Anthony J. Leggett


Interview, December 2003

Interview with two of the 2003 Nobel Laureates in Physics, Anthony J. Leggett and Alexei Abrikosov, 9 December 2003. The interviewer is Joanna Rose, science writer.

The Laureates talk about inspiration, the necessity of experimenting (2:18), what makes people become discoverers (7:44), quantum mechanics (12:07), their views on quantum computers (16:02), and challenges for the future (19:01).

Interview transcript

Professor Alexei Abrikosov, and Tony Leggett, welcome to the Nobel e-Museum and also to this interview. We’re very happy to have you here. I’d like to ask you the first question. What are the sources of inspiration to you, as a scientist, Dr Leggett?

Anthony J. Leggett: I think that’s a rather difficult question to answer, actually. I think in the scientific discovery, luck plays an enormous role, but I think one thing one can be fairly sure about is that if you’ve not been thinking about the problem continuously and perhaps even when you’re lying awake at night for some time, perhaps some weeks or even some months, then it’s unlikely that you’ll get the sudden flash of discovery that makes it work.

And what would you say, Dr Abrikosov?

… for me the inspiration was always experiment …

Alexei A. Abrikosov: My answer, for me the inspiration was always experiment. Some experimental facts which were strange, could not get an immediate explanation, and so on. These were always my source of inspiration, and I think that only that. Yes, I am very closely connected to experiment. Not mathematics, not models, nothing but experimental data. And so of course after that, what Tony said, the thinking and so on, even sleepless nights, that is of course how it comes. However …

It’s a part of scientific research?

Alexei A. Abrikosov: Yes. However, my ideas I just take from experiments.

Anthony J. Leggett: Yes, I would certainly agree with that. I always find that the main stimulus to theory is some curious experimental result that seems totally outrageous and unnatural. And one tries to understand it.

So somehow the experiment is ahead of the theory.

Anthony J. Leggett: Often it is, yes. Then if one’s lucky, one may be able to make predictions about some experiment which has not been done, one would like it to be done and come out the way you say.

But there is one problem in your fields of research in super conductivity, I would say, that you can’t predict the super conductors that would work in high temperatures, like room temperature. What do you think about that?

Alexei A. Abrikosov: You see, there was some experiment which was actually performed in the United States by an American physicist and a Russian visitor, which inspired these experiments where they tried to find high temperature super conductivity. They failed eventually. However, they inspired me to some extent, and therefore I even published some model, how high temperature super conductivity could be achieved. No such, actually, it was not achieved on that path but actually nobody tried to find it on that path, and so therefore I still have hopes that that is at least one of the good paths for searching high temperature super conductors.

And Dr Leggett, what about prediction?

Somewhere out there I would take a large bet that there are substances that will be super conducting at a room temperature …

Anthony J. Leggett: Well, if you … there are about 100 elements known, if you consider a compound which involves six of these elements, then crudely speaking there are, let me think, a trillion such compounds. Nature has never made most of these compounds. We will certainly not be able to make most of these compounds in any reasonable time. Somewhere out there I would take a large bet that there are substances that will be super conducting at a room temperature. We just don’t know where they are in this immense space. Once we have a generally accepted theory of cuprate super conductivity, I think we may be in a much better position to go and look for them.

So as a theorist, you have a lot of work to do for that. You can’t rely on experiment.

Alexei A. Abrikosov: No. I can tell you the following, that it happened both in the past. People first discovered super conductivity was of course an occasional discovery. People did not expect anything like that. And in many cases, and with other super conductors, it was also, in most cases, it was like that. But not always. And just with these super conduction cuprates it wasn’t like that. Namely, Alex Müller, with whom I spoke many times on that, and discussed this subject, Alex Müller had an idea, actually …

Also another prize winner.

Alexei A. Abrikosov: … had an idea, yes, and he actually knew what he was looking for. It was a pretty detailed, I would say, expectation. And as soon as such a substance appeared it was made by Raveau in France, so he thought that’s it, and so how to transform that substance which was a semiconductor, how to transform it into a super conductor, that he already had in mind. And he did that with the help of Bednorz who was a crystallographer with him, he did it in a pretty short time, I would say. And so he got it, and so therefore Tony is correct how many compounds, possible compounds there can be.

So therefore I think that the reality is that people must have some guiding idea that reduces enormously the amount of such experiments or checks and so on. And after that, so therefore I am optimistic in this respect. I think that the high … the room temperature super conductivity will be discovered much earlier than people think. What concerns the cuprates, contrary to Tony, I believe that I understand everything about them, and can explain all the phenomena which are known about them, and so on, and on the basis of my knowledge I can say that these substances are already exhausted and we cannot expect to get much higher critical temperature with these substances.

