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Interview of David Wineland by David Zierler on October 27, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/44983
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In this interview, David Zierler, Oral Historian for AIP, interviews David Wineland, Philip H. Knight Distinguished Research Chair at the University of Oregon. Wineland recounts his childhood in Denver and then Sacramento, and he describes his early interests in math and engineering. He discusses his undergraduate education at University of California Davis and then Berkeley, where Frederick Byron played a formative role in his development as a scientist, and whom he followed to Harvard for graduate school. Wineland discusses working in Norman Ramsey’s lab, and the significance of Dan Kleppner’s demonstration of the hydrogen maser. He discusses his postdoctoral research at the University of Washington where he worked with Hans Dehmelt on making accurate measurements of the electron g-factor, and the opportunities that led to his career at NIST in Boulder. He describes the excellent research environment and instrumentation that made precision measurements for clocks feasible and the important of Shor’s algorithm for his work. Wineland explains the difference of accuracy and precision as those words apply to atomic clocks, and the societal benefits of achievement improvements in this field both for land- and space-based applications. He describes the day he learned that he would receive the Nobel Prize, the collaboration he enjoyed with Serge Haroche, and his post-Nobel work in quantum information. Wineland describes his reasons for moving to the University of Oregon. At the end of the interview, Wineland assesses the current and future prospects of true quantum computing and the societal benefits that this advance could confer, and ongoing developments that can further improve atomic clocks.
OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is October 27, 2020. I'm so happy to be here with Professor David Wineland. Dave, thanks so much for joining me today.
You're welcome. My pleasure.
OK, so to start, would you please tell me your title and institutional affiliation?
Yes. Let's see. Well, I have a couple. My main title is Philip H. Knight Distinguished Research Chair, and I’m also a Research Professor in the Department of Physics at the University of Oregon.
When did you arrive at Oregon?
Yes, I started in January of 2018.
Yes. So, I don't know whether you know a little bit of my history. I was at NIST in Boulder, Colorado for 43 years, and then came here in 2018.
Well, Dave, let's take it all the way back to the beginning. Let's start first with your parents. Tell me a little bit about them and where they're from.
Well, my dad was born in Douglas, Arizona. But his father was a train conductor, and they ended up moving to California. This was Durham, California, a small town. My mother was born in Denver, Colorado. She grew up there and later met my father in Denver.
Where were you born?
I was born in Wauwatosa, Wisconsin. This was during the time of World II. During the war my father was working for Allis-Chalmers, and he was doing mechanical tests on things like superchargers for airplanes, and towards the end of the war, he was working on jet engines, jet planes.
Did you grow up in Wisconsin?
No, no. After the war, we moved back to Denver where my parents had met. My dad was really more interested in civil engineering, so he worked for the Bureau of Reclamation in Denver. In 1947, we moved to California where he became Chief of the Bureau’s Sacramento Design Office. In 1958 he took a job with the California Department of Water Resources and that's where he finished out his career and we still lived in Sacramento.
What were your ages when you lived in Sacramento?
I was born in '44, and we remained in Denver until '47. So, I was 3 years old when we moved to California. I have a sister, she's two years older than me. So that's where we both grew up.
Through 18 years old? You were in Sacramento your whole childhood?
That's right, yes.
Did your father involve you at all in his career? Did you have a good understanding of what civil engineering was as a boy?
I wouldn't say a good understanding. I knew basically what he did; he worked mainly on waterworks, so dams and canals. In fact, in California, there's a famous canal, The California Aqueduct, that goes from Northern California down to near LA, and he was chief design engineer for that and spent a lot of time on that when he was in California. So, I knew basically the kind of things he did, but I didn't know the details.
Did your mother work outside the home?
Before she met my dad, she worked for an insurance company in Denver. And that's where they met. But when we moved to California, she had retired from that to stay home to take care of my sister and me.
Dave, you went to public schools throughout?
And when did you start to get interested in science? Was it early on for you?
Yes, I was first interested in math as a young kid. And I think the thing that really did it for me was when I took a physics class in my senior year of high school. That's when I really got interested in physics. I thought it was cool the way that relatively simple math could explain the world we see around us. I wasn't really locked into physics at that point. In fact, when I started college, I started as a math major. But I was also taking physics classes and became more interested in that.
What were some of your other interests in middle school and high school?
I always liked mechanical things. So as a kid, it was model airplanes. And then after I got into my teen years, my buddies and I were interested in cars and motorcycles. In fact, I got my first car when I was 14. I couldn't drive it yet, but my dad helped me get it, and I tore it down, fixed some things, and built it back up. That sort of thing. I did motorcycles for a long time and raced them, up through my post-doc position.
Through graduate school and as a post-doc, that was what I did on many Sundays.
Dave, how high up in your class did you graduate in high school? Were you near the top?
I was probably near the top. I wasn't the top student though. There were certainly kids that were better than me. The thing is, although I was always interested in school, I had my other interests like cars and motorcycles. So that's how I spent a lot of time. And my parents were very generous in terms of letting me do pretty much what I wanted. My parents were products of the Great Depression. They were lucky they had jobs through the Depression, and they realized the importance of that. So, ingrained in me was the notion that I would go to college, just so I would have a secure job. That was really important. So, if I kept up my grades, they gave me pretty free reign and I spent a lot of time playing with cars and motorcycles.
Dave, what colleges did you apply to? Do you remember?
Yes, I applied to the University of California, and for the first two years I went to the Davis campus, which is near Sacramento. And actually, I applied at Caltech and Stanford, but they didn't want me. My father had gone to Caltech. His family was of very modest means, so he had to pay his way through college. In fact, he went his first two years to Caltech, and then he took a year off to work again, so he could get some money to pay for his last two years. He graduated in 1931.
