Falling dominos and future computers
IBM scientist Don Eigler, shown here at his lab with Neon, one of the service dogs he trains.
Read or Listen: Building computer circuits at the atomic scale
Read or Listen: Scientist finds deep mystery, beauty in atoms
IBM scientist Don Eigler describes computers of the future and playing dominos with atoms and molecules.
Salazar: Why do you do research in nanotechnology?
Eigler: The guiding principles of my research are in some sense, two–fold. I work as a member of the IBM team, and our job in IBM’s Research Division is to create the future of information technology.
What does that have to do with nanotechnology? If you look at what actually does computation, these days, computation is already being done in nanometer–sized structures, structures of the order of, for instance, 93 nanometers. And in the future, we want to be able to do it in smaller and smaller scale structures. That’s what has traditionally driven all of the progress which makes information technology what it is today. And we see that as something that we want to continue. We want to drive our industry foreward. So we’re very much motivated by the demands of our industry. I certainly am.
But then I’m also driven by my own curiosity and about questions that have to do with fundamental science, in particular, fundamental physics. I’m trained as a physicist. So I’m very curious to understand the properties of nanometer–scaled structures and new ways to learn about those physical properties and new ways to use those physical properties in the real world.
Salazar: Tell me about the quantum corrals.
Eigler: The quantum corrals came about because we were interested in doing some experiments and we wanted to do these experiments on top of a copper surface. And it turns out that when we looked at the copper surface, we were surprised. Suddenly, there were these waves. And we had not been anticipating these waves. It turns out that in retrospect, the waves should have been expected. Retrospective is amazingly accurate! What we learned was that the waves were due to electrons that were trapped on the surface. They’re called surface state electrons, and we could see how these electrons were interacting with the various atoms on the surface. And so we decided that if we can see what the electrons are doing, which is really valuable for us, let’s put them inside of a small structure. Let’s trap them inside of a small structure. And so the small structure, that we decided build was a structure that was made from individual atoms on top of the copper surface. So we put those atoms into patterns that were in many ways like a fence to hold in animals. That’s why we call these structures corrals. The animals were the electrons. And what was cool about this was that we got to trap electrons inside of a really small structure that might be five nanometers across or seven nanometers across, and then to study the properties of these electrons inside these small structures.
And where that ties in to our business is in the future, we are going to be doing computation inside of structures where the quantum properties of the electrons in these structures will become really important, in terms of how they operate. This gave us a leg up into understanding and investigating how electrons will behave in these really small structures, where the wave properties of the electrons were really important.
Salazar: So this quantum corral acts something like a wire?
Eigler: Well, actually, it’s exactly not like a wire. That’s the really exciting thing about it. And that’s one of the reasons that we are exploring these nanometer–sized structures. Let me tell you how it’s not like a wire. We use wire inside a computer chip to send information all over the place, the active transistors to the next active transistors. And if you think about how a computer chip works, we use transistors to do the logical operations, and we use wires to send the information from place to place. The way the wires in our computer chips work, they work kind of like a hose, or a pipe, carrying water. The thing that they conduct is electrical current, instead of water like a pipe. But conceptually, the way that we use them is the same way that you would use pipe, or a hose, to let water flow. So you could get a piece of information from here to there, for instance you could turn the flow of water on and off and on again, in a pipe, and at the other end of the pipe you would see that the water was flowing or not flowing. And we could say if the water was flowing, that would represent a logical “one”, and if the water was not flowing, it would represent a logical “zero.” That way you could transmit information from place to place. Now that works very well, and it’s very conventional. But it turns out that when you build structures that are small enough, the electrons that are flowing in the wire don’t really need to flow. Instead you can utilize their wave properties. When you try to transmit information with waves, it’s very different than what happens when you transmit information by sending water down a pipe, or electrons down a wire. Let me give you a specific example to emphasize that point.