Yes. So you have to find something entirely new. Do you agree with that?

Alexei A. Abrikosov: But with some idea.

Anthony J. Leggett: Well, I think, yes, I agree with part of that, at least. I would tend to, without wanting to express myself about whether we at the moment have a satisfactory theory or not, I would tend to agree with the belief that within the cuprate family it’s not very likely we’ll get much higher transition temperatures. What I do believe, however, is that there may be ways of understanding the cuprates, which will lead us to other classes of materials which might be room temperatures in the conductors. So I’m also optimistic.

So maybe, somehow you need the genius. I’d like to ask you what is it that makes some people do the discovery and others who work as hard as the discoverers don’t do that?

Anthony J. Leggett: A large element of luck. Somehow, I suppose the people who do make big discoveries are ones who somehow manage to free themselves from conventional ways of thinking and to see the subject from a new perspective. But how you quantify that I wouldn’t know.

I understand.

… there must be luck, of course, definitely, but there must be also something else …

Alexei A. Abrikosov: I can tell you from my point of view it is the following: Of course, there must be luck, of course, definitely, but there must be also something else. And this else is the knowledge. A person must keep in his brain a lot of, I would say, different knowledge about different materials and so on. He must have that all in his brain, and if he has that so then at the proper moment it will click. And I can give you the same example of Müller, actually, how he got is idea. He worked all his life on perovskite ferroelectrics.

Ferroelectrics having a perovskite structure, and he learned a lot about these substances, and he knew that there exist different phase transitions, very interesting phase transitions, and he learned quite a lot about that. But that subject was interesting for a limited amount of specialists, so in most cases he met only, as he told me himself, he met only boredom, polite boredom, I would say. But studying that, he actually knew everything about perovskite materials. And he actually managed to develop some intuition within these materials. And when one day, a substance was discovered, it was not these /- – -/ cuprates, but it was vismut lead and what else? There was strontium oxide. Yes. Such a thing. So it was a perovskite, and it was a super conductor. And this super conductor, it had two strange features. A pretty high, for those days, critical temperature, something like 14 Kelvins, and on the other hand a pretty small density of electrons.

And according to the Bardeen-Cooper-Schrieffer theory, the higher the density the higher the critical temperature. And that was strange, and so therefore he tried to understand how this could happen, you see? And so, and it was a perovskite material and so on, and so therefore he invented some idea of explanation, that actually the reduced electron density, it reduced the screening of the electric charges and in this way it enhanced interaction between electrons, which led to the formation of /- – -/ cuprates. That was his explanation. And so then he started searching such materials, so that was the thing. So his knowledge about the perovskite actually was very important for just bearing that idea.

It’s also good luck if something doesn’t work as you expect, as I understand. What would you say about that?

Anthony J. Leggett: Well, yes. Again, some of the most stimulating experiments, to a theorist, are those which don’t come out as you confidently expected them to.

I have another question. It has been said that in super fluidity, for example, you can see quantum mechanics in macro scale. You can actually see quantum physics …

Anthony J. Leggett: Yes. Well, I think one has to be a little careful about that statement, because the statement that one can see quantum mechanics on a macroscopic scale is very ambiguous in its meaning. In the sense that in a super fluid or a super conductor you have a very large number of atoms which are all, or electrons, which are all behaving similarly, and therefore effects which normally occurred at atomic level are enormously amplified. That certainly is true in super conductors and super fluids.

The famous example, of course, is a cat inside a closed box …

However, there is a much more dramatic prediction, namely that under certain circumstances, if you apply quantum mechanics consistently, you reach a description of the world in which there is a quantum super position of two macroscopically different states. The famous example, of course, is a cat inside a closed box. If you apply quantum mechanics consistently to the situation you appear to arrive at a description in which that cat is neither alive nor dead, but in a quantum super position of the two states, until some further observation is made. And that’s very different …

You mean that the cat is both alive and dead?

Anthony J. Leggett: Well, it’s difficult to express the result in classical terms, but if you take the interpretation of quantum mechanics seriously and you apply the same interpretation at the level of the cat as you do at the level of the atom, then you do seem to reach the conclusion that it is not definitely in one state or the other until observed. And that, of course, is the famous quantum measurement paradox or Schrödinger‘s paradox. That’s a very different situation from what one normally gets in the sort of standard applications of super conductivity and super fluidity.

I understand. But what about quantum mechanics? Isn’t it bizarre that you have those super positions somehow, or whatever you call it, but somehow it doesn’t follow the  logics.