What caused you to transfer halfway through?
Oh, it's just that although I liked the Davis campus, it had traditionally been an agricultural school and was still was largely that when I attended. Berkeley was the big-time for physics. So, I wanted to be a little closer to that, which is why I transferred.
Now, you said initially, your major was math. Did you transfer over in physics while you were at Davis? Or did you realize you needed to go to Berkeley to do physics?
I don't remember actually when I switched majors. It didn't really matter very much but by the time I got to Berkeley, I was a physics major.
Now, did you get advice from some of your professors at Davis? Did they see abilities in you where they said, "You'd be best served at Berkeley"? Or how did you make that decision?
I don't think there was any particular encouragement from my professors there to switch. I think somehow I just got it in my head that it was the right thing to do. And at the University of California, it was very easy to switch, so I could easily do it.
What year did you arrive at Berkeley?
So, I said two years and two years. Actually, halfway through my third year, I went down to Berkeley, transferring in the winter term to go there. So that would've been start of '64 when I went there. I finished my junior year there, as well as my senior year.
So probably by the time of your senior year, things got to be pretty interesting on campus with the counterculture and the anti-war movement.
Oh, yes. My roommate got arrested. Although I was very sympathetic, I didn't want to jeopardize school, so I stayed away from the protests pretty much. But yes, it was an amazing time. Kind of a surreal time with police around and things. It was crazy.
Dave, who were some of the professors at Berkeley that you really admired or perhaps became close with?
There was one in particular; his name was Frederick Byron. He had graduated from Harvard and was there on a junior faculty position. He eventually went back to the University of Massachusetts. I took a classical mechanics class from him, and it was really a tough class. But he was good at stimulating us to dive in. So, he was probably the most influential person. He'd gone to Harvard, so that's why I applied to Harvard for graduate school.
Were there any formative lab courses or lab work that you became involved with at Berkeley?
No, there may have been possibilities to work in a real lab, but I was too focused on keeping my grades up. So, I didn't really do any of that. There were some laboratory classes, but you didn't really get immersed in things doing that.
Dave, by the time you graduated, did you have a good idea if you wanted to focus on experimentation or theory in graduate school? Or you were wide open to both?
No, I thought I was going to be a theoretical physicist, partly because I didn't really know what experimental physics would be like. But when I went to Harvard, it was pretty clear that to be a successful theorist, it would be best if you were really good at math and I didn't feel I was that good. Anyway, so I looked around at what people were doing in labs, and as I said, at first I didn't really have an idea what it would be like. But it became clear when I started looking at what people were doing in the labs that experiments were a much better match for me. And like I said, I'd done things with my hands outside of school, so it just seemed really appealing to be doing that.
Was there a particular professor at Harvard that you were specifically interested in working with when you applied?
Yes, there was Norman Ramsey, who's a very famous atomic and molecular physicist from the 20th century. And I liked the people that he had as graduate students. It seemed like a real nice, friendly group and the work appealed to me. Around that time, Norman and his colleague, Daniel Kleppner, had invented and demonstrated the hydrogen maser, which is still used as a reference clock these days. That kind of high-precision work really appealed to me.
And what was Ramsey working on? What was his group involved with at the time you joined?
Well, he'd gone through his earlier years, doing atomic and molecular beams. At Harvard he had a couple experiments on molecular beams and as I mentioned, he and Dan Kleppner had recently demonstrated the hydrogen maser. So, he had both of those projects going, the masers and the molecular beams. And, in fact, in the mid-fifties he had written a text called Molecular Beams, and it was kind of the bible for a lot of us to learn about atomic and molecular physics.
What was the big excitement around masers at that time?
As I say, what appealed to me was the high-precision aspect of it. To achieve high precision and high accuracy you had to think of all the environmental factors that were perturbing the atoms and things like that. I don't know, that detective work just appealed to me and it’s how I got started in atomic physics.
What was Ramsey like as a graduate advisor? What was his style?
Actually, Norman was heavily involved in Fermilab when I was at Harvard. I forget what his exact title was, but he was one of the founders of the lab and headed the group that managed the construction and operation of the lab. So, in fact, while I was a student in his group, Norman was at Fermilab much of the time. However, he was very good about showing up for Friday meetings. Almost without fail, he would show up for our group meeting. But that was about it in terms of interaction. He kept tabs on what we were doing and was careful about that. But as far as learning the physics, it was from the other students, and we basically helped each other along.
So when you developed your own dissertation topic, you essentially did it on your own. It wasn't that he handed you a problem to work on?
Well, no, he suggested it. In fact, this was a time where there was a lot of work going on with hydrogen masers and Norman just thought it would be nice to have accurate measurements of all three hydrogen isotopes. So, my project was to build a deuterium maser. It wasn't a dramatic experiment or result, I would say, but it certainly taught me the techniques and was very good that way, I think. And it was completely my experiment, too, which was kind of nice. Although, as I say, my fellow grad students, we were helping each other out a lot and that was really nice too.
Why deuterium? What can a deuterium maser do or measure?
Well, for all three isotopes, deuterium, hydrogen, and tritium, the masers were based on the hyperfine transition in the electronic ground state of the atom. So, the atomic physics is very much the same for the three atoms, but the apparatus was a little different. The main difference with deuterium is that it has a longer wavelength. Its frequency is 327 megahertz, whereas hydrogen is at 1420 megahertz which is fairly close to that of tritium, as well.
Dave, I'm curious, in the world of masers, what were some of the theoretical implications that allowed masers to come into existence? Or to flip that question around, in what ways may masers have furthered theoretical work in atomic physics?
