## Physics for the 21st Century

# Manipulating Light Featured Scientist Paul Kwiat

**Interviewer: What first got you interested in science?**

**PAUL:** Well, I guess when I was in high school I knew I wanted to do something sciency. I liked understanding the way things worked. And I didn’t really like biology. I didn’t have a head for memorizing lots of names. And at the time, I didn’t really care if the fruit flies had vermillion eyes or wide eyes or something like that.

But then my physics teacher was a really fun guy, and he just loved understanding things, and he would break things all the time. I remember he accidentally broke a light, he was playing with a slinky and the end came through it and knocked out the fluorescent light. And he just was unflappable. He just said well, what can we learn about this. How much energy is stored in the spring?

I really liked that sense of just kind of playing with things and trying to figure out what we can learn from them. And I like the fact that you could somehow predict the future. At least that’s what I thought back in high school.

**Interviewer: How did you become interested in the physics of light, and the entanglement of light?**

**PAUL:** When I was in graduate school I had thought that I was going to be studying solid-state physics, trying to understand why some materials are hard, and some are soft, and some conduct, and some don’t conduct. And then I saw a professor who was talking about the so-called Gedanken experiments, thought experiments, but he wasn’t just talking about them—he was actually doing them in his laboratory. He had these very simple setups using photons, and that’s how I got involved in that.

And at that time, I actually didn’t think that there was any future in it. I thought this is just really neat stuff, and I’ll never get a job in doing this afterward, but at the moment it was just really cool to be able to play with this really basic quantum physics that really just flies in your face of any kind of classical intuition.

I thought I have to do it and the rest of my career be damned. I’ll become a vacuum cleaner salesman or something like that. And then I just got very lucky that the whole field of quantum information sort of sprang up around me and suddenly it was very important to know how to make entanglement and how to measure it and how to apply it. And so, it’s been really neat seeing how those applications keep growing and growing. And so, what I’m looking forward to seeing ten years from now: where are we going to be? What’s the next thing? What’s the next super hyper-ultra entanglement thing going to be?

**Interviewer: How do photons play into your current research?**

**PAUL:** So, light is all around us. Every day when we’re outside, we’re being hit by millions and billions and billions of photons. So, these particles of light are kind of streaming all around. What we do is in a very controlled setting is produce very, very special pairs of photons. And I’m interested in just how those one or two photons behave. And they can behave in really strange ways that are almost inconceivable using classical physics.

In our research, we’re concerned with the properties of light and the properties of photons. There’s a property that you don’t tend to see sort of directly by looking at light, which is the polarization of the light. And the polarization that describes the direction that the light is wiggling, if you will, is oscillating. If the light is say propagating along my finger, then it could be wiggling horizontally, and we’d say that it was horizontal polarized, or it could be vertically polarized, or it could be some super position of horizontal and vertical—it could be diagonal or anti-diagonal or basically anything in between. We can use that degree of freedom to encode information. And, we can also use the direction of the light to encode information, or we can use the color of the light to encode information. So, all of these properties are useful and interesting to us.

In my group, we have two main research thrusts; one of them is developing resources: things like high efficiency detectors, entangled photon sources, hyper-entangled photon sources—and then once you have those resources, the other thrust is applying them immediately to all kinds of things like very fundamental tests of quantum mechanics, and trying to understand what these states are like. How they behave? How to treat them? How to measure them? Or how to apply them to problems in quantum communication, in metrology, and quantum computing?

**Interviewer: What’s the difference between encoding information in light, and encoding quantum information in light?**

**PAUL:** So, in a classical communications when we have say a fiber optic cable, and we send lots of information by sending pulses of light down that cable, each pulse of light maybe has a million photons in it roughly. But all the information there—it’s really classical in the sense that you can measure that pulse, and it doesn’t change the information that’s being encoded.

What we’re doing is instead sending single photons down the fiber and we use some property of the photons to encode information. So, we use the polarization to encode information, and if we have horizontal polarization, we can call that a zero. If we had vertical polarization, we can call that a one. And because we can send arbitrary super positions of horizontal and vertical we can send arbitrary super positions of zero and one. So, we have these quantum bits or qubits that can fly down the cable. And because of the fact that we can have that superposition state, that enables us to do things in information processing like sending completely secure messages that would be impossible to do with classical communications.