Such an object should not exist according to classical laws …

Alexei A. Abrikosov: I must say, I am in a sense much simpler. The existence of liquid helium that is actually at low temperatures and that it doesn’t solidify at ambient pressure is a quantum phenomenon. That is. It is a paradox. Such an object should not exist according to classical laws. And so, already here it starts.

Anthony J. Leggett: Well, yes, I would agree. And I think that is one of number of cases one could quote in which one sees in one sense or other the macroscopic effects of quantum mechanics. But I’m including the difference between a liquid and a solid, as a macroscopic difference. But I do think there’s a big difference between this kind of case and the genuine Schrödinger’s cat kind of situation, which is one which we have not yet been really able to probe directly in experiments, although we’re working towards it.

But there have been experiments done in York, for example, that actually show that you can have two states at the same time.

Anthony J. Leggett: Well, what they actually show is that everything is consistent with that assumption. They’ve not shown that that is a necessary assumption, and I believe myself that it is very important to go on and do a further, a new generation of experiments which, if they do agree with the prediction of quantum mechanics, will definitively exclude the hypothesis that this object was definitely in one state or the other when it was observed. I believe such experiments are possible and there are people who are thinking about doing these experiments.

There was a big problem even for, I would say, the most famous scientist in the world, Albert Einstein; he could never accept quantum mechanics, because it was too bizarre for him. Do you think it is real, somehow?

Anthony J. Leggett: I personally think it’s entirely possible that in the year 3000 we will still believe that quantum mechanics is the whole truth about the world. If we really do still believe it in the year 3000, then I think in some sense our attitude towards the physical world at the everyday level will be radically different from what it is today, because we will really have had to face up to this weirdness, which by that time I’m confident will have been amplified to the everyday level. I think it’s at least equally probable and perhaps more so, that as we go from the level of the atom to the level of the cat, we will find that somewhere along the line quantum mechanics breaks down and some new theory of which we can have at present no conception will take over. I am personally hopeful that it’s the second thing that happens.

The year 3000, so it’s almost 1000 years ahead. OK. What about quantum computers? It would be also …

Alexei A. Abrikosov: What Tony was telling you, it was exactly about quantum computers.

That’s what you think?

… there is no reason in principle why one should not build a functioning quantum computer …

Anthony J. Leggett: Well, if quantum mechanics does describe the whole universe at all levels, then it seems, as far as I can see, that there is no reason in principle why one should not build a functioning quantum computer. I think, however, one may well find that the practical difficulties of doing that are just so enormous that in the end people will conclude that it just isn’t worth it. That although the price tag on a quantum computer that can factorise, say, a 500 digit number, is very large, it’s not infinite, and at some point people may just conclude that it isn’t worth the effort.

It will be too expensive, you mean?

Anthony J. Leggett: Yes. Well, or just take too long and involve too many people etc, etc, yes.

Do you have any other guesses?

Alexei A. Abrikosov: No, no. I don’t. Somehow, this topic, I never loved, and so therefore I always decide, I never go to conferences on quantum computing, and so on, so that’s not for me. That’s not for me.

It’s the weird, or too far from experiment, or why?

Alexei A. Abrikosov: Well, it’s far from experiment, and from my point of view, it’s dull. I don’t know, my taste is so, I like objects, you know, I can see and I can feel them.

Anthony J. Leggett: I would think it’s probably fair to say that at least right now the challenge of quantum computing is not throwing up any very deep new conceptual questions. It’s a matter of in some sense engineering, and so whether it’s a matter of taste, whether you regard that as interesting or not.

I understand. So what is the challenge for the future, do you think, in your field?

Alexei A. Abrikosov: In my field, first of all, if you speak like that, that we have many challenges, actually. And every time I am working on something and that is maybe a small problem from your point of view, but usually one should not divide the problems into small and large problems, because every small problem can become a large problem, or eventually, you know, develop into something. So therefore one must just, if one has a problem, one has to solve it. And that’s all. That is the main important thing. Of course, the general challenge is high temperature super conductivity, room temperature super conductivity, in my field at least, yes, but however, I understand very well that I am alone unable to solve it. Yes, and so it requires an effort of many people and for some time, yes, and experimental efforts, actually, not just theoretical attempts, yes? A theorist can give an idea where to search. However, he cannot predict that just this and this substance will be the one. No. No way.

… the funding system for science for this particular thing must be changed entirely …

And so just when I was in Washington, there was such a session that was dedicated to 50 years since Eisenhower gave a talk at the United Nations about peaceful applications of atomic energy. So then I said there, I spoke about it, room temperature super conductivity, and I said that in order to reach that goal, and it is reachable, I am absolutely sure about that, then the funding system for science for this particular thing must be changed entirely, because it is a long term project and you cannot expect immediate, immediate