What kind of universe do we live in? Dutch physicist Ronald Hanson has provided perhaps the best answer to date — and Albert Einstein wouldn't like it.
The question revolves around a phenomenon called quantum entanglement, which predicts that changing one particle instantaneously changes the other — even if they are on opposite sides of the galaxy, 100,000 light-years apart.
Einstein called this idea "spooky action at a distance." And he dismissed it, arguing that nothing could move faster than light, so entanglement couldn't be real. Instead, he proposed that unknown "local factors" must determine the strange properties of these so-called entangled particles.
So how has Hanson proven him wrong? He separated a pair of entangled particles far enough apart that local forces could not act on both at the same time, measured their properties, and found that the particle properties correlated.
It is the strongest proof of quantum theory to date, and raises all sorts of questions about the nature of the universe. The Kavli Foundation put some of those questions to Ronald Hanson and Renato Renner, a theoretical physicist who studies quantum cryptography, in a Google+ Hangout. (Watch the original video.) The following is an edited transcript of their conversation.
About the participants:
Ronald Hanson is the a professor in the department of Quantum Nanoscience at Delft University of Science and Technology and a member of the school's Kavli Institute of Nanoscience. He has conducted the strongest test of quantum entanglement yet.
Renato Renner heads the Swiss Federal Institute of Technology (ETH), Zurich's Institute for Theoretical Physics and the Quantum Information Theory (QIT) Group. He is a leader in applying quantum physics to data security.
The Kavli Foundation: Let me start with an obvious question. What do we mean when we say a particle is entangled? When I weave my fingers together, are they entangled? Is this what we're talking about?
Ronald Hanson: Entanglement and the theory of quantum physics is really different than that. If particles get entangled, they lose their identity. So before they are entangled, you can assign them certain properties. They can have definite values. But once they are entangled, they now have only an identity together as a whole. The weird thing is that this entanglement bond stays there even if you pull the particles apart. So even when you place those particles on opposite sides of the galaxy, they will behave as if they are actually one particle.
TKF: Renato, is that pretty much what you see?
Renato Renner: That's a very nice explanation, and it includes certain things that we learned after Einstein made his remarks about faster-than-light communication. He argued that nothing moved faster than light, and so separate particles couldn't possibly communicate with each another.
One thing we've learned is that entangled particles lose their identity. Indeed, they even lose their properties. You can no longer say that an entangled particle has a certain, specific property. For example, before it was entangled, a particle might have been spinning up or down. Once entangled, it loses this property. [Einstein's Unfinished Dream: Marrying Relativity to the Quantum World ]
That's something you don't find in the classical world. It's a special property of the quantum world, and something that we have to learn to accept in some way.
TKF: How does entanglement actually happen? Is there a physical mechanism linking these particles together? I mean what would it look like? Do we have any idea?
R.H.: Entanglement doesn't happen automatically. It requires interaction between particles. So two particles must either be close together, or they have to interact with some third or fourth particle, a technique we used in our experiments. But we have to establish some link by the usual physical laws that we know.
The peculiar thing is that once this entanglement has been established, it will stay there. So then — without any interaction, without any rope connecting the two particles, without anything — you can move them apart and the entanglement will persist. It's really the state of the particle. It has nothing to do with physical laws or dynamics.
TKF: And yet they're connected.
R.H.: Yes, when we do experiments, they seem connected. But maybe we should qualify that. They are connected in the sense that if we take a measurement of one, it correlates with a measurement on the other, even though the measurements can be done so fast that there's no time for communication between the particles.
TKF: This suggests we could communicate at faster-than-light speeds, right? The galaxy is 100 thousand light years wide but this happens instantaneously. So, is something moving faster than light?
R.R.: No. Nothing is moving faster than light. Of course, the first explanation we would come up with is that it takes something moving faster than light to explain that behavior. But then when you think more deeply, you realize that there may be other explanations.
For example, we just said that entangled particles lose their individual properties. But let's momentarily assume they still have their properties. Then the only explanation would indeed be that something is moving faster than light.
But let's think of it in a different way, and say they simply don't have these properties. These properties only come into existence when we observe them. Then there is no reason to think of a physical mechanism that communicates information about one particle's property to the other particle, because neither particle has that property to communicate. The property appears only at the time we observe it.
TKF: So if this is not a matter of communication, does it suggest something about the structure of the universe? In other words, what makes this behavior possible? Do we have any clue? Do we even know where to look?
R.H.: This is a very good and deep question, I think. It boils down to what Einstein was trying to do, find a theory underneath quantum mechanics that is more intuitive in our world.
I think it's fair to say that people are still debating about the implications of Bell's inequality, the equations that define whether the behavior of entangled particles reflects quantum or classical physics. That is what we were testing in our experiment. But there are different ways of arriving at Bell's inequality, and different premises that go into it. You can come at it from different angles.