**Interviewer: What is quantum information?**

**PAUL:** So, quantum information is a field that came up roughly twenty years ago, and the idea is to use really weird bizarre features of quantum mechanics to do things in information processing that would be difficult or impossible otherwise. So, things like solving certain computer problems that would be basically impossible—that would take the age of the universe on any kind of classical computer to solve, or sending completely secure messages that are probably secure by laws of quantum mechanics.

And we use photons as our carriers of quantum information. Quantum information can be understood by comparing it to classical information. Classical information is based on bits—”bits” stands for binary digits, things that can be zero or one. In any computer, there are lots of these bits and any one of them is a zero or a one. In the quantum case, we have the extra possibility that we can have super positions of zero and one at the same time. So, the bit acts like it’s not really zero, it’s not really one, it’s somehow both of those at the same time. And that enables you to have a great parallelism in order to solve certain problems.

The dilithium crystals of all quantum information processing, if I can use a Star Trek term is the phenomena of entanglement, and Schrödinger, he was one of the founding fathers of quantum mechanics, he described entanglement as the quintessential quantum mechanical feature that really separates it entirely from classical lines of thought. In our lab, we are largely focused on producing entangled pairs of photons and actually extending that so that they’re entangled in as many ways as possible, and that we would call hyper-entangled.

**Interviewer: Can you explain the concept of entanglement?**

**PAUL:** Entanglement is the word that describes the quantum mechanics of when you have more than one system. And somehow their properties are coupled to each other and correlated with each other no matter how far apart those two systems might be.

But if we imagine, for example, that we each had a coin, and we each are flipping our coin, I’m likely to get heads or tails and you’re likely to get heads or tails. There’s no connection between the two of them. On the other hand, if these were quantum mechanically entangled coins, then it could be the case that when I flip my coin, I’m equally likely to get heads or tails. You’re equally likely to get heads or tails. But whenever I get heads, you always get heads; whenever I get tails, you always get tails. And that’s a connection that exists no matter how far apart those coins are, and in that sense, it’s a non-local connection. And it’s that connection that enables us to do really interesting things in communication and teleportation and cryptography. When you have a quantum computer that’s working all of the internal states are these extremely complicated entangled states of all the different particles interacting.

One feature that’s unique about entangled particles is that each particle itself doesn’t have definite properties. So, I have two particles; particle one—it doesn’t know what energy it has, it doesn’t know what direction it has, and it doesn’t know what polarization it has. It is the same thing with particle two. It doesn’t have a definite energy or direction or polarization. But if these are entangled, as soon as I know the energy of one particle, I know the energy of the other one. As soon as I know the direction of one particle, I know the direction of the other one. As soon as I know the polarization of the one particle, I know what the polarization of the other particle is. And that’s true even if these particles are on opposite sides of the galaxy, at least according to our understanding of quantum mechanics. I guess it’s been tested up to about a hundred miles separation or something like that so far.

**Interviewer: How do you actually create entanglement in the lab?**

**PAUL:** We need to have two particles. The way that we get those is we take a laser and we put it into a special crystal, and that crystal has the property that takes the high energy photons that are going in and occasionally will split them into two daughter photons. And when we produce these pairs, they can be entangled in different properties. So, they can be entangled in energy because we take the energy at the parent photon, and that has to be divided between the two daughter photons—and there’s lots of ways to do that. You could divide them so that each daughter photon has the same amount of energy, or so that photon one has more energy, and photon two has less, or vice versa. There’s this whole range, each photon alone doesn’t have a definite energy. Each photon doesn’t have a definite color. It’s like the one photon is red and green and blue all at the same time. And the other photon is blue and green and red at the same time. So, if one of them is red, the other one is blue, or both of them are green, or one of them is blue—the other one is red. Okay, so that’s one way they can be entangled.

The photons are entangled with energy. They’re also entangled in their directions. So, when I have my parent photon coming through it produces a pair of photons, and that pair of photons is emitted along a cone—a cone that’s centered around this parent laser beam, and each photon is emitted around the whole cone. There’s no definite direction for either of the two photons.

