量子糾纏－愛因斯坦的最終謎團

Professor Aspé, Dr. Klauser and Professor Seilinger, you have been awarded the 2022 Nobel Prize in Physics. There was an unusual amount of attention on the ceremony for the 2022 Nobel Prize in Physics. The occasion honored the verification of a phenomenon that Albert Einstein once famously dismissed as spooky action at a distance. Quantum entanglement. It's a mysterious phenomenon apparent on a subatomic scale. Quantum particles become entangled and remain synchronized no matter what. Even if they are at opposite ends of the universe, the particles behave in exactly the same way. It's as if they are connected by a telepathic link. Scientists say quantum entanglement suggests the universe is fundamentally unknowable, beholden to countless threads of fate. As a physicist, can you say you understand free quantum entanglement? I do not. I can say quite the opposite. No, I do not understand quantum entanglement. And that is, you know, it's shaking all our understanding of what space is. Is quantum entanglement real? Or is the phenomenon a misunderstanding? This conundrum befuddled many of the 20th century's greatest minds. Some who pursued it saw their lives ruined. When you're really thinking deeply, you begin to believe all sorts of crazy things. Whatever I speculate will probably be wrong, because nature has much more fantasy than we have. This is the story of the 100-year search for quantum entanglement. But that he plays dice and uses telepathic methods is something that I cannot believe for a single moment. The mystery started with an issue identified by Einstein, that epic defining genius. He was deeply skeptical of a certain theory sweeping the world of science at the time. Quantum mechanics is at best an incomplete theory, and at worst, completely wrong. Quantum mechanics had been presented as an explanation to the nature of things in the subatomic world. For example, place a screen with two slits in front of a wall and fire tiny particles at it. The traces they leave on the wall won't match the shapes of the slits. Instead, they scatter randomly and eventually create a striped pattern. Here's how quantum mechanics explains this phenomenon. When no one is watching, the particles act like waves. They pass through the slits as waves and turn back into particles only upon reaching the wall. Wait, particles are waves? Well, it's a little complicated, but physicists call a physical entity with this odd property a quantum. Scientists didn't understand quantum mechanics, but they used it to make calculations that conveniently explained a variety of phenomena seen on the subatomic level. The theory quickly came to be relied upon by all the leading physicists around the world. Quantum mechanics may seem difficult to understand, but a traditional children's game might be able to provide a visual explanation. In Red Light, Green Light, players aren't allowed to move while the caller is watching. But when his back is turned, they can move about freely. Now, let's play this game on the quantum scale. The moment the caller covers his eyes, the player's positions become impossible to follow. They replicate all around the playground. This is what physicists mean when they refer to something as a wave. The players may or may not be anywhere. They are in a state of what's called superposition. But the moment the caller opens his eyes, they stop replicating. The players stop in random locations. This is what Einstein could not accept. Take billiards. The balls don't turn into waves when we aren't looking. Asked whether I really believe that the moon exists only when I look at it. The year is 1935. Einstein publishes a paper highlighting major flaws in quantum mechanics. In it, he presents a thought experiment that takes place in extreme conditions. It could be set at the farthest corners of the universe. But this scale is a little too large to grasp. So let's return to Red Light, Green Light. But this time, we'll mix in the two-slit experiment. Let's call this new version the which-way game. Here are our main characters. The twisties act just like quantum particles. And there's also a watcher. He stands in front of a gate with two doors and keeps track of which side the twisties enter. The twisties randomly choose which side to go through, left or right. And the watcher notes which side they go through at the moment of their passage. Suppose a group of twisties enters this gate at one end of the universe, while another group enters a separate gate at the other end. Because the twisties act randomly, sometimes they pass through the same side. Other times, they don't. But something strange happens when their movements are calculated using quantum mechanics. Quantum particles behave like waves. When waves collide, they mix and become indistinguishable. That's what happens when the blue and yellow twisties collide. They also mix and become indistinguishable. Then, they each head toward the gates at opposite ends of the universe. Their movements are supposed to be random. But quantum calculations show they pass through the same side at the other gate. In other words, the moment a twisty is observed going through the right side of a gate at one end of the universe, its twin will then have to go through the right side at the other gate 100% of the time. Picture it like this. In quantum entanglement, the results are always synchronized no matter where you are. It's like telepathy. But how is it possible? Einstein wasn't convinced. He didn't believe such unexplainable synchronization could exist. This was how quantum entanglement, the great scientific mystery of the 20th century, was born. I think quantum entanglement is one of the strangest, strangest notions in modern physics. That's why people have dug in generation after generation to try to make sense of it. The 1930s. World War II breaks out. Science and technology are increasingly used for military purposes. Quantum mechanics are applied in the development of everything from nuclear weapons to transistors. Researchers who know how to use it are celebrated. They win the Nobel Prize almost every year. Einstein's objection is irrelevant. Questioning the nature of things has become an inconvenience. But