If you think about sound moving around in a room, you can talk to somebody across the table. Suppose you have four people sitting across the table. You can talk to the person who’s on the other side of the table, and the other two people can also carry on a conversation. As long as you know how to listen to the other person across the table from you, you can get what we call two channels of information flowing across the same volume of space in front of you. And that’s because you’re using the wave property of sound. Waves can pass right through one another. But if you’re using water pipes to send the information, guess what? You can’t have the water pipes pass through one another.
So if you look at how we build our computer chips today, you’ll find that we might have as many as twelve layers of wires, built with quite a bit of expense, on top of the active silicon. Why do we have so many layers of wires? It’s because we have a lot of information to move around, and we’re using the wires just like pipes. We can’t let the pipes intersect with one another. But if you can send information using the wave properties of electrons, you can send multiple channels of information right through the same volume of space. We demonstrated that using quantum corrals. That’s something that we hope that we’ll be able to exploit in the future, when we start building really small computer circuitry.
Salazar: And that leads to my next question about something else you’re working on, using molecules of carbon monoxide as something like a domino to flip, and potentially, to use in computation.
Eigler: What I think that you’re referring to there are the molecule cascade logic circuits that we built a couple of years ago. Again, serendipity played a role in our research. We were interested in building quantum corrals, where the walls of the corral were going to be played with carbon monoxide molecules. We had a hunch that carbon monoxide would make some very nice corral walls. And indeed, it does. But in addition to that, my colleague Christopher Lutz was assembling structures from carbon monoxide, when he noticed a curious thing happen. He built a particular–shaped structure, and after a little bit of time one of the carbon monoxide molecules had hopped from one position on the surface to a neighboring bonding site, about one atomic diameter away from where it initially was. And this was a very simple structure that had been built from just three carbon monoxide molecules. So Chris went on to investigate what was the cause of this. And he understood that, and he realized that he could build a longer and longer structure where the hopping of one carbon monoxide molecule would cause the subsequent hopping of another molecule, would cause the subsequent hopping of another molecule, and on and on. So it’s kind of reminiscent of standing up a linear chain of dominos and knocking one over, causing the next one to topple, and the next one to topple, and the next one to topple.
Now Chris is trained as a computer scientist. So when he saw this happen, he says, “aha, that’s information transport.” You can think of looking at the end of the chain of dominos and say that the end domino is standing up, and call that a logical “zero.” If it’s fallen down, it’s a logical “one.” So you can transport a bit of information using dominos. You truly can. And then Chris went on from there. What he found was that there were arrangements of these “molecular dominos,” if you like, that he could use to perform logical operations on information. And he went on from there to discover the full set of logical operators that are needed in order to do computations, just like a regular computer, to go on from there to building a fully operational, three input sorter. That’s the name of a particular kind of logic circuit. These are remarkable. They are far and away the smallest logic circuits ever built, much smaller than anything else going on. But what has really excited us was not that we’re going to be making computers out of carbon monoxide molecules on top of metal surfaces at low temperatures that are operated by a scanning tunneling microscrope. But it pointed a way to the future of computation that would be very different from the kind of computation that most people are thinking about. And that’s a kind of cascade, or domino style computation, but done not through the motion of molecules, but by the interaction of what we call electron spins with one another. And that’s where our research has taken us today.
Salazar: The fun for me in playing with domino chains is in knocking them down, and less so with picking them up. Is that the limitation with these molecular cascades?
Eigler: You’ve got it. That’s the challenge that we face with the carbon monoxide molecule, is that we can do a calculation once. But just like dominos, once they’ve fallen over, you have to stand them up. And with the carbon monoxide molecules, we had to move them back to their original positions one–by–one, which is a little bit time–consuming. But now, imagine instead of using regular dominos, that the dominos that we use had hinges so that they didn’t fall all over the place, when they fall over they fall over in a very controlled way. And suppose that I gave you an anti–gravity button, so that you could press the anti–gravity button after the dominos had fallen, and then all of the dominos would stand back up again. Then you could do another calculation. And you could go on and on and on. So our challenge was to think of a way to reset our dominos, and the thing that looks very promising there is to use the magnetic property of electrons, what physicists call the “spin” of the electrons. This is what gives the electron its magnetism and its angular momentum. And to be able to create a cascade of spin motions, and then reset that cascade using a magnetic field. And if we can do that, then we might have the opportunity to do a calculation, and then reset it, and then reset it, etc …
Salazar: How far are we from seeing something like this in a computer?
