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

From Einstein's initial skepticism and Bell's experiments to the 2022 Nobel Prize, quantum entanglement has become the pillar of the physical mansion. So what is quantum entanglement and why it takes scientists decades to establish the existence of quantum entanglement? Knowable podcast host Adam Levy and guest physicist Nicholas Gissing answer the mystery.

Adam Levy: what does quantum physics tell us about the nature of reality? Can two particles at great distances influence each other? Is the universe really "ghostly"? I'm Adam Levy, and this is the magazine Knowable.

In this season's program, we will discuss how and when scientific ideas developed. In each episode, we explore a theme that has changed the way we think. In this episode, we focus on a topic that has actually existed in physics for about 80 years, which is actually a phrase, "ghostly teleaction".

Einstein used this impressive phrase in expressing his absurd emerging ideas of quantum physics. In this episode, we will explore what made Einstein feel too "ghostly" to accept. And a method proposed by Northern Ireland physicist John Stewart Bell that can eventually be used to test the universe and verify whether Einstein's suspicions are valid.

Before that, however, we need to discuss the changes that took place in the field of physics in the early 20th century. At that time, it became more and more clear that the universe did not work as physicists had previously described.

What is wave-particle duality? Classical physics describes the world with deterministic laws. These laws govern the way waves and particles, such as light and electrons, operate. However, these laws can not explain a large number of novel phenomena and experimental results at that time. Therefore, the classical view of the universe is gradually replaced by the view of quantum mechanics, which is used to describe the strange properties of microstructure.

Nicholas Gissing: at first, this is what people call wave-particle duality.

Adam Levi: this is Professor Nicholas Gissing of the University of Geneva. He devoted his life to the study of applied physics and quantum physics, and spent decades exploring the application of quantum physics in communication.

So, what is wave-particle duality?

Nicholas Gissing: suppose you have a particle that can be either an electron or an atom. All these particles sometimes behave like a particle, such as a billiard ball, but sometimes they behave like a wave. We can say that the wave-particle duality of particles really opens the prelude to all quantum science. Until the 1960s, or even later, wave-particle duality was still the main concern of scientific problems.

Adam Levy: the elements of the universe are neither waves nor particles, but they can have the characteristics of both in some way. This concept completely subverts physics. Even though this may have been a major concern, the strange properties of quantum physics pose much more than that.

Quantum physics predicts that particles can be entangled with each other. This means that multiple particles can converge and form a fundamental connection. Even if these particles are later separated far away, we will no longer be able to describe their properties independently. The observation of one particle can significantly affect the observation of another particle.

Nicholas Gissing: quantum entanglement means: now, if I measure the first particle or take some other action, the second particle will be affected in some way, or the second particle will "tremble".

Adam Levy: this view challenges the core of Einstein's idea that no information can travel faster than the speed of light in the universe, so no object can have an instantaneous effect on other objects that are separated from it. However, quantum physics believes that measuring a particle in an entangled state can have an immediate effect, or tremor, on other parts.

Nicholas Gissing: yes, so like many people at the time, Einstein realized that if you measure on one side, the other side will be affected in some way. We can give an example: one of the particles may be in Geneva, where I am sitting now, and the other may be in the United States as far away as the United States. Even so, the entanglement of these particles makes it impossible for you to describe them separately. Einstein just couldn't believe this, so he thought that quantum entanglement was impossible.

Adam Levy: in fact, in 1947, Einstein wrote a letter to Marx Bonn. In the letter, he jokingly called the non-locality we are talking about as "ghostly teleaction". This term has been used to this day. But at that time, did Einstein fight against these ideas alone, or did he actually have a positive discussion among physicists?