Well, I think the theory was basically understood although not at the level of precision of the actual devices. However, there was some interesting side-experiments we could do. For example, during the time I was working on my thesis experiment, a paper appeared where a group claimed that they were seeing a gravitational dipole moment in a proton. In atoms like the hydrogen isotopes, the basic effect is that if there is a gravitational dipole moment in the atom, its hyperfine frequency would see a shift depending on which way its angular momentum is pointed relative to the direction of gravity. There was a claim that this group had seen this basic effect. That was really interesting to me because the deuterium maser would enable a much more sensitive test of the effect. The experiment didn’t show the effect, but it was just fun to do that kind of test. However, I would say the masers were generally not a big factor for theoretical atomic physics.
Was there any talk at that point of lasers coming into development? Or this would be later on?
No, the earliest lasers existed then. In fact, the first masers were based on ammonia molecules in a molecular beam. These masers were the first demonstration of a maser or laser principle. There's really not much difference other than the wavelength.
Was there a sense that at some point masers and lasers would coexist? Or was there an understanding that lasers would come to overtake masers?
Well, I don't know whether I'd say it that way. In fact, as I mentioned earlier, hydrogen masers are still used as reference clocks. So, these days, clocks based on lasers are more accurate, but it took a long time for that to happen; it only happened in the last ten years or so. So yes, I don't think there was, in some sense, any kind of competition. For a long time, the microwave clocks were the king and laser-based clocks weren't as accurate. There’s also the fact that since they operate at different frequencies a laser or maser might be better for a specific application.
Dave, who was on your thesis committee?
Well, it included Norman and Frank Pipkin. Those were the two atomic physicists. And I'm forgetting now who the third person was. Funny story about this is, the day I was going to make my thesis presentation, I showed up, but Norman hadn't showed up. So, Frank checked with his secretary, and she called him. He was in Chicago! So, we had to postpone a few days.
Dave, how did the post-doc at the University of Washington come about for you?
Well, towards the last couple years of my thesis work, I was reading about things other people were doing. Hans Dehmelt at the University of Washington had been doing some high-resolution, accurate experiments on the spectroscopy of trapped ions and that work really appealed to me. So, I applied to work with him and was accepted. As you probably know, Hans Dehmelt's most famous experiment was a precision measurement of the electron g-factor. So, when I went there, he really wanted to focus on that and he convinced me to work on the electron experiments. This was fine with me because it was pretty much the same kind of physics.
During the time I was there, it became clear that to make really accurate measurements of the electron g-factor, you wanted to have one electron in the trap. If you had more than one, then the mutual perturbations on the electrons would be a problem. So, probably the most interesting thing for me was to be able to isolate single electrons in the trap and start to manipulate them. Although I left before the accurate measurements of the g-factor were made, the single electron business was kind of a precursor to those experiments.
What were some of the practical implications of this research?
Well, this work was maybe of more importance in terms of physics, but the kind of traps we were using for the electrons can also make very precise mass ratio measurements. I would say that's been one of the main applications, these very precise mass ratio measurements between different atoms, different isotopes, things like that. You could maybe argue how practical that is for the high precisions that were attained, I think it was more interesting just for fundamental physics, not really for more practical applications.
How long were you in Seattle total?
I was there four and a half years, a relatively long haul as a post-doc. And then I took the position in Boulder at NIST.
Now, did you enter the academic job market? Was your initial plan to apply for more traditional faculty type jobs?
Yes, that's what I had my heart set on from early on. I got a couple offers during the last two years that I was at Washington. They were from reasonable places, but it was clear that they didn't have money to really support research and it would've been very hard to get going. What was nice at NIST is, first, the person that hired me, Helmut Hellwig, my group leader at that time, was very forward-thinking. Up to that time, the Time and Frequency Division at NIST was very applied. They did calibrations and things like that. But there was really very little basic research. My group leader really pushed to get basic research going in the lab.
So, that was great for me, because for at least the first couple years I didn't have to worry about money. And once the lab got rolling, then we were able to get outside support as well. After that, it was clear sailing. I guess all I'm saying is that it turned out much better for me to go to NIST because of the support. And a big factor was, I think, that my boss was very set on the idea that the lab should being doing basic research as well as the calibrations and things like that. If that support hadn't been there, I probably wouldn't have stayed.
I was curious, in 1975, if your sense was that NIST was fully established in Boulder at that point, or was it still in sort of development and building mode from the big transfer from Washington DC?
No, no, it was certainly firmly established. I forget when the Boulder Labs were established. I think it was 1953. So, the Time and Frequency Division was certainly firmly established by 1975. There were other divisions that were there too, versus being in Gaithersburg on the east coast.
And did you spend any time in Gaithersburg or in Washington DC? Or everything for you was in Boulder?
Basically, everything was in Boulder. I visited Gaithersburg. There would be occasional meetings there and things like that. But all the work I was involved in was really in Boulder.
I'm curious how well-connected you felt to your colleagues in academia, if you felt like, from a research perspective, you would be publishing and attending conferences, basically the same as if you were in a faculty position.
Yes, I think that's right. I never felt any separation. Most of my colleagues outside of NIST were in academia. They never treated us like second-class citizens or anything like that, so I think we were on equal basis with colleagues outside of NIST. And a lot of the physics was very much the same, so there was a lot of mutual interest, I would say.
And did you have any relationship with the physics department at the University of Colorado at all?
Oh, only a little bit. We certainly plugged into the research they were doing, but there wasn't any comprehensive research relationships between our group and those at Colorado during the time I was there.
Did you take on graduate students at all?
