From Einstein’s initial doubts and Bell’s experiments to the 2022 Nobel Prize, quantum entanglement has developed into a pillar of the physics edifice. So what is quantum entanglement, and why have scientists spent decades establishing its existence? Knowable Magazine Podcast HostAdam Leviwith guest physicistNicholas GissingSolve the mystery for us.
Adam Levy:What does quantum physics tell us about the nature of reality? Can two far apart particles affect each other? Is the universe really “spooky”? I’m Adam Levy and this is Knowable magazine.
In this season’s show, we discuss how and when scientific ideas developed. Each episode, we explore a topic that has changed the way we think. In this episode, we focus on a topic that has actually been around in physics for about 80 years, which is actually a phrase,“Spooky action at a distance”.
Einstein used this impressive phrasing when formulating his emerging ideas in quantum physics, which he found deeply absurd. In this episode, we explore what made Einstein too “ghostly” to accept, and how Northern Irish physicist John Stuart Bell proposed a method that could eventually be used to test the universe and prove love. Methods to determine whether Einstein’s suspicions have basis.
Before that, however, we need to discuss the changes that took place in physics in the early 20th century. At the time, it became increasingly clear that the universe did not behave as physicists had previously described.
What is wave-particle duality?
Classical physics uses deterministic laws to describe the world. These laws govern how waves and particles, such as light and electrons, behave. But these laws could not explain the large number of novel phenomena and experimental results that appeared at that time. As a result, the classical view of the universe was gradually replaced by that of quantum mechanics, which was used to describe the bizarre properties of microscopic structures.
Nicholas Gissing:In the beginning, that’s what people calledwave-particle duality.
Adam Levy:This is Professor Nicholas Gissing from the University of Geneva. He has dedicated his life to the study of applied physics and quantum physics, and has spent decades exploring the applications of quantum physics in communications.
So, what is wave-particle duality?
Nicholas Gissing:Suppose you have a particle, which can be an electron or an atom.All these particles sometimes behave like one particle, like a billiard ball; but other times they behavebehave like a wave. We can say that the wave-particle duality of particles truly kicks off all quantum science. Until the 1960s, and even later, wave-particle duality remained a major concern in science.
Adam Levy:The universe is made up of neither waves nor particles, but it somehow possesses properties of both. This concept turns physics on its head. Even though this may have been the main concern, the bizarre nature of quantum physics poses far more problems than that.
Quantum physics predicts that particles can interact with each otherentangled. This means that multiple particles can come together and form fundamental connections. Even if these particles are later separated by large distances, we can no longer describe their properties independently.The observation of a certain particle willSignificant impactto the observation of another particle.
Nicholas Gissing:Quantum entanglement means: now, if I measure or do some other operation on the first particle, the second particle behaves in a certain wayaffectedor in other words, the second particle “trembled”.
Adam Levy:This view challenges the core of Einstein’s conception of the universe, namelyAll information cannot be transmitted faster than the speed of light in the universe, therefore, no object can have an instantaneous impact on other objects that are separate from it. However, quantum physics holds that measuring one entangled particle can have an immediate effect, or tremor, on other parts.
Nicholas Gissing:Yeah, so Einstein realized, like a lot of people at the time, that if you take a measurement on one side, it’s going to be affected in some way on the other side. We can give an example: one of the particles may be in Geneva, where I sit now, and the other particle may be far away in the United States. Even then, the entangled nature of these particles makes it impossible to describe them separately. Einstein just couldn’t believe this, so he thought quantum entanglement was impossible.
Adam Levy:In fact, in 1947, Einstein wrote a letter to Max Born. In the letter, he nicknamed the non-locality we are now discussing “spooky action at a distance.” This statement is still used today. But at that time, was Einstein fighting alone against these views, or was there actually an active discussion among physicists?
Nicholas Gissing:That’s right.So I think Einstein invented a lot of accurate and creative sayings, like“God does not play dice”,besides“Spooky action at a distance”.“Spooky action at a distance”It does reflect the idea that touching particles in Geneva will cause particles in the United States to tremble. There is indeed such an action at a distance. Einstein said: “This is too weird. It can’t be real. It can only be a ghostly effect.”
I’m surprised that physicists didn’t pay more attention to this problem at the time. The vast majority of physicists at the time were indifferent to these questions and issues. And there was no way to translate this theory into experiments at the time. Some even believe that this is a purely philosophical question with no physical consequences. We will soon find out that they were completely wrong, even though these views were the prevailing ones at the time.
Adam Levy:Shortly after Einstein’s death, John Stuart Bell proposed a test in 1964 that might solve the problem.Today we call itBell test. What is the core of this test?
Nicholas Gissing:Suppose you 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. This way you get two possible measurements. When you flip these coins, you may get heads or tails. Now, if you flip these coins the same way on both sides, you will always get either heads or tails.
Adam Levy:This is the specter predicted by quantum physics. Einstein couldn’t believe this. Even if two entangled particles are separated, they will always show the same measurement result, because what you are measuring is the property of their whole. This is like saying that someone guarantees you that you will get the same result when you toss two separate coins. It seemed as if both particles had instantaneously agreed to exhibit a head-up measurement.