Nicholas Gissing: that's right. So I think Einstein invented a lot of accurate and creative expressions, such as "God doesn't roll the dice" and "ghostly teleaction". The "ghostly teleaction" does reflect the idea that touching particles in Geneva can cause particles in the United States to tremble. There is indeed such a super-distance effect. Einstein said: "this is too weird, it can not be true." this can only be ghostly. "

To my surprise, physicists did not pay more attention to this problem at that time. At that time, the vast majority of physicists paid no attention to these problems and issues. And there was no way to turn this theory into an experiment at that time. Some people even think that this is a purely philosophical problem with no physical results. We will soon find that they are completely wrong, even though these views are the mainstream at the time.

Bell tests Adam Levi: shortly after Einstein's death, John Stewart Bell proposed a possible test in 1964. Today, we call it the Bell test. What is the core of this test?

Nicholas Gissing: suppose you now have two particles that are maximally entangled and in a highly entangled state. For ease of explanation, you can think of them all as coins. In this way, you will get two possible measurements. When you flip these coins, you may get heads up or heads up. Now, if you flip these coins in the same way on both sides, you will always get heads up or tails up.

Adam Levy: this is the ghost predicted by quantum physics. Einstein could not believe this. Even if two particles in an entangled state are separated, they will always show the same measurement, because what you are measuring is their overall properties. It's like saying that someone assures you that when you toss two separate coins, you will get the same result. It seems that both particles instantly agreed to show positive measurements.

But there may also be a less ghostly explanation. For example, the particles may have reached a secret plan before they are separated, deciding which side to show up after being thrown. In this way, there will be no more quantum entanglement, just particles putting their plans into practice.

Since John Stuart Bell first proposed a method to verify the ghostly nature of quantum entanglement, a few experiments have proved the existence of quantum entanglement in half a century. Here is a picture of theoretical physicist Bell at CERN (1982).

Nicholas Gissing: and John Bell came up with a genius idea that changed the way you flip a coin a little bit. Now let's assume that one coin is still tossed in the usual way, while the other coin is thrown in a slightly different way.

Adam Levy: that is to say, we can measure these two particles in a slightly different way. In that case, will they still agree on a measurement with each other, for example, they all show face up?

John Stuart Bell is a theoretical physicist in particle physics. He took some time to study quantum theory and realized that this slightly different way of flipping a coin could finally solve the problem. Whether there is a real entanglement between particles, or whether Einstein denies whether the idea of quantum "ghosts" is correct. If Einstein is right, there is no quantum entanglement, but a secret plan has been reached between the particles in advance, then physicists will be able to repeat the coin toss experiment over and over again. Find the upper limit of the frequency at which two particles have the same measurement results.

Nicholas Gissing: but in quantum mechanics, you can violate this upper limit, which is what we call the Bell inequality.

Adam Levy: so if quantum physics is correct, the particles are really entangled together, and the "ghostly super-distance interaction" does exist, then in the process of repeating the experiment, the two particles are more likely to show that they are both positive or negative.

This is because these particles are indeed connected together, and they can have an instantaneous effect on the results of each other's coin flip experiments. In other words, the Bell experiment can show which view is correct: the entangled state of quantum prophecy, or Einstein and his suspicion of this grotesque behavior.

Nicholas Gissing: this is indeed the method proposed by Bell. It turned the whole discussion into a potential experiment.

Experimental verification of quantum entanglement Adam Levy: however, Bell himself did not complete the test he proposed. When on earth did someone actually conduct an experiment that clearly shows that particles are indeed entangled in such a ghostly way?

Nicholas Gissing: yes. So John Bell is a theorist who doesn't know how to finish the experiment. We had to wait another 10 years until the 1970s, when John Krause first got the experimental results that when the measurement methods were slightly different, the two particles had exactly the same results under high probability.

Adam Levy: John Krause completed the Bell test by manipulating calcium atoms to obtain two photons that are clearly in an entangled state. These photons, similar to flipping a coin, are measured in a slightly different way, which here means to measure the direction of polarization of light, that is, the wobble direction of light waves. Based on such experiments, Krause was able to observe the frequency at which two photons produce the same measurement. The experimental results do contradict Bell's inequality, which indicates that these photons are indeed entangled. Einstein's hypothesis seems to be wrong.