Each photon is emitted into all directions and has no direction associated with it. But as soon as you measure the direction of one of them you know the direction of the other one. That means that they’re entangled in direction. So, if you measure that one of them is emitted along 3, and the other one is emitted along 9, or if one of them is emitted at 12, and the other one is emitted at 6, that’s an example of them being entangled in direction.

**Interviewer: Can you explain the polarization entanglement?**

**PAUL:** We’re also able to create the photons so that they entangled in polarization. And the polarization is this wiggle direction of the photons. the crystal that we use when we produce a pair of photons are both wiggling in the same direction. They’re both say horizontally polarized. And it doesn’t matter where along the cone they’re emitted, they’re still both horizontally polarized. If we rotate the crystal by ninety degrees, then instead of producing two horizontally pairs of photons we’ll produce two vertically polarized pairs of photons. And again, it doesn’t matter where on the cone they’re emitted, they’re both vertically polarized. And the trick to getting polarization entanglement is we take the two crystals and we just put one right on top of the other. We sandwich them together, and we dry both of them. We pump both of them. And now there’s a quantum mechanical uncertainty as to whether or not that birth process takes place in the first crystal, giving two verticals, or if it takes place in the second crystal giving two horizontals. And the upshot of that is that the photons are entangled in polarization.

**Interviewer: How can one photon know what the other one is doing?**

**PAUL:** I’m not sure I can give a good explanation for that. The fact that they’re in this entangled state means they have this non-local correlation—so that it doesn’t matter how far apart they are, they’re in the same quantum state. They’re guaranteed to always have the same polarization—so, as soon as I measure one, I automatically know what the other one is. I don’t really know how I can say how those correlations exist, they simply do. In fact, Einstein called that spooky action at a distance: the fact that this one can know immediately what the other one knew—and he didn’t like that at all. In fact, he thought that there was maybe some underlying theory that would explain how that could be but as far as we know there is no underlying theory. It’s just the way it is.

**Interviewer: But there’s this moment before measurement where they don’t have a value. Correct?**

**PAUL:** There is a moment before measurement where the only thing you can say about the two photons is that they have the same polarization, but neither one has a definite polarization. It would be like us saying that I have the same amount of money in my wallet that you have in your wallet, but neither of us has a definite amount of money in our wallet, which is crazy, because, of course, I have a definite amount of money in my wallet. I don’t happen to know what it is at the moment. But I know approximately what it is and I know that it’s definite. It’s not that my measuring my wallet causes there to be a definite amount of money there. But that’s because my wallet is not quantum mechanical. In the case of the photons, the photon doesn’t have a definite polarization. It’s when I make a measurement of it that immediately causes it to have a definite polarization, and then that causes, because of the entanglement, the other photon to have the same definite polarization.

**Interviewer: So now what do you do with these entangled photons?**

**PAUL:** So, okay, so once you produce these entangled photons what good are they? What can you use them for? Well, they’ve been used for a lot of different things in quantum information processing. My group and other groups have used them for quantum cryptography—where we send one photon to each of two parties, and the fact that the photons have correlate polarizations allow those parties to make a completely secure key that they can use to encrypt any message in a way that it’s impossible to eavesdrop on.

So, that’s quantum cryptography. They’ve also been used for quantum teleportation where you can send a quantum state from one place to another place, but without actually having it go the intervening distance. And the way that works is that both the sender and the receiver have to already have an entangled pair. And then they can do this quantum teleportation.

They’ve also been used in quantum computers. In some sense, when you have a quantum computer that’s running the internal states—these are complicated massively entangled states of all of the different qubits, of all of the different quantum bits. And so, people have used photons to do small quantum algorithms. At the moment, the world record is up to six photons. We have a long way to go before we’re at thousands of qubits, but on the other hand, a few years ago we only had two qubits. So the fact that now we’re at six, we’re definitely seeing progress on that front. All of those examples were all sort of done using just entanglement in one property—say entanglement and polarization, or entanglement in energy.

One of the things that my group is studying now is hyper- entanglement. In hyper-entanglement, the photons are actually entangled simultaneously in all of these properties at once. So they’re entangled in the energy that they have; they’re entangled in the directions that they’re going; and they’re entangled in the polarization that they have. And that enables us to do things that would be very difficult otherwise and really simplifies some of the other experiments.