in 1952, one gifted scientist reminds the world. Ah, here's a picture of the young David Bohm. When he was probably in Princeton. David Bohm. He studied under J. Robert Oppenheimer, the father of the atomic bomb. With a recommendation from his mentor in hand, Bohm was appointed an assistant professor at Princeton University at the age of 29. It was while teaching there that Bohm started to feel the same uneasiness about quantum mechanics as Einstein. Essentially, they ignore the problem. They say that the main purpose of physics is to calculate results, you see. What they say is that when students first take quantum mechanics, they find they can't understand it. Bohm found a new approach to the wave equation used for quantum mechanical calculations. He discovered it could be rewritten in this formula. The first line was the equation for motion, a familiar tool for physicists ever since Newton introduced it over 300 years ago. But the second line represented a new force that had never been seen before. It affected quantum particles and could be felt across the entire universe in an instant. This new force helped explain the mystery of quantum entanglement. Let's go back to the which-way game from earlier. Bohm's new force would influence the twisties in the same way no matter where they are in the universe. They are affected by the same force, so they move in the same direction, left or right. This allows the twisties to be synchronized at all times, even if the blue and yellow ones aren't mixed together. Bohm believed this explained the mystery of quantum entanglement. But the theory was met with fierce criticism from his peers. Even his own mentor, Oppenheimer, rejected it. We consider it juvenile deviationism. Why was the response so harsh? Bohm's theory assumed the existence of a force that had yet to be discovered. The problem was no method could prove or disprove its existence. It could not be tested experimentally. His idea was dismissed as an armchair theory. Bohm fell into despair and eventually left Princeton. He bounced around universities in Brazil and Israel. He tried to maintain his belief in the existence of the new force, something that could be felt throughout the whole universe. But gradually his ideas changed. I began to feel that something beyond science would be needed. I think that my own interest in science, and many of them interest in science, is not entirely separate from what is behind the interest in religion or in philosophy. That is to understand the whole of the universe, the whole of matter, you know, how we originate and so on. He grew increasingly drawn to mysticism and the paranormal. It was something to strive towards. And I have a feeling that it didn't do David Bohm much good at the end of life, because there was still the conflict. And I think he felt, you know, as his energy was dissipating, he felt... I think he felt sad, I tried to... Bohm became a cautionary tale. Physicists started to believe that pursuing quantum entanglement could cost them their careers. They distanced themselves from the topic. But in the end, it was Bohm's theory that brought about a revolution. For me, they were a big thing. Bohm said, yes, by example. Here is an example of doing exactly that. I was... it had so many queer features. But it swept away the impossibility of corruption. It reopened the question. And that amazed him. Yeah, yeah, so this is a poster of my hero, John Bell. I think, you know, in my hierarchy he stands higher than even Einstein and Newton. And I just wanted to show you the two heroes that I have portraits of. One is Marie Curie, and the other one is John Bell. CERN, or the European Organization for Nuclear Research. It's a global hub of quantum study, and known for the discovery of the elusive Higgs boson in 2012. This is where Bell conducted his research. He worked alone, and in secret, to unravel the mystery of quantum entanglement. I still remember this opening sentence. So John Bell started by saying, I'm a quantum engineer, but on Sundays I have principles. Bell was deeply disturbed by the attitude of physicists toward Bohm. Why hasn't there been, for all entirely and so on, bursts of dogmatic value or some kind of devised openings under Bohm? And the extraordinary change that occurred with Bohm's paper. So, my argument is that Einstein's argument is absolutely correct. Bell had been transfixed by Einstein's ideas since his youth. On his days off, he worked to resolve his idol's fundamental question on quantum entanglement. He looked for ways to prove or disprove the theory. Quantum entanglement is a phenomenon in which particles synchronize without any reason. Remember, the blue and yellow twisties mix together when quantum entanglement occurs. But Bell thought of a way to synchronize their actions without mixing them together. What if the two twisties already had instructions about which gates to enter before they set off? If they followed the instructions, their movements would be synchronized no matter what. If he could prove the existence of such instructions, Bell could disprove quantum entanglement. But how could he confirm the presence, or absence, of such instructions? Bell came up with an observation method. There must be at this interface between the system and the apparatus. So then I could find the possibility. The idea can be explained by a new setup to the which-way game. First, we increase the number of gates from 1 to 3. We name them A, B, and C, and give each gate-watcher the power to randomly switch their positions. Then we distribute instructions to the twisties, who move in pairs. Each instruction tells them which gate, A, B, or C, and which side, left or right, to go through. The two twisties follow the instructions. Why did Bell use this setup? Because he knew it would allow him to create a formula that shows whether quantum entanglement exists. Bell's inequality would later lead to a revolution in quantum mechanics. This is what happens when the formula is applied to the game. The gate-watcher keeps track of the number of twisties that pass through each side. Let's give it a try. It goes like this. No matter what instructions are given to the twisties, the inequality holds. But