Nicholas Gissing: this is definitely an amazing result. But John Krause faces more than one problem.

First, on the other side of the coast of the United States, others carried out the experiment after John Krause, but he got a different result. Obviously, one of these two results is wrong, but how do you know which one is wrong?

In addition, in the 1970s, no one would take it as a serious physical problem. Because of this, no university is willing to promote John Krause to professor. His career was seriously hampered because he was the first person to complete the Bell test.

Then there was another experiment, but that was in the 1980s. So you will find that every major experiment takes another 10 years or so.

The experiment was carried out by Alan Aspe and his colleagues in Paris, France. Their experiments are better and of higher quality, though they take longer. To some extent, they solved this problem and proved the point of view of quantum mechanics. Since then, quantum entanglement is no longer theoretical, but has been established as a real feature of nature.

Adam Levy: these experiments finally proved the point of view of quantum mechanics and denied Einstein's suspicions, indicating that "ghostly teleaction" is a natural phenomenon. But I don't think it's over. At that time, there were still some unexplained areas that needed to be addressed, that is, what we called loopholes.

Instead of having an instantaneous impact on each other's measurements, particles may also have some hidden tricks that allow them to make plans in advance to deceive the test. Physicists call this possible particle deception a local variable. Can you describe the efforts made to eliminate these loopholes and determine that quantum entanglement is a true feature of nature?

Nicholas Gissing: in Alan Aspe's experiment, the main loophole comes from the process of generating photon pairs in the experiment.

Of course, you won't send them to Geneva or the United States, but you will launch them at both ends of a large laboratory, keeping them about 10 meters apart. Then you need to measure them.

However, in real experiments, when you make these measurements, you may not get any results, simply because the photons are lost on the way, or the detection results are limited by the efficiency of the detector itself. A photon is easily lost or undetected. So you can really think that it is these hypothetical local variables that determine when photons can be detected. You may be able to explain the results of the experiment, but everything is guided and driven by local variables.

Therefore, this point still needs further testing. Nearly 30 years after Alan Aspe, additional tests were conducted in 2015. It took us a long time to get a single photon detector with good enough performance.

Adam Levy: this test effectively uses a combination of photons and electrons to bypass the detection problem, allowing the measurement of pairs of entangled particles more than a kilometer apart. Since then, a number of other forms of Bell tests have emerged to fix other loopholes in quantum entanglement. These tests use almost everything from satellites to computer games, constantly pushing the limits of the field.

But is it too paranoid to say that physicists conduct these experiments simply because particles may somehow plot to deceive the results, leading us to think that they act like ghosts?

Nicholas Gissing: physicists are really paranoid. However, the absence of Bell inequality means that this characteristic of quantum theory is not only a feature of a certain theory, but also a feature of nature. So it changes the world view that physics shows us. This huge change is a conceptual revolution, so the local variables that can determine when photons can be detected, even if hypothetical, deserve our attention.

Therefore, I think in-depth research is really meaningful. Remember, physics is really just an experimental science. It's not enough to think about theory. You have to do some experiments. So I think this kind of work is very meaningful. But I also agree that almost no one, and almost no experimenter, believes he can falsify quantum mechanics. Although this kind of work is also meaningful.

What does quantum entanglement mean to our lives? Adam Levy: but is it all theoretical? In other words, can the understanding of the quantum world and the methods of completing these experiments bring some practical applications to our increasingly mundane lives?

In fact, as early as in Einstein's time, this was a purely philosophical problem. John Bell turned it into a feasible experiment, which was completed by John Krause for the first time. Alan Aspe was the first to complete the conclusive experiment. In the 1990s, people suddenly realized that this kind of abstract thinking does have a practical impact. Because in fact, if you can always, or almost always, get the same and random results from two particles, you will get randomness at a certain distance. The randomness of this non-locality is very close to an encryption key.