**Interviewer: Can you explain the advantages to hyper-entanglement vs. entanglement?**

**PAUL:** I think there are at least three. One, there’s a very simple sort of notion that you just have more information that you can carry per photon if you can use multiple properties of the photon. If you only use polarization you’re limited to just a single bit that you can carry in that photon. If, on the other hand, you include direction, well there are a lot of different directions that the photon can point in. So, there’s actually multiple bits of information in the direction plus the bit of information that you get from polarization. And then if you also include the energy of the photon, that’s yet more bits of information that you can send in one photon. That’s one advantage is being able to put more information on a given photon.

The advantage of hyper entanglement for quantum logic is that if you have quantum bits sitting on three separate photons, it’s difficult to get them to interact with each other. It just turns out that they don’t interact strongly with each other. On the other hand, if you can get all three qubits to be sitting on the same photon, then it turns out it’s relatively easy to get them to do quantum logic with each other. It’s easy to get them to interact because they’re all on the same photon.

**Interviewer: Is it difficult to hyper-entangle?**

**PAUL:** First off, I should say that hyper-entanglement doesn’t completely save the day because you run out of these extra properties. I mean there’s only a few properties. So, there’s only so much information you can actually do with one photon or two photons. So, you definitely need to be moving to experiments where there are four photons and six photons and twenty-seven photons. And we’re working down that path.

In quantum computing you’re able to do things more easily using hyper- entanglement and some of my colleagues in Australia have shown that you can do quite complicated logic circuits that normally would require twenty components and seven photons. And instead you can do them with only three photons and only seven components or something like that. So, there’s a great savings. And so, try to implement some of those systems is definitely the next step in that direction.

**Interviewer: How is your research addressing the big questions in your field right now?**

**PAUL:** I think one of the big questions in quantum information at the moment is can one actually make a quantum computer? Can you make a large-scale quantum computer—not one that just has three qubits or six qubits, but one that has hundreds of qubits in it? And there are lots of different people that are working in different systems— working with atoms or ions or super conductors. We happen to be working with photons. We think they have a lot of advantages. Because they don’t interact much with the environment, they’re quite clean. They don’t decohere. You can preserve these quantum super-positions. Also, it’s very easy to produce the photons at very high rates. And we can manipulate them quite easily, and we can detect them very efficiently.

**Interviewer: So, what value is there to what you’re doing now?**

**PAUL:** I’m asking myself that every morning. Okay, I think there are three answers to the value of what we’re doing at the moment. The first answer is that thus far we haven’t seen any limits to how far we can push photons for quantum computing. And so, until we see limits, we’re just going to keep pushing until we find out what those limits are. And maybe there won’t be limits. So, the value is to keep going with something that’s working until it stops working.

The second value is that as we learn more about these basic quantum mechanical things like entanglement and hyper-entanglement and what that actually means, we have a much better understanding of all of physics or at least of quantum physics. And so, that’s a good thing in and of itself just to understand.

And the third benefit, which is related to the second benefit is that there’s a high likelihood that we’ll come up with other applications that we haven’t even thought of yet because a lot of the applications that I talked about five years ago didn’t exist. I mean people hadn’t thought about them. And so, because this is such a new field, and new things are coming up all the time, the better understanding that we have of the basic principles, the better we’re going to be able to see or to realize those other applications when they come along.

**Interviewer: Will quantum computers replace classical computers?**

**PAUL:** So, quantum computers are much, much more powerful than classical computers, but only for very specific problems. And so, a quantum computer is not likely to ever replace a classical computer. One thing people use computers for all the time now is maybe writing a web blog. And that means you’re making copies of that information for lots of people. And one thing that’s very different about quantum information than classical information is that you cannot copy it. That’s why it’s useful for quantum cryptography because you can’t copy it. An eavesdropper cannot copy that information. So, in that sense, you’re never going to be using quantum computers to do web blogging, for example. But what quantum computers are good for is they can solve some mathematical problems like factoring numbers into their prime constituents. That’s like saying fifteen is equal to three times five- and the reason you care about that is that that problem is at the heart of all of classical encryption. So, whenever you encrypt anything on the internet, it’s relying on that being a hard problem; whereas, it turns out it’s an easy problem if you have a quantum computer. Quantum computers are also good at doing simulations and basically solving quantum mechanics problems.