that's not the case when quantum entanglement is assumed to be real, and the twisties are mixed together. The characters move in sync for no reason. Only the left side of the equation grows, and the inequality doesn't hold. In short, the formula makes it possible to determine if quantum entanglement is real or not, simply by counting the number of times the twisties pass through each gate. Bell had come up with a method to test the validity of the famous armchair theory. In my opinion, the situation which is revealed or concealed by quantum mechanics is a strange one that we have not really digested. Most theoretical physicists don't worry about this. It's a little bit like riding a bicycle. You know, you can ride a bicycle without knowing very much how it's done, and without being able to tell anybody else how to do it. And I think that most theoretical physicists use quantum mechanics rather in the same way as they ride a bicycle. But I also think it's important that a number of people should be thinking about what it all means. That the future of physics will be increasingly unintelligible. And this I don't believe. And I don't believe there are good arguments for taking that position. The result of his weekend research came to be known as Bell's inequality. He was not a known name as an experimental physicist. And so he published his findings in an obscure journal. Was any attention paid to the paper in particular? Not at all. I think it took until four or five years later. And then I got the letter from John Clauser. I encountered, just sitting on the shelf at the Goddard Institute for Space Studies where I was writing my PhD thesis, I found John Bell's paper in a very obscure journal. I think it only published one issue and immediately went out of business. And I read it and I was just overwhelmed. It's a sort of a mess. What all would you like? Let's see. I got, this is an experiment here that I was building to phase contrast imaging with soft x-rays. For the primary purpose is for medical soft tissue imaging, mammography. Something like a new repair built. All of this stuff. All of this hardware here I built. John Clauser has brought a DIY attitude to experiments since he was a young student. Quantum mechanics was very mysterious. I just simply could not understand it. And I couldn't understand why I couldn't understand it. Clauser failed his quantum physics class twice. He decided he needed to review the basics and started reading up in the library. That's where he came across Bell's paper. Clauser began suspecting that quantum entanglement wasn't real as Einstein had said all those years ago. He decided to conduct experiments to prove it. I also went around to various faculty members at Columbia and they all said, oh that's silly, nobody would want to do that. But I realized that this is a very important experiment to do and I wanted to do it. Clauser wrote to Bell, who became a source of encouragement for him. No, I don't know of any experiments and the experiment that you suggest is very much worth doing. He was a very charming and delightful guy. Clauser ignored his teacher's advice and negotiated with other universities. One agreed to host his experiment. He came up with this setup. The center of the device heats calcium atoms and produces two light particles called photons. Clauser fired them in opposite directions and watched their movements. His observation equipment included a device that switched the path of the photons. In this experiment, the photons are the twisties and the observation equipment is the gatekeeper. But this was just the beginning of a long journey. The project had little funding. Clauser had to look through the garbage at various labs almost every day to gather the materials for his experiment. Photomultiplier tubes. They detect photons. I used a bunch of them. To get the hardware. Couldn't have done the experiment without it. Either had to make it or find it or borrow it or steal it or whatever to do the experiment. And so, little of all of those. He designed everything from the electronic circuit to the photon generator and built it all from scratch. It took him two years, but he finally completed the setup that would make it possible to conduct the world's first experiment on quantum entanglement. And his results went against Bell's inequality. Clauser showed the world that quantum entanglement, famously denied by Einstein, could exist. I was disappointed. I mean, I was a great fan of Einstein. I was very disappointed that I did not, that I had proven him wrong. But his findings didn't lead to an improvement in Clauser's career fortunes. The experiment was dismissed as flawed and unimportant. So I hung around there until 76, got there in 69. Got a war out by welcome. Everybody said, well, you got all the results that everybody expected. They haven't done anything important. You need to look for a job elsewhere. So I left there. In the end, Clauser moved on to a completely different field. He didn't get a chance to continue his studies. Momentum was once again stopped, just when it seemed as if the mystery of quantum entanglement was about to be unraveled. But then, a major development. It is 1976. A small meeting on quantum entanglement is held in Sicily. Alain Aspect is the organizer. He quietly invites a handful of researchers who are interested in the topic. They include Clauser and another young scientist. Alain Aspect. He will go on to win the Nobel Prize in Physics alongside Clauser. He first explained to me that everybody would criticize the subject, that this subject has no interest, and that he would not advise me to work on this subject unless I had a permanent position. And I had a permanent position. So I told him that I had a permanent position. And at that point, he said to me, so we can talk about science. Aspect tried to improve Clauser's experiment. After countless rounds of trial and error, he identified a loophole. Let's once again explain it in terms of the which-way game. The loophole would allow the twisties to notice when the watcher has switched the gate. They would then secretly agree to make the same movements. Aspect came up with a way to fix this. You have to change something on this