Wechat official account: Origin Reading (ID:tupydread), author: Wei Fengwen Wu Yi

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