Q & A with F. Duncan Haldane, 2016 Nobel Laureate in Physics

The Daily Princetonian sat down with University professor F. Duncan Haldane. Haldane was awarded the 2016 Nobel Prize in Physics for "theoretical discoveries of topological phase transitions and topological phases of matter." He joins the likes of Philip Anderson, Joseph Taylor, Daniel Tsui, and David Gross as University faculty members who've received a Nobel Prize in physics. Haldane is the Eugene Higgins Professor of Physics.

The Daily Princetonian: As you mentioned in an interview with the Royal Swedish Academy of Sciences earlier today, "we have a long way to go to discover what's possible. A lot of these things were things one wouldn't have initially dreamed were possible." What dreams do you have for the future of the emerging field of topological physics? How do you hope to see your discoveries applied in the future?

F. Duncan Haldane: Well, I’m not sure how far it’s going to go, but it’s taking off in lots of different directions. I mean, my own thing, which I would like to see turned into something really practical, is that you can make signals and information and energy flow around the boundary of something in one way only, which means … you can make the boundary curve and do all kinds of things and it doesn’t disturb it, so you have a kind of almost guaranteed way of sending stuff without the information being destroyed. That’s the most remarkable thing. That was already discovered in what’s called the “quantum Hall effect,” but there you needed these giant magnetic fields, a huge institution with giant amounts of power and a tiny little sample.

The remarkable thing, which, I guess, is the second of the two things I kind of cited, was that they just didn’t need all these giant magnetic fields, and you could actually get it just to the property of a kind of crystal structure. The fact that you could get all these interesting effects, just as a kind of materials property in principle, was what generalized everything to just thin layers to three-dimensional systems and all the stuff that Bob Carver and people are making here. But this thing, where everything goes in one way without the possibility of turning back, has always appealed to me and is something that should have some useful technological [application], maybe for transmitting bits of information between units of a quantum computer or something. And so created materials now start to do this.

Now, people have made stuff that should do it with light also. So far, they’ve made it by building the crystals to be about the centimeter size ... and it works with microwaves. And I think that’s probably the most potentially useful thing, because to be able to send light beams around rather bent tracks — light usually wants to go in straight lines — you could put it in optic fibers and they can bend a bit, but there is always some chance of getting scattered. If you make a kind of topological optic fiber that couldn’t — that didn’t — degrade, that would be something amazing. And right now, the principle is being demonstrated by building things about little units and making a crystal where the basic structure is about [the size of a quarter], rather than light, which is microwave scale. So I would be very happy to see something emerge from this so-called topological photonics, which is kind of one of my babies.

Otherwise, we’re trying to understand something called entanglement in quantum mechanics, and that’s turned into one of the things I started quite recently, about five years ago. That was something called the “entanglement spectrum.” So that’s one of the most deeply mysterious areas of quantum mechanics and that’s one of the most actively researched areas right now — the issue of entanglement, which is what Einstein calls “spooky action at a distance” that he didn’t like — Einstein came up with the idea of entanglement to prove that quantum mechanics was wrong. He was saying quantum mechanics predicts this weird stuff, which can’t possibly be right. But his proposed experiment was great, and quantum mechanics does do all that weird stuff, right? So we don’t really know how far we are going to go with quantum technology, but I think quantum information processing is in its very infancy, but I think it’s got a long way to go, and it’s got a lot of potential. And a lot of people are putting money into it right now.

Engineering departments, our [electrical] engineering department, has people working with the thing. So it’s really spread out from physics ... Mathematicians are working on this too, trying to invent ... the programming language for quantum computers and protocols and things. So there’s all kinds of developments in this and somehow there is a very fruitful marriage of new ideas coming from quantum information theory and material theory. It’s been a very fruitful cross-fertilization.

DP: You have been with the University since 1990. How has being on Princeton’s campus affected you? Do you find your work is at all shaped by the presence of students in your daily life?

DH: It’s a great department, great colleagues, great students, and very supportive. We’ve managed to build up — when I came, there was not so much condensed matter … Now we’ve built up a very focused group on all these new kind of physics. Now the Chemistry department moving next door is very good, because we have a very fruitful collaboration with people in chemistry who are making these materials. So for all this stuff to progress, it actually needs cooperation; there’s an underlying deep mathematical principle to this, but rather abstract. Then there are these kind of toy models which could demonstrate the system — you could actually do a calculation with and show this stuff could be real. And then there are the people who transform this into actual — you know, they grow some crystals that do this unexpected stuff and that’s been a very fruitful collaboration. So Princeton’s built up a very good group and we have a lot of graduate students that were mentioning this stuff. It’s a very good atmosphere.

DP: You said this morning that you consider topological physics to have been “completely overlooked” in the past. What initially fueled your pursuit of this lesser known field?

DH: Well, I stumbled into it. I mean I found with these little toy models, simplified models of matter and when trying to understand how they behave, I discovered the things did something strange. I didn’t know at the time this was topological, right? So I discovered they did some quite unexpected things, were possible, and then later it merged — both my work and David Thouless’ work too. And the kind of mathematical — this stuff was very robust, it was stable against messing it up, and it turned out to be that the principle was this topological stability. There were various kind of mathematics — so a lot of very powerful mathematics about this, very abstract mathematics had happened. In fact, earlier no one had made a connection to the physics. Then there was a huge insight when someone pointed out that this stuff was behaving that way because it related to a mathematical structure. So, I guess the mathematics is too complicated to an abstract, a mathematician probably wouldn’t have known what this actually meant, this abstract theorem.

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So it turned out that the underlying principles then started emerging, [about] why these things were possible. So it required one to interact with others — in fact, I got inspiration from talking with [Edward] Witten [GS ‘76]. I learned something about particle theory which I didn’t know and turned out to be precisely relevant to these things, and then there were these — David Thouless learned from. Actually, from someone who used to be, at the time, at Princeton; Barry Simon [GS ’70] was a mathematical physicist professor who was joint with mathematics and physics at Princeton at the time. He actually pointed out to David Thouless, the other person who got this prize, that what he found could be translated precisely into a known and deep mathematical example. It takes lots of different people to come together, and I don’t think anyone could do it by themselves.

DP: What advice would you give to students looking to research emerging fields?

DH: I would say go with — well, if you go with the exciting field there’s more competition, of course. Of course, the thing to do is go with the field that is going to become exciting next year, then you’re in it at the beginning. It’s hard to say. I think you just have to follow your gut feeling, basically. When you start research, you don’t know where you’re going to go, you don’t know how it’s going to turn up, and a lot of it is grasping the opportunity. One hopes the opportunity, something, comes up. My advice to people is to go to lots of seminars, when people come around, at least a few of them will be interesting, right? Some of them you might not even understand the work, but I don’t think it matters. You’ve got some feeling about what people are interested in and you need to get a feeling for what’s going on and what’s good at the time, and maybe things work out. But I think you just have to follow your instinct in some way.

DP: Is there anything else you would like to add?

DH: Take full advantage of Princeton, of the opportunities. At Princeton, we have lots of people coming through, giving talks, and you’ve got a lot of exposure to what’s going on. I suppose when you’re starting out, talk to as many professors as you can. It’s hard to say, you can’t really tell what’s going to happen, and life is making the most of whatever turns up, really. I think you can’t really go wrong with a Princeton physics degree, even if you end up on Wall Street. It gives you a lot of valuable training for thinking about problems. And maybe you don’t end up discovering some fantastic new thing. Whatever. Lots of people end up doing lots of different jobs after physics, but it gives you some kind of training for looking at problems and trying to think about them.