Einstein's angle was that there are "local" factors that act on both entangled particles, and that these particles have "real" properties even before we observe them. If we see a blue marble, for example, we believe intuitively that the marble was already blue before we looked at it.
If you start with Einstein's concepts of locality (local forces) and realism (real properties) you end up at Bell's inequality. And Bell's inequality was violated in our experiment. So we have to drop either locality or realism to make it work.
TKF: If I understand you properly, researchers use Bell's inequality to test whether Einstein was right or wrong about local factors and realism. Bell's inequality says what? Is there a simple way to explain it?
R.R.: Building on what Ronald said, I would explain it in the following way: You make certain assumptions. One assumption is that particles have real properties. Another assumption is that nothing travels faster than light. These are your two starting assumptions.
Now if you combine them and do the mathematics, you arrive at Bell's inequality, which tells you that certain things that you can measure are smaller than a certain number.
What Ronald did is that he measured these things, and he found out that they are not smaller than this particular number. They are actually larger. So we conclude that one of the two assumptions has to be wrong.
Both of these assumptions are things that we would naturally think are clearly true. Ronald's experiment proves one of them has to be wrong. We still have a choice as to which one we think is wrong, but one of them has to be wrong.
TKF: While we've been talking, several listeners have sent in questions. One wants to know whether higher dimensions could account for the linkage that connects entangled objects. Is this a possibility? Is this something we even know?
R.R.: People often ask about higher dimensions when they try to find a mechanism to explain entanglement. However, as I said before and what Ronald's experiment essentially shows, is that we have to give up one of our assumptions about local forces or realism. If we give up the assumption of realism — that things have properties — then higher dimensions is a valid explanation.
We could also explain Ronald's experiment if we give up the assumption that there is nothing faster than light. If we do that, however, and this is an important point I try to stress all the time, then we are in trouble with our assumption that we have free choice.
For example, in Ronald's experiment, he has to choose what he measures. He has to choose between different measurements. Now, if you give up the assumption that nothing is faster than light and you assume there is some higher dimension in which things can propagate much faster, then we can no longer hold onto our belief that we have free choices. But that's something we probably don't want to give up.
TKF: This goes to the heart of what Einstein meant when he said, "God doesn't play dice," right? He thought particles had properties, even if we didn't know how to figure out what they were. You're saying this experiment shows that they do not have a property until we measure them, and then they both have the same property at the same time. Am I getting that right?
R.H.: Yes. There's still two options open: Either God plays dice, in which case reality as we've just defined it does not exist, or signals do go faster than the speed of light and can actually talk to each other by some unknown force. So these are two possible explanations of the experiment. Or as Renato said, we don't have free will. Everything we are doing now, the people that are watching this, they're just running like a clock in a fully deterministic way without any free will.
TKF: So God playing dice is essentially what gives us free will. There's a certain amount of not knowing what's going to happen next.
R.R.: Yes. One needs to carry out an experiment in such a way that what you are going to measure is not predictable. That way, the outcomes of the experiment are also not predictable. One way to achieve that randomness is through the free choice of what we do. This is an assumption, but then, we get things that are not predictable from the experiment. So we invest in some free choice and we get some randomness back. This is, as we may discuss later, something that is extremely important for cryptography and data security.
TKF: Well, I was just about to ask you about security. That's your interest and you've suggested we could use entanglement to protect privacy. How?
R.R.: As Ronald said, when particles are entangled they lose their properties. They only get properties once we measure them. So, for example, entangled particles can be polarized either up or down. Because these properties are not there before they're measured, no one can see them. So, in some way, you can encrypt or hide data because, for a certain time, that information simply doesn't exist.
To be a bit more specific, we start by entangling two remote particles, Alice and Bob. When we measure them, the results will be correlated. Two observers will both see, for example, either a zero or a one. But because this random bit did not exist until we measured Alice and Bob, no one could have possibly stolen or even predicted it.
They not only get a correlated random bit, they get a random bit which is fundamentally unpredictable by anyone. This is exactly what you need in cryptography: a way to make sure that you and I have certain random bits that we both know but no one else knows. Entanglement achieves exactly that.
TKF: Those random bits would be how I know who you are?
R.R.: Yes. We could use that common knowledge to authenticate each other or to hide messages so only the two of us could see them. We could use it as a cryptographic key to encrypt messages. For example, you might ask me an important question where the answer is either yes or no. So I tell you that when our random bit is "zero," I give you a plain answer, but when it is "one," I give you an answer that is the opposite of what I mean.
Now, if I answer, "no," someone who does not know the value of our random bit cannot know whether the answer was actually "no" because our random bit was a zero, or "yes" because our random bit was one and I flipped it.