Yes, most of our graduate students were from the physics department at Colorado. In fact, we benefitted significantly from a joint NIST/Colorado program that enabled CU grad students to work in our labs. We had students from a few other universities, but most of the students came from CU during the time I was there. That's still true today, the students in the group are mostly from University of Colorado.
And did you have any opportunities to teach? Was that ever available to you?
Yes, I did. I taught a little bit, just more or less for fun. But, as you know, it just takes time to do a good job at that and I wanted to focus more on the research. So, I would occasionally fill in for people when they were away, but no, I never really got into serious teaching.
And given the dual commitment, as you were saying, from your boss between specific measurement, calibration measurements work and also basic science, where did you fit into that in terms of the way you developed your research agenda? Did you work in both fields pretty equally?
No. I spent a relatively small fraction of the time I was there working on calibrations . So, I had the luxury of being able to focus on the basic research, just trying to perfect it.
And who were some of your key collaborators in the early years at NIST?
Well, let's see. When I first went there, there'd been a new cesium beam atomic frequency standard constructed. So, one of my first tasks was to basically do the calibration measurements on it. That took, realistically, about a year and a half to do everything that was necessary. When I was with Hans Dehmelt, we published an abstract for a meeting, which was one of the first two proposals for laser cooling. At the same time, there was a paper by Art Schawlow and Ted Haensch that basically proposed the same thing. What was nice for me is that the cooling could be very important for atomic clocks. The main reason is that moving atoms experience Einstein’s time dilation frequency shift when seen by a stationary observer in the lab. So, by cooling them, you could suppress that shift. And so, fundamentally, there was a real interest in doing the cooling.
So, as I said, my group leader pressed the administration to get support to try the cooling. I worked with a colleague, Bob Drullinger, and he knew about lasers; I didn't know anything about lasers when we started. Anyway, we teamed up together, and the support from my group leader enabled us to get the laser and some basic equipment. This was certainly one of the highlights of my career. It was great working with my colleague Bob, and we could just totally focus on the experiment. Also, what was really nice is that we started with an empty lab, and I think it was only five months later, we had our first result. That was really an exciting time.
What did that result tell you?
Well, I don't think there was any doubts about the basic idea. But it was a big thrill to be able to do it. And, in fact, laser cooling is important. Basically, all high-accuracy atomic clocks rely in part on being able to laser cool the atoms to a very low temperature.
Now, did you have a good idea all of the value that laser-cooled ions would have? In other words, in terms of spectroscopy, in terms of atomic clocks, in terms of all kinds of quantum possibilities? Or do you just do the experiment and then figure out what you can apply it to? How does that work?
Well, it was clear about the value for clocks; again, to reduce the time dilation shift. But we didn’t anticipate all of the future applications going in.
And if you could explain the science there a little bit, why? Why are laser-cooled ions–why is there that obvious value in terms of clocks?
Well, as I mentioned, if the atoms are moving, their internal clocks appear to run slower when we observe them in the lab. The velocities in an atomic beam can be fairly high on a scale that is interesting, and it was hard to calibrate the time-dilation shift out very precisely. So, by just lowering the temperature, you can almost get rid of it. That was the obvious application in clocks and also for very high-resolution spectroscopy. A nice milestone in our group was later achieved with our first optical clock. This project was led by my long-term colleague, Jim Bergquist, and was based on a 282 nm quadrupole transition in laser-cooled, singly ionized mercury 199. After many years of hard work, in 2006, Jim and his team achieved a systematic uncertainty of 7x10-17. This was significant because it was the first realization of an atomic clock that had a lower systematic uncertainty than Cesium.
Where I really got interested in quantum information and related things, was also at NIST. A nice story in connection with quantum information at NIST is that is in the early days of our quantum information studies, my main colleague was Chris Monroe. You probably know that name. He's very famous in quantum information now.
One focus we had was to try to make so-called spin-squeezed states for clocks. They’re entangled states, but they have the characteristic that in principle, they can increase the precision of clocks. One way to say it is if you have N atoms in a particular entangled state, it's like having one atom that runs N times faster. That increases the measurement precision because of that clock rate increase. So anyway, we were trying to do experiments towards making these entangled states. We had a couple of ideas on how to do that. But what really got us going, this was around '95, was that Peter Shor, he's now at MIT, came up with an efficient algorithm for factoring numbers using a quantum computer. This had important implications because some of the most popular encryption protocols, such as RSA, derive their security from the difficulty in factoring large numbers. Peter’s algorithm was one of the first important potential applications of a quantum computer.
In the summer of ’94, there was an international conference on atomic physics in Boulder, and I was one of the organizers. We were able to have Artur Ekert, who is a famous theorist in quantum information and quantum communication, come to the meeting and present a lecture on Peter’s factoring algorithm and quantum computers. In the audience at this meeting were two of Chris’s and my theoretical colleagues, Ignacio Cirac and Peter Zoller from Innsbruck. One of their strengths is that, besides being very good technically as theorists, they’ve also kept their eyes on experiments and they knew very well the kind of things ion trappers could do and what we couldn't do. And so, within weeks after hearing Artur’s lecture, they came up with a scheme for a prototype quantum computer based on trapped ions, and they were nice enough to send us a preprint of a paper describing it.
Since Chris and I had been working on things very close to this, trying to make entangled states for clocks, it was natural for us try to implement their ideas. So, within a month or so, Chris and I with the rest of our team had results on the key part of their scheme for implementing entangling logic gates. That was certainly serendipitous; we were in the right place at the right time. So that was a lot of fun, too, to be able to do it so quickly.