side fast enough in order to change it before any information has been transmitted to the other side of the experiment. If I change something in less than 40 nanoseconds, then what I do here will not be known by the apparatus on the other side. If the watcher changes the gate at an extreme speed, even if the twisties plan together where they will enter, the final positions will have changed before they arrive. But was there a device that made such super-fast changes possible? There wasn't. So how did Aspect solve the problem? Aspect used water. He created a device that uses ultrasound to make the water quiver and cause a diffused reflection. It works just like glasses cleaners. The device made it possible to randomly allocate light into either of the two routes. It created something like a pop quiz for the particles. In 1982, Aspect releases the results of the experiment. They go against Bell's inequality, once again proving that quantum entanglement is real. I explained to everyone who wanted to listen to me why Bell's theorem was interesting. Gradually, people said, yes, it's true, it's interesting. Fifty years after Einstein raised his initial doubts, physicists finally start to take quantum entanglement seriously. The results suggest that telepathy-like synchronized action between objects at opposite ends of the universe, which Einstein doubted, is possible. Does this mean that objects don't exist until they are observed, as assumed in quantum mechanics? Physicists named the Erie synchronization a non-locality and began to consider it a serious issue. Bell continued to search for the reason for the non-locality. He died of a stroke in 1990 at the age of 62. He always said, I do not understand this non-locality. It's a mystery to me. It's a mystery to me. For a physicist, if you have the feeling there is something in it, which I don't understand, but I want to discover this. I want to know it. This is an internal feeling, you see. A new era was dawning. This is a quantum computer. By now, the set of possible applications has expanded really dramatically. So then I thought that this definition actually is equivalent to the definition of an eavesdropping. Developing the qualities as a test for eavesdropping. Even in the world's largest IT companies, we are trying to create a revolution. 2010, 30 years after the experiments by Clauser and Aspec. Physicists accept that quantum entanglement probably exists, and they are looking to apply it to modern technology. But one researcher continues to examine the fundamental questions. Anton Zairin. I mean, I personally, I'm still interested in the foundations. I'm not interested to do work on applications, because that is nice, but it cannot answer the fundamental questions. Zeilinger feels that Einstein's question still hasn't been fully answered. All of the past experiments were conducted inside labs. None have yet proved that synchronization caused by quantum entanglement can actually occur in objects at opposite ends of the universe. Zeilinger comes up with an idea on a grand scale. If you do it with the cosmic sources, then the common cause has to be, you know, billions of light years ago. He decides to use celestial objects for the experiment as a way to improve the accuracy of the observation device. He connects the switching equipment to telescopes. He uses two objects called quasars, which have existed since the early days of the universe. The quasars are located light years away from each other. The research team uses the fluctuation of the light from the quasars to determine the timing of the switch. This is in order to make the results resemble those from an experiment run at the actual far ends of the universe. 2018. The team is able to overcome technical hurdles and finally conduct the experiment. It puts the probability of quantum entanglement being real at 99.9999999999999 percent. In other words, it is a near certainty. The experiment proves non-locality is real. It shows quantum entanglement has been a part of the universe since early days and continues to occur everywhere. It turns out the ambiguity Einstein referred to may be a basic reality of the universe. The result puts an end to the 80-year-old question of whether quantum entanglement is real. The question has long been considered to be just philosophical. It was rather late, but finally it worked out. I would like to know Einstein's reaction to the experiments. I would like to know what he would say now. We as humanity have to change our view of the world. But that will probably take a long time. And it will happen because the young ones become convinced. Today, the discovery of quantum entanglement is revolutionizing our perception of space. One theory says only the surface boundary at the edge of the universe is real, and everything inside is merely a holographic projection. And according to that theory, we could say that it arises from the entanglement of these particles on the boundary. So here, the quantum degrees of freedom are like the people, and there is a lot of entanglement between nearby ones and a little bit less between the ones that are further away. That's the type of pattern that constructs these kinds of space-times. Does that mean we are holograms? John Clauser conducted experiments to test Bell's inequality. He has retired from academia and now dedicates his time to sailing. It turns out a scientific approach works for this too, and he's won many races. I am confused and confusing. But at least now I have advanced my confusion to put greater details on why I am confused. It was the best I could offer. Einstein's rejection of quantum entanglement is considered his biggest mistake. The question was brushed aside many times after being raised 80 years ago. But now it's bringing about major changes to the world. The question which is open now is, why? Why is the universe so strange? That we don't know yet, but maybe someday. Some young person comes and says, ah, this is it, you know. I have it, I know it, you know. And then I will say, oh, really, great. Quantum entanglement, a phenomenon that defies common sense. The truth at the heart of the mystery still awaits us, but only if we're ready to think outside the box.