You can think of an encryption key as a password. What is a password? The password must be the same at both ends, such as you and Amazon or your bank, or some other institution, or anyone you want to communicate secretly. Note that the password must be random.

Through quantum entanglement, we have obtained the cipher with such characteristics. In addition, based on the theory of quantum entanglement, we can also know that if two particles are in a highly entangled state, they cannot be entangled with any other object. They cannot be entangled with a third particle.

Therefore, this ensures confidentiality. So, if you are entangled with the bank, you can get the same random password on both sides. And you can be sure that no one will ever have a copy of your password, which is exactly what you want. So in spirit, cryptography and Bell inequality are very close to each other.

But this is a complete revolution, and people suddenly realize that these strange quantum correlations, Bell inequalities, and the possible existence or absence of local variables are actually encryption keys. Therefore, they are very useful in the information society we live in.

Adam Levy: the 2022 Nobel Prize in Physics was awarded to the work to achieve the Bell test. What is the significance of awarding such an honor to the work?

Nicholas Gissing: I think the 2022 Nobel Prize in Physics is not only a recognition of the three winners (John Krause, Alan Aspe and Anton Zelinger), but also recognition of the whole field. This field, which has been neglected for decades, has finally gained the highest level of recognition. I must say that the Nobel Committee made a huge mistake in not awarding the Nobel Prize to John Bell. He should have won the prize, but he died too early, or the Nobel Committee acted too late, or for some other reason.

Therefore, I am personally very satisfied with this award. Of course it was awarded to three individuals, but on top of the three of them, the award did recognize an once controversial area. The best example is John Krause. He did a good job in the Bell test for the first time, but never got a part-time job in any university. Many of these jobs were ignored until he won the Nobel Prize 50 years later. So this is really amazing. I don't know if this often happens: an area has been neglected for such a long time, probably for decades.

When I started working in this field myself, I could only schedule my work after 9 p.m., and I couldn't make a living on it alone. But now the Nobel committee has recognized it. That's good.

Adam Levy: now, is the argument finally over? Can we point out with certainty today that the universe is indeed ghostly, without any loopholes, and that particles will not plot to deceive the Bell test?

Nicholas Gissing: yes, but I just wouldn't call it "ghostly". I don't think anything is ghostly, on the contrary, it's all very real. I mean, this has become a routine test in the lab, and you even have some students doing this kind of exercise in the lab. So this is a very solid fact.

Adam Levy: what do you think of the great progress we have made in understanding quantum behavior in the past few hundred years?

Nicholas Gissing: what I want to say is that over the past 30 years, it has been very useful for me to understand quantum behavior by knowing that these non-local quantum correlations can naturally generate encryption keys. You can see this change.

At first, people were talking about wave-particle duality, but now few people will talk about it. People are talking about quantum entanglement. Quantum entanglement has really changed people's minds and made quantum mechanics so popular. Today, I don't think there will be any articles, books or lectures in this field that will not mention quantum entanglement. Therefore, quantum entanglement is finally recognized as the essence of quantum mechanics.

Adam Levy: in fact, quantum entanglement, an incredible "teleaction" that Einstein thought, is now at the heart of many of our quantum technologies: whether it's the new encryption technology used to ensure the confidentiality and security of your communication with the bank, or the long-term exploration of building practical quantum computers. Today, physicists and journalists still often refer to quantum entanglement as a "quantum ghost".

It took physicists a long time to reflect on Einstein's complaints about quantum physics, and longer to prove him wrong, and finally pointed out that quantum entanglement is a basic way in which the universe works. even separated particles have some kind of synchronization. However, today's physicists are actively putting "ghostly teleaction" into practice.

Author: Adam Levy & Nicolas Gisin

Translation: wnkwef

Revision: seventeen

Original link: Quantum entanglement's long journey from 'spooky' to law of nature

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Adam Levy

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.