This, of course, is somewhat before the real internet revolution of the late 1990s. I'm curious if you ever thought about some of the ways that quantum information might be applied to the coming internet revolution.
Well, I never really got into that aspect. But even before '95, before Shor's algorithm, people were coming up with algorithms showing how you could implement secure communication with quantum particles and photons in particular. These things were going on a few years before, in the early 90s.
There was a lot of hype with the idea of doing factoring and the government agencies had to be interested because of the implications for secure communication. There were even discussions about, should they have a Manhattan Project to take it over? That sort of thing.
And that didn't happen, which I think was good because there was so much basic physics that needed to be understood before a useful quantum computer could be made. We still don't have a useful factoring machine. It was also good for me because we didn't have to sink our teeth into this very directed practical problem. We could continue with basic research on how, in our case at NIST, to improve the quality of the logic gates and things like that.
Were people, at that time, talking about achieving true quantum computing? Or that was way off into the future?
Well, it got oversold by some people, I would say. But I think for most of us, it was clear it was going to be a very hard problem. And particularly for factoring on a useful scale. For factoring, probably the interesting scale is to be able to factorize at least a 200-digit decimal number, which would be very difficult. We still can't do it with our current quantum computers. So, I think it became clear after the dust settled, after the big hype, that it was going to be a very hard and very long-term problem. And what was good, is that people at the agencies appreciated this as well and they knew it would be crazy to close it off because the problems were just so hard to solve. You still needed to have a lot of open basic research. Fortunately, the agencies have continued to support open research on this and I think it's been great for the field. And I wouldn't have liked to be behind closed doors anyway, so it was great for me personally that they kept open research.
And how did this research lead to quantum teleportation of information with massive particles? How did that come about?
Oh, it was kind of a natural thing to do. First of all, the teleportation had previously been demonstrated with photons. So, the basic physics was pretty much the same. But teleportation could actually be very useful in a trapped ion quantum computer. First of all, to do something interesting, you need a large number of ions. To transfer quantum information around your processor, you can’t use conventional classical means. So, one way to transfer information between different locations in your processor is to have an entangled qubit pair factory and then distribute the partners of these pairs to different locations in the processor. You can then use the teleportation protocol to transfer the quantum information between these locations with the aid of these entangled particles. So, the group made a demonstration along those lines. It still hasn't been done on a scale useful scale for quantum computing yet, but people still think along those lines for the future, I would say.
And this research was directly connected to your work on atomic clocks?
Yes, nearly all the experimental techniques we use are the same. For our qubits, we want their transition frequencies to be very stable so we choose ions that have transitions that can also be good for clocks. And the manipulations we do to prepare and measure the states are essentially the same. We do use different ions for clocks from the ones we use for our quantum computer experiments. This is just because they'll have certain characteristics that are a little easier to handle in one application versus the other. But the basic techniques are largely the same. If you walked into one of the clock labs, it would look nearly identical to one of the quantum computer labs. Very much the same.
Dave, given how precise atomic clocks were already by the time you got involved in this project, what tools do you have at your disposal to be able to measure and confirm that the atomic clock that you developed was even more precise? How do you even do that at those small scales?
It's a combination of things. If you have, say, three precise clocks, you can inter-compare the frequencies. What’s important is that if one clock is fluctuating more than the others, you can find out which one. Then you have to track down what's causing the fluctuations. To achieve high accuracy we need to suppress the environmental or systematic effects that shift the clocks’ frequencies. For this we have to rely on some auxiliary measurements. An example is that, almost always, a clock’s frequency will be shifted by an external magnetic field. And so, we use different transitions in the atom, which are much more sensitive to magnetic fields to measure these fields. After we have calibrated the fields, then we can then calculate how much they shift the clock’s transition frequency.
So, it's a little bit of bootstrapping there, but we can be pretty confident what we're doing with these calibration measurements. There are a lot of similar auxiliary measurements that we have to do. I mentioned the time dilation shift. The best clocks now have an inaccuracy of about one part in 1018. NIST’s laser-cooled aluminum ion clock project, headed by David Leibrandt, has this level of inaccuracy, which is limited by the uncertainty of time-dilation shifts from residual ion motion. Some other groups working with ions or neutral atoms have also reached this level of accuracy.
I'm curious, in the world of atomic clocks, the nuances of the words accuracy versus precision and how those differences are relevant for your field.
Well, it's sort of funny. I think, just in terms of us working on experiments, accuracy is based on how well we can understand the systematic shifts. Precision is based on signal-to-noise ratio, which is improved by using more atoms and averaging longer. There was a debate about the term “accuracy” within NIST a few years ago. Some argued that accuracy only applies to how well you could determine the cesium hyperfine transition frequency since the definition of the second is the duration of 9,192,631,770 oscillations of the Cesium hyperfine transition.
And so, it was argued that the only accurate clocks are cesium clocks. But this seems too restrictive to me and I think most bench scientists view accuracy as a perfectly natural term for describing how well you can understand systematic frequency shifts.
Dave, beyond the motivations of just pursuing the basic science, what are some of the practical applications of achieving greater and greater precision with atomic clocks? What industries or other entities might recognize the value, even at this scale, of improvements in the field?
For centuries, clocks have been used for navigation. In the old days, it was people on ships with their sextants, looking at the angle of stars relative to the local level. From those measurements and also knowing time, they could determine longitude on the surface of the earth. Now precise navigation is done with satellites. An oversimplified picture of how you navigate with satellites is, if you have a network of satellites, and if the clocks on the satellites are synchronized, and you, on earth, have a clock that's synchronized to those clocks, then by some protocol -- it's more complicated than this -- but suppose you agree that the satellite will emit a pulse of radiation every second. Your distance from that satellite is given by the speed of light times the delay time before it reaches your position.