Eigler: Yes, we’re making very rapid strides in the direction of being able to create a spin cascade. So a couple of years ago, we took an important step, which was to demonstrate that we could measure how much energy it took to flip the electron spin on a single atom. And from there, we learned how to assemble rows of magnetic atoms and to investigate and learn about the magnetic configuration of individual atoms put together. So we’re now at the point of learning how to engineer what we call the “energy landscape” of very small magnetic systems, and how to be able to create a change in the spin configuration at one end of this magnetic system and to have that change in spin configuration propagate, or cascade, to the other end so that we can use that for transporting information. But what makes this very different from a wire is that in a wire, the electrons move and there’s energy dissipation, or heating, that occurs when the electrons move down a wire. Here, the electrons aren’t moving at all. All that’s happening is that the spins are changing. Now there is energy dissipation. But it’s much, much less than what happens when you move the electrons. And it’s small.
Salazar: How small are we talking about?
Eigler: The limits that we see, our building blocks are individual atoms. And we don’t see building computer circuits that are smaller than an individual atom. If somebody can do that, wow. I think that’s a realistic limit. But we can see building computer circuits, at least in our laboratory, which have length scales on the order of, or sizes of the order two or three atoms across. I should say that things are engineered at the atomic level, at the length scale of individual atoms, or about 2/10ths of a nanometer. That would mean that in the area of one nanometer by one nanometer, you might be able to have two, possibly three or four different logical operations in such a small area. That’s incredibly dense. Now we don’t know how to manufacture that. But you have to start somewhere. Where we start is in our research laboratories. And that’s where we’re at right now. Right now, we’re in the age of exploration, which for us scientists is the most exciting time.
Salazar: You’ve described yourself as a “gizmologist.” How do you balance nuts–and–bolts with the nearly invisible world of nanotechnology?
Eigler: For me, it’s not a balancing act. It’s like breathing. You just do it. First, I don’t think it’s hard to understand. Every problem in physics might seem hard to understand when you stand off to the distance and all you see is the whole thing together. But as you start to build mental models of things, really good physicists find ways of breaking tough things down into very simple nuts and bolts problems. And one of the great moments for any scientist is when you have that moment of insight that takes something that you thought was complex and you suddenly say, “wait a second, this is just nuts and bolts. I know all about nuts and bolts.” And that’s a very powerful, very exciting moment for any scientist. We live for that. That’s the juicy stuff. The gizmologist about me, I love to design and build things. I have since I was a little kid. And also, I love nature. I love asking questions about nature and trying to understand the natural world around me. Everywhere you look in the natural world, whether you look at a very grand scale, or a very small scale, you find extraordinary beauty. And the closer you look, the deeper the mystery, the deeper the beauty. That’s the physicist in me. And it’s also, being a gizmologist and a physicist are completely complimentary. Physicists have always had to design and build their own experimental apparatus because there was no experimental apparatus that would allow them to get answers to the kinds of questions that they were asking. It’s a great long–standing tradition in physics. So being a physicist and a gizmologist are bred for one another. There’s no conflict at all.
Salazar: Thank you for taking time out today. Is there anything else that you’d like to share with the readers of Earth & Sky?
Eigler: The question that I hear that is very widespread that I hear people asking is, “is this nanotechnology stuff real? How’s it going to impact my life? What is the value that’s being created that we hear all this hype about in the media? To me, that’s a very important issue. I wish we had more time, but the short answer is that every time that we explore a realm of what’s physically possible, we find that there are really beneficial and valuable uses. And what we’ve discovered for certain in the last 20 years is that there’s this tremendous wealth of behavior and opportunity that comes from nanometer scale structures. And from that, I conclude that there’s going to be tremendous value.