If we share a common random bit that no one else knows, I could answer you in such a way that only you can decrypt it. No one else would ever be able to guess what this answer was. That's essentially the idea of it.
TKF: But you would need more than one bit to do this, right?
R.R.: Yes. That's what makes quantum cryptography so strong. Quantum cryptography, and in particular the type of experiments that Ronald carried out, gives us a way to constantly produce new random bits from entangled particles. We can use them to create keys to encrypt information.
In conventional cryptography, I get the password once. Maybe my bank sends it to me. It's like passing me an envelope. But whenever I use it, an adversary could try to guess what it is. The more messages I send, the more information my adversary learns about it. At some point, it's used up. But in quantum cryptography, we can constantly produce new secret keys without having to meet and exchange that secret envelope.
TKF: That sounds difficult to hack, but today's security codes are pretty good too, aren't they? I've heard they generate 340 undecillion — a number so big, I've never even heard anybody say it — 340 undecillion possible keys. It would take 800 million times longer than the age of the universe for our computers to crack them. What kind of threats are you envisioning here?
R.R.: I've never heard this number either.
TKF: It's 10 to the 36nd power . Which is pretty high. So isn't quantum security overkill?
R.R.: There are at least two answers to this. First, quantum technology will produce something else which we did not talk about, namely quantum computers. We don't know when the day will arrive, but there is a possibility that 20 years from now, we will have a quantum computer. It's known that most known public-key cryptography systems, the type we use every day on the Internet, can be broken in very short time using a quantum computer. In fact, a quantum computer could decrypt a message as quickly as an honest user could do it. So once quantum computers exist, the public key systems we use to communicate with banks and stores will be completely insecure. They can be broken in no time.
The second way to answer this question is that it takes so much time to crack today's security codes because we are referring to classical computers. Classical computers essentially go through every possibility and check all possible keys. If there are too many, the computer needs a very long time to do it. But quantum computers use a completely different approach. There are very clever algorithms that are known today and which I will not describe that can break this type of key within seconds.
TKF: Ronald, I know some of your work involves quantum computing. Are you building the type of computer that Renato needs to protect us against?
R.H.: That is indeed one of the goals of my research. We have a fairly new institute here in Delft called QuTech. One of our primary goals is to build a large-scale quantum computer. You could indeed use it to break cryptographic codes.
I'd also like to add two things to what Renato said. We may not have a quantum computer tomorrow or even in 10 years. But you have to realize that whatever we communicate now can be stored and deciphered 10 or 20 years from now, when we do have a quantum computer. So when thinking about information security, you already have to take into account that quantum computing may arrive in the future.
What is so nice about quantum cryptography is that you don't have to know what happens inside your device. It's really a black box approach to cryptography. As long as the box at the sender and the receiver violate Bell's inequality, then you know for sure that the key you are generating is secure. That's really powerful. It doesn't rely on other people not having enough computational power or enough access to your system. It relies on the fundamental laws of nature.
TKF: I've got a terrific question from a listener. Ronald, your experiment separated quantum particles by about one mile. Is it possible that there are local forces that might extend one mile, or 100 or 1,000 miles, or even several light years? Is that a possibility?
R.H.: The question asks if there is some length scale involved in quantum entanglement, and once we go beyond a certain distance it breaks down. So I think this is a good time to mention that in the experiment, it's not so much the distance between the entangled particles that matters, but the distance combined with the time it takes us to measure the particles. We do our measurements so fast, there's not enough time, even at the speed of light, for the signal to go from one particle and influence the measurement of the other. So the mile is important because we are dealing with a time scale of a few millionths of a second. That's the time that it actually takes us to do the measuring.
The other answer is that we don't really know. If we believe quantum physics, there should be no length scale involved in entanglement. It should also work if the two particles are separated basically by the size of the universe. Of course, such experiments have never been done. So the real answer is that, experimentally, we don't know.
TKF: One of our listeners has asked whether we could use quantum entanglement for communication.
R.H.: Yes. Actually, quantum entanglement allows you to teleport information, what we call quantum information. You can teleport quantum states over distance. Teleportation here really means that the information disappears on one side and at the same time reappears on the other side.
This sounds like faster-than-light communication, but one caveat here: Besides teleporting the quantum signal, you also have to send a classical signal to decipher what you have teleported. It's actually a very useful concept for a future quantum version of the Internet that we're trying to build, but it does not allow faster-than-light communication.
TKF: So we would be unable to, say, get our pictures of Pluto back to Earth instantaneously.
R.H.: Right. With all that we know of nature and all the experiments that have been done so far, this seems to be impossible.
TKF: One of our listeners asks about relativity. Imagine we have one entangled particle here on Earth and another one in a craft that's moving at almost light speed, fast enough that time dilates. Will both particles change instantaneously?