By having a network of satellites whose positions are known, you can get your three-dimensional position. But now that we're starting to get accuracies of 1 part in 1018, that could give us very precise navigation, down to the 1 cm level. One application that people talk about is, for example, if we could navigate down to the centimeter scale, we could maybe detect earth strains: the earth's surface shifting of relative positions separated by kilometers, and this might be an indicator, or a precursor signal of earthquakes as the strain builds. This relative navigation might also be based on measuring changes in the gravitational potential frequency shift at the surface of the earth. For example, the change in gravitational potential frequency shift for raising a clock one centimeter on the surface of the earth is about 1 part in 1018. It's still possible to get very precise navigation using the clock timing, but people are dreaming about using the gravitational frequency shift to be able to measure this as well. Actually, this is an effect that satellites have taken into account for a long time. For example, the gravitational potential frequency shift on the surface of the earth, due to the earth, is about 1 part in 108. So to do precise navigation, we have to understand this effect precisely.
Now, your explanation for the value of atomic clocks in navigation, of course, is an earth-based value. I wonder if you've seen some of your colleagues in astrophysics or astronomy see value in atomic clocks in their efforts to navigate the universe, as it were.
I'm not sure about navigating the universe, but for example, when people look for exoplanets, they basically look for, say, a planet orbiting around a star or some other object. Basically, there's a Doppler shift due to the motion, and they can very precisely measure changes in the Doppler shifts due to the orbiting planet. That’s one use that's come into play in the last decade or so for astronomy and it’s a pretty big area by now. As a side note, when we started thinking about the gravitational potential frequency shifts in our experiments–you can ask, "Well, what's the gravitational potential frequency shift from cosmological bodies?" So just for fun, I estimated the shift from the Virgo galaxy cluster. The Virgo galaxy cluster is about 50 lightyears away, but it's so massive that the gravitational frequency shift on the surface of the earth from this cluster is about 5,000 bigger than the gravitational frequency shift from the earth.
In practice it doesn't usually matter because anything near the surface of the earth will be shifted the same way so that shift drops out of the problem. But it's still interesting to think, how would you define the ultimate frequency standard on a cosmological scale? If there's such huge shifts from these cosmological bodies, it's just an interesting question to ask how you could define an ultimate standard for the universe. But what I was going to say initially was that for GPS, early on they appreciated this as a big effect. I forget the numbers on the satellites, but the gravitational red shift on the satellites is significantly smaller than it is on the surface of the so this is an effect that they have to take into account in comparing clocks and satellites.
Dave, as your research was becoming increasingly recognized with some of the most prestigious awards in science, leading, of course, up to the Nobel Prize in 2012, I'm curious at this time if you thought about, of all of the science that was being done, what was it about your research that was striking such a chord with your colleagues and these committees? What do you think was the significance that got this wonderful recognition?
I don't know that I can give a good answer. I think we all appreciate that physics, like many other fields, can be a bit fad-ish. I think there's certainly an element of that in quantum information. For example, for the Nobel Prize, Serge Haroche and I were cited for being able to manipulate photons and atomic particles and study their quantum properties. That may be a good example of fad-ishness in physics, because I think many people find it interesting just to be able to do such manipulations. Certainly, it's interesting and fun for students, too, because we were able to carry out several “thought experiments” or “gedanken experiments” posed by the inventors of quantum mechanics such as people like Einstein and Schroedinger. For example, we could demonstrate the elements the EPR Paradox and Bell’s inequalities. So, it has been great fun to do that and I think really intriguing for the students.
Did you collaborate with Serge Haroche at all? Were you aware of what he was doing at the time?
Yes, for sure. Actually, we never collaborated on papers or on experiments together. One way to say how our experiments were different but complementary is that we were manipulating atoms with electromagnetic fields from laser beams, and he was manipulating electromagnetic fields with atoms. Interestingly, most of our experiments with atomic ions rely on coupling the internal states of ions to their motion. When we put a single ion in a trap, it is well-described as a three-dimensional harmonic oscillator. A group of N ions in the trap behaves like an N-atom molecule with 3N modes of vibration oscillation. To a very high degree, we can describe each of these modes as a harmonic oscillator.
In Serge Haroche's experiments, the relevant harmonic oscillators are the modes of photons contained in an electromagnetic cavity where the photons can be manipulated by coupling to the internal states of atoms that are passed through the cavity. So, one thing that is interesting is that when we write down the Hamiltonian for the ion experiments, we would often write down the same equations that Serge Haroche would write down; it’s just that his harmonic oscillator was physically different than ours. In a lot of cases, the same physics was at play.
To say a little bit more, I met Serge in the early 1980’s but I’d followed his work through the literature from much earlier. Serge got his physics education in Paris, where they take pride in their very elegant presentations and written work. And so, from early days, his papers were always inspiring to read. More recently he coauthored with his Paris colleague, Jean-Michel Raimond, a great textbook on quantum information using atoms and photons, which is kind of a bible now for people in the field. So, I certainly got to know him through his work and because of the commonality of our experiments. We developed a friendship, too. We have done things with our wives and gone to places together, and so it's been a lot of fun. It was a great thrill to be able to share the prize with him because I've known him and his work for so long.
Dave, what was the day like when you learned the news of the Nobel Prize?
Well, surreal for sure. For some people, it's kind of a sport to stay up and see who wins the Prize, that sort of thing. I had never done that so when the call came to Colorado, it was a little bit before 4:00 AM. The Swedish Academy calls the people that win the award literally about five or ten minutes before they hold a press conference announcing the winners.