However, there may be a less spooky explanation. For example, the particles may have come up with a secret plan before they were separated, deciding which side to show up when thrown. Then there would be no quantum entanglement, just particles putting their plans into action.
Since John Stuart Bell first proposed a method to verify the spooky nature of quantum entanglement, a handful of experiments have proven the existence of quantum entanglement in the half century. Shown here is a photo of theoretical physicist Bell at CERN (1982).
Nicholas Gissing:And, John Bell came up with the genius idea of slightly changing the way the coin is tossed. Now let’s assume that one of the coins is still thrown in the regular way, and the other coin is thrown in a slightly different way.
Adam Levy:That is, we can measure these two particles in slightly different ways. In this case, will they agree on a measurement result with each other, for example, both show right side up?
John Stuart Bell is a theoretical physicist in particle physics. He took some time off to study quantum theory and realized that this slightly different way of tossing a coin could ultimately solve the problem. Is entanglement really happening between particles, or is Einstein right in denying the idea of quantum “ghostliness”? If Einstein is right and there is no quantum entanglement, but a secret plan reached between particles in advance, then physicists will be able to find two particles that have the same characteristics in the process of repeated coin tossing experiments. The upper limit on the frequency of identical measurement results.
Nicholas Gissing:In quantum mechanics, however, you can violate this upper limit, which is what we callBell’s inequality.
Adam Levy:So if quantum physics is correct, particles are indeed entangled, and “spooky action at a distance” does exist, then in the process of repeating the experiment, the two particles are more likely to show that they are both face-up or both. For the reverse side up.
This is because these particles are indeed connected together, and they canInstantly affect the outcome of each other’s coin toss experiments. In other words, the Bell experiment could illustrate which view is correct: quantum predictions of entangled states, or Einstein and his skepticism about such weird behavior.
Nicholas Gissing:This is indeed the approach proposed by Bell. It turned the entire discussion into a potential experiment.
Experimental verification of quantum entanglement
Adam Levy:However, Bell himself did not complete the test he proposed. When did someone actually perform an experiment that clearly showed that particles are indeed entangled in this spooky way?
Nicholas Gissing:Yes. So John Bell was a theoretician who didn’t know how to perform experiments. We can only wait another 10 years, until the 1970s,John KrauserFor the first time, experimental results were obtained that when the measurement methods are slightly different, two particles have exactly the same measurement results with a high probability.
Adam Levy:John Krauser is passedManipulating calcium atoms, to obtain two photons that are obviously entangled, thereby completing the Bell test.These photons, similar to flipping a coin, are measured in a slightly different way, meaning in this case thatMeasuring the polarization direction of light, that is, the swing direction of the light wave. Based on experiments like this, Krauser was able to observe the frequency at which two photons produced the same measurement. The results of the experiment indeed contradict Bell’s inequality, which shows that these photons are indeed entangled. Einstein’s hypothesis appears to be wrong.
Nicholas Gissing:This is absolutely an amazing result. But John Krauser also faced more than one trouble.
First, on the opposite coast of the United States, someone else followed John Krauser and performed this experiment, but he got a different result. Obviously, one of the two results is wrong, but how do you know which one is wrong?
Plus, in the 1970s, no one took it as a serious physics problem. Because of this, no university was willing to promote John Krauser to professor. His career was severely hampered by the fact that he was the first person to complete the Bell Test.
There was another experiment later, but this was in the 1980s. So you will find that every major experiment takes another 10 years or so.
This time the experiment was conducted byAlan Aspeandhis colleaguesexistParis FranceFinish. Their experiments were better and of higher quality, although they also took longer. To a certain extent they solved this problem and proved the idea of quantum mechanics. Since then, quantum entanglement has ceased to remain a theory and has been established as a real feature of nature.
Adam Levy:These experiments ultimatelyProving the point of quantum mechanics, and denied Einstein’s suspicions, showing that “spooky action at a distance” is a naturally occurring phenomenon. But I guess it doesn’t end there. At the time, there were still some unexplained areas that needed to be addressed, which we called loopholes.
Rather than having instantaneous effects on each other’s measurements, particles may have hidden means that allow them to plan ahead to trick tests. Physicists call this possible particle deception local variables. Can you describe the efforts to close these loopholes to establish that quantum entanglement is a real feature of nature?
Nicholas Gissing:In Alan Aspe’s experiment, the main loophole originated from the process of generating photon pairs in the experiment.
Of course, you wouldn’t send them to Geneva and the United States, but you would launch them to opposite ends of a large laboratory, keeping them about 10 meters apart. Then you need to measure them.
However, in a real experiment, when you are making these measurements, it is veryMay not get any results, simply because photons are lost along the way, or the detection results are limited by the efficiency of the detector itself. A photon can easily be lost or undetected. So you can really think that it is these assumed local variables that determine when the photon 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 still requires further testing. Nearly 30 years after Alain Aspe, additional testing was conducted in 2015. It took us a long time to get single-photon detectors that performed well enough.
Adam Levy:The test effectively used a combination of photons and electrons to bypass detection problems, allowing measurements of entangled particle pairs separated by more than 1 kilometer. Since then, numerous other forms of Bell tests have emerged to patch other holes in quantum entanglement. These tests, using everything from satellites to computer games, continue to push the limits of the field.
But wouldn’t it be too paranoid to think that physicists were conducting these experiments simply because particles might conspire in some way to fool us into thinking they were acting like ghosts?
Nicholas Gissing:Physicists are indeed paranoid. However,Bell’s inequalityThe failure of means that this feature of quantum theoryNot just a feature of a theory, is a characteristic of nature. So it changes the world view that physics shows us. This huge change is a conceptual revolution, so the local variables that determine when a photon can be detected, even if only hypothetically, deserve our attention.
So, I think it really makes sense to do in-depth research. Remember, physics is really just an experimental science. It’s not enough to just think about theory, you have to do some experiments as well. So I think this kind of work is very meaningful. But I also agree that almost no one, and almost no experimentalist, believes that they can falsify quantum mechanics. Although such work is also meaningful.
What does quantum entanglement mean for our lives?
Adam Levy:But is this all just theory? 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 Einstein’s time, this was a purely philosophical question. John Bell turned it into a viable experiment and was first completed by John Krauser. Alan Aspe was the first to complete the conclusive experiment. By the 1990s, people suddenly realized that this kind of abstract thinking did have practical consequences.Because in fact, if you could always, or almost always, get the same random result from two particles, you would getRandomness at a certain distance.And this kind of non-local randomness is very close to aencryption key.
You can think of an encryption key as a kind of password. What is a password? The password has to be the same on both sides, say you and Amazon or your bank or whatever, or whoever you want to communicate confidentially with. Note that the password must be random.
Through quantum entanglement, we have already obtained a code 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 become entangled with a third particle.
Therefore, this ensuresConfidentiality. So, if you’re entangled with a 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 again, which is exactly what you want. So in spirit, cryptography is very close to the failure of Bell’s inequality.
But it was a complete revolution, and people suddenly realized that these strange quantum correlations, Bell’s inequalities, and the possible presence or absence of local variables were 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 for the work that led to the realization of the Bell Test. What is the significance of such an honor for this work?
Nicholas Gissing:I think the 2022 Nobel Prize in Physics is not just for the three winners (John Krauser,Alan AspeandAnton Zeilinger)’s recognition is also a recognition of the entire field. This field has been neglected for decades and is now finally being recognized at the highest level. I have to say that the Nobel Committee made a huge mistake in not awarding the Nobel Prize to John Bell. He should have received this award, but he died too early, or the Nobel committee acted too late, or for other reasons.
So, I am personally very satisfied with this award. Of course it was awarded to three individuals, but above them all, the award did acknowledge a field that had been so controversial. The best example is John Krauser. He did well on the Bell Test for the first time, but was never able to secure an official position at any university. Much of this work was ignored until he was awarded the Nobel Prize 50 years later. So it’s really amazing. I don’t know if it happens often that a field is ignored for such a long time, maybe decades.
When I started working in this field myself, I could only make a living after 9pm. But now, the Nobel committee has recognized it. This is a good point.
Adam Levy:Now, is the debate finally over? Can we say with certainty today that the universe is indeed spooky, without any holes, and that particles do not conspire to cheat the Bell test?
Nicholas Gissing:Yes, but I wouldn’t call it “spooky.” I don’t think anything is spooky, on the contrary, it’s all very real. I mean, this has become a regular test in the lab, and you even have some students doing this exercise in the lab.So this is a veryconclusivefact.
How to view the achievements of quantum mechanics
Adam Levy:What do you think of the tremendous progress we have made in understanding quantum behavior over the past few hundred years?
Nicholas Gissing:I would say that for me, over the past 30 years, understanding quantum behavior started with knowing that these non-local quantum correlations can naturally generate cryptographic keys, and that’s been very useful stuff. You can see the change.
At first, people were discussing wave-particle duality, but now, almost no one will discuss it anymore. People are talking about quantum entanglement.quantum entanglementReally changed people’s perspectives and made quantum mechanics so popular. Today, I think there is no article, no book, no lecture in this field that does not mention quantum entanglement. Therefore, quantum entanglement was finally confirmed to be the essence of quantum mechanics.
Adam Levy:In fact, quantum entanglement, what Einstein thought was incredible “action at a distance,” is now at the heart of many of our quantum technologies: whether it’s new encryption techniques used to keep your communications with your bank private and secure , or the long-term exploration of building a quantum computer with practical uses. To this day, physicists and journalists still often refer to quantum entanglement as the “quantum ghost.”
Physicists took a long time to reflect on Einstein’s complaints about quantum physics, and an even longer time to prove him wrong, eventually pointing out that quantum entanglement is a fundamental way the universe works, even ifThere is also some synchronicity between separated particles. However, today’s physicists are actively putting “spooky action at a distance” into practice.
Author: Adam Levy & Nicolas Gisin