R.H.: That's a good question for Renato to answer.
TKF: Give it to the theorist.
R.R.: Yes. Let's say I measure a particle in one lab. One would imagine that the change happens immediately to the sister particle in the second lab as well.
However, the only thing that we actually know is that when you take a measurement in one lab, you will see a correlated outcome in the other lab.
So you shouldn't think of it as a change that happens and spreads. It's more that if you take a measurement at the same time, which is now relative to the reference frame of the observer doing the measuring, then you see this correlation. From the particle's point of view, the speed of the particle or time dilatation is not relevant. What is relevant is that you're sitting in a lab, someone is sitting in another lab, and we take a measurement at the same time in our reference frame.
That said, there was recently an experiment carried out by Nicolas Gisin's group at University of Geneva that tried to look at this problem. He moved one observer relative to the other in a very clever way, so that each observer, from his point of view, appeared to take the measurement first. The experiment showed that despite the fact that they both seem to make the measurement first, they both see the correlation between the particles.
If we were to interpret this experiment in a way that assumes some sort of signaling between the particles, we would have to assume that the signal would have had to travel into the past. This is a strange explanation. It's much better to assume that there's no signaling, and try to come up with other explanations.
TKF: Speaking of strange explanations, let me ask you a question that's interested me: We entangle photons. We entangle electrons. We entangle atoms. Is there any reason we could not entangle — watch out, Starbucks — a cup of coffee?
R.H.: That's a very interesting question, and one that is on many physicists' minds. For example, my colleague, Gary Steele at Delft's Kavli Institute is trying to create quantum superpositions with very small objects, and many other people around the world are actually exploring this. They want to see what happens if we make things bigger, more massive.
Who knows? Maybe we will have to rethink the laws of physics that we know up to now. Quantum physics has very good laws for describing what happens in the world of small particles. But if we look at our everyday lives, we never see such things. Your cup of coffee from Starbucks is always in one place. It's not in two places at the same time, and it does not appear to be entangled with something else.
So where is the transition in scaling up from the quantum world of small particles to the classical world we perceive, where things have well-defined properties and seem to be in the right places? Quantum theory defines these two domains and gives them different laws, but we do not fully understand how to connect these two worlds. Some people call this the measurement problem or the measurement paradox. But it really has to do with what happens when we go from a quantum mechanical picture up to some measurement result that we see on the screen. Maybe there is some clue when we scale up to bigger particles. Who knows?
R.R.: Whether the laws of quantum physics also apply to large objects is one of the most important questions we should answer in physics. We could go even further, and ask, "What happens if humans are entangled? What happens if I'm entangled with a particle?" To add to Ronald's reply, one possibility indeed is that there is a transition point. Once we get to large enough objects, they become more classical and behave the way we think and perceive them.
There are also other explanations. One explanation is that everything is actually entangled. If I just look at an object, I become entangled with it. But because I am now part of this entangled thing, I don't perceive it as entanglement. It would only look entangled from the outside. To me, the object looks as if it has a well-defined property.
We don't know which of these explanations is the case. So we have at least two consistent explanations. One says everything is quantum, things are entangled, and we are entangled with one another. The other, maybe less adventurous, says that when things get larger, they become classical and all these strange behaviors disappear. Deciding between these two possibilities is something I think we should do in the future. Trying to do experiments with larger and larger objects is a nice way to achieve that.
TKF: That would be an interesting set of experiments. We're going to wrap this up since we're already a few minutes over, but I do have one final question. If Einstein could join us today, how do you think he would react to this experiment? What avenues of research do you think it might suggest to him?
R.H.: If you look at Einstein's life, he started out overthrowing the physics of his time. Relativity theory was a complete break with everything that was known before. But by the time quantum physics really matured, he seemed to have grown more conservative. When he analyzed quantum entanglement, he didn't say, "Let's take this quantum entanglement and see what the properties are and what it can tell me about the world." Instead, he said, "I think the world should be local and obeying realism and therefore quantum theory is incomplete"' So on this point, he seems quite a bit stubborn.
My guess is that even after seeing the results of our experiment, he would not be convinced. He would still say, "My theory is still there." But we don't really know, because he did not get to see Bell's inequality, and maybe that would have convinced him. I'm not so sure.
R.R.: I would assume, obviously, that Einstein is an intelligent, rational person. He would look at the possibilities. As we discussed, there are not many possibilities left. All the options require us to give up a certain view we had of the world. As Ronald said, Einstein was actually very good giving up certain intuitions people had about the world at that time. Ronald's experiment closes the possibilities that Einstein thought were true. I would assume that he would give up those possibilities, and try to find the theory that explains things in accordance with this experiment. This would probably be a very unconventional theory, and I would expect Einstein would be able to do that.
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