Were you asleep? Did they wake you up with a phone call?
In fact, I would've slept through the phone call, but my wife woke up and then woke me up. Anyway, it was a very brief call. They were very nice and congratulatory, and all that. They said they would get in touch again, and they told me I was sharing it with Serge. And so, one of the first people I spoke to was Serge, and we and our wives talked a little bit together. What was interesting during the calls, is that you could tell people were trying to call in from the clicks on the phone. After the call from the Swedish Academy, the next call was from a reporter from Brazil. He also knew there'd be other reporters trying to call, so I just talked to him for a very brief time.
And so, I think I took one other call, then I said to my wife, "This is going to be crazy. I'm just not going to take more calls." So, I was certainly awake by then. And then around 5:00 in the morning, the reporters started showing up on our doorstep, people from Denver and other places. I didn't want my wife being bothered, and I said, "I'm going to go to work." Of course, NIST, not surprisingly, arranged a press conference for that morning, which was broadcast widely, so that took care of a lot of the reporters. Anyway, phone calls were nevertheless still piling up on my work phone and there was a lot of people hanging around that day as well.
One interesting story was that at about 5:30 in the afternoon that day, I was getting set to go home. Before I left the office, the phone rang. And I thought, "Oh, well maybe I'll just take this one call." I'd not taken hundreds of other ones before that. The person asked, "Is this David Wineland?" I said, "Yes" and he said, "This is Air Force One." It was President Obama calling! It was really cool and great to have that happen. I did eventually reply to the other calls that I’d received.
A few weeks later, the White House put on a reception for all the US laureates. And so, my wife and I got to meet President Obama in person there, which was really neat.
Dave, the Nobel Prize offers a level of recognition that's unrivaled, of course. And with that recognition comes a platform, if you choose to step on it, to talk about issues that might be important to you that have nothing to do with the research for which you were recognized. So I want to ask if you thought about that, and if you've developed any strategies for ways that you use that voice and that platform, or ways that you've refrained from using that platform in the years since this recognition.
Well, first of all, I haven't, myself, really instigated any major things like that. I've endorsed a number of statements that are signed by other laureates as well. Many of them have to do with human rights. For example, there’s a really great person from Iran who’s trying to bring awareness to the rest of the world how many people are being mistreated there. So, he's enlisted a lot of us to sign on to messages that he's produced. And there's other similar things like that, that I've done. Usually, I want to check a little bit to make sure the request is bona fide, which takes a little bit of time. But I think most of the things I've been approached on are from very honest people just trying to do good things, so it's easy to sign on, I would say. I wouldn't say I've done a huge number of things like that, but it’s been nice to do.
I'm curious about the longterm effect of the Nobel's recognition. Has it been valuable for your research? Or has it largely been a distraction away from your research?
I certainly get a lot of invitations to speak. Maybe not so much nowadays, but there were hundreds every year for the first few years. I would try to be careful and only accept a handful of those. But part of it is that for someone in my position, I feel obligated, for example, to my fellow atomic physicists, to go around and talk about and promote our field, that sort of thing. So, I think there's that kind of unwritten responsibility and I was happy to do it. But I didn't do nearly as much traveling as I could have. I would do about two trips a month, during that heavy time. I don't get asked too much anymore.
Post-Nobel, for your last six years at NIST, what were some of the projects you were involved in?
It was mainly the two things we've talked about, the clocks and the quantum information stuff. When I left NIST, I still had graduate students and I became their co-adviser; I wanted to see them through their theses. I’ve done that and the last two graduated within the last few months. So, I stayed close to those projects, but I’ve been gradually withdrawing from the NIST work. First of all, I just can't keep up on everything and I don't deserve to just be put on the research papers without really contributing. I'm not directly involved with the clock work anymore. I pulled away from that maybe a year or two ago and am slowly pulling away from the quantum information experiments there as well.
After I came to the University of Oregon, we were able to hire an assistant professor, David Allcock, who was Oxford-educated, and was a post-doc at NIST during the time I was there. Although I'm still working a bit with the people at NIST, I'm mostly trying to help David get going. And as I tell people, I'm his post-doc, so that's my career right now. It's his lab, and I'm helping as much as I can, working with the students and so on, but he's really the leader of the group.
Dave, what were some of your considerations in leaving NIST and joining the University of Oregon Department of Physics?
Well, there was some family considerations. I honestly wasn't looking around, but someone promoted me to this job here, and I already knew some of the people here. So, I came out, interviewed, and a lot about it seemed right. I talked to a couple of other people at different places just to get a sense of the lay of the land. A big factor in coming here is, first, we liked the setting, and I liked the people in the department a lot. So, it was an easy thing to do. I certainly miss my colleagues at NIST, but given the circumstances, it was the right thing to do.
And how well have you transitioned into the traditional professor's life of teaching classes and taking on graduate students?
Well, I haven't done any teaching yet. I've done a little bit substituting here and there, but I haven't really taken on regular course teaching. My main focus right now is to try to help David Allcock get his group going and I'm working with his students on theory that's relevant for the experiments. I do think about teaching and I think it would be fun to do, but I'm just so busy right now and there's not a requirement for me to teach, so I'm holding off for now.
Well, Dave, now that we're right up to the present, I'd like to ask you for the last part of our conversation one broadly retrospective question about your career and then one that sort of looks forward to the future. And so I want to ask first if you can reflect broadly on the ways that NIST provided you with an ideal research environment, both in terms of the collaboration, both in terms of the funding environment, and perhaps most importantly, NIST's unparalleled access to instrumentation and laboratory work. In what ways was NIST generally the ideal place for you to do the kind of work that you wanted to do in your career?
What I found was, as I said, the person that hired me in was very supportive. But I think more generally, the people I had as bosses over the years were all extremely supportive. One person we all really liked was my former physics laboratory director, Katharine Gebbie. To paraphrase, her motto was, "Find good people, support them, and let them go." And so, it was like that for all the time I was at NIST. I just felt totally supported. I mean, we couldn't do crazy stuff, but basically, we could decide our path and having that freedom was really great. We always got support from NIST, but we also got support from the outside. Being at NIST, we couldn't get support from NSF for basic research. They just kind of have a rule about that. But for DOD agencies, they were very supportive.
Well, Dave, for my last question, I want to ask, looking forward, there's so much excitement right now in the sense that we're really on the cusp of true quantum information systems and true quantum computing. And given how central your research is to these advances, I wonder if you could sort of use your powers of extrapolation a little bit and explain, what's it going to take to get from where we are now to true quantum computing? How will we know when we get there? And what's that going to change for us as a society?
Well, those are pretty sweeping questions. I think one thing to say is it's still a very hard problem, but I think it's going to happen. The killer application that got all this going in '95 was Peter Shor’s factoring algorithm. However, he feels that the classical cryptographers are now coming up with encryption that, as far as he knows, a quantum computer can't crack. So maybe that original application will eventually go away. Of course, there's presumably lots of stored data that could be decoded, so a quantum computer might still be useful in that sense. In any case, I think we’re still far away from useful decryption.
One thing a lot of us think about, certainly not an original idea, is that maybe we can use our "quantum computers" to do simulation. Still a bit of pie in the sky, but just to give you the gist of things people think about is that, maybe with our quantum computers, we could simulate the quantum dynamics of a molecule that might be useful in drug therapy, something like that. And so, a dream is to be able to maybe have this quantum computer that would allow you to simulate the action of these potentially useful molecules without actually having to synthesize them in the laboratory. That would be a wonderful application, I think. Another thing people think about is the use of quantum computers in finance and things like that. I can't really comment much about that.
Another thing, which is maybe one of the original proposed uses, is along this idea of simulation. There’s a story about Richard Feynman that apparently in the early 80s, there was a meeting on reversible classical computation that he attended. -- This story may have gotten expanded over the years -- but supposedly he was asked or made the statement that the ideal reversible machine is a quantum system. He’s also recognized as being one of first people to say that a quantum machine could efficiently simulate other quantum systems. He was interested in complicated many particle–nucleons, describing the dynamics there. But there are all these other potential applications I think other people are thinking about. So, I would say many of us think that simulation may be the most important application of quantum computers the future. And we're not there yet but, as you know, there's now a lot of companies trying to develop quantum computers. Some are well-established like IBM, and Honeywell recently formed a trapped ion group. My former colleague, Chris Monroe, has co-founded a startup company, IonQ, for trapped ion quantum computing which should be very competitive.
You’ve probably heard the term quantum supremacy. Recently, there was a group at Google that implemented an algorithm that used all the horsepower of their Josephson-junction quantum computer to perform the algorithm. They could show that the algorithm was much more efficiently run on the quantum computer than simulating it on a classical computer. The algorithm wasn’t practically useful, but it established the power of quantum computers over classical computers for certain problems. I think what'll really set things going is if someone with their quantum computer can solve a problem that tells us something new about nature or solve a useful problem that is intractable on a classical computer. I don't think it's unreasonable that this stage will be reached in the next decade. Maybe not, but I think there's a real chance that that's going to happen.
It's amazing how much support there is within the private companies, but now also from the National Quantum Initiative. There's a huge amount of money that's going into the support of basic research so that's been a big step forward. As I say, I'm optimistic we will have useful machines in the longer term. It may still be a ways off but it might even happen in my lifetime. We'll see.
And, Dave, just to establish a historical marker, to be clear, there's still room for improvement with atomic clocks. They can still get better even from where they are now.
That's right. In principle, there's no limit. The two requirements are precision and accuracy, as we discussed before. In the NIST single ion experiments it can take many days of averaging to reach a measurement precision of 1 part in 1018, so increasing the number ions will be of significant practical value. This is underway in several labs. Increasing accuracy is just a statement about how well you can control the systematic perturbations to the transition frequency. In principle, there's no fundamental limit to how well you can do there.
Always room for improvement.
Oh yes, for sure.
Well, Dave, I want to thank you so much for spending this time with me. It's been great to have you be able to talk about your contributions over the course of your career, and this will just be a wonderful resource for historians and all kinds of people who will access this in the Niels Bohr Library. So, thank you so much for spending this time with me. I really appreciate it.
One thing I did want to say, it was part of your earlier question, but I didn't get all the way through. I think one of the nice things about NIST is they really supported the group environment. For example, you hear a lot of stories where, at universities, they want people to become individual stars. I think NIST was very good in terms of supporting groups. It worked great for me and our group, I would say, because although I was officially the group leader when I was there, I wasn't the guy on high giving orders, and we always ran things by committee. And I think that was just a great situation. We got along very well, I made some great friendships with colleagues, and the congenial attitude made for such a nice environment.
Which, of course, suggests that the Nobel Prize, in some indirect way, was a recognition not just for your work but for your work in the context of the collaborations that you had at NIST.
Oh, yes, absolutely. No question. That's right. In fact, the work the Prize recognizes would not have been realized without these collaborations. I wish there was more time to highlight the contributions of all my colleagues at NIST; there were many, all important to the success of our group.
Right. It's important to get that for the historical record, so I appreciate that as well. OK, Dave, thank you so much. It's been great spending this time with you.
Yes, well thank you. I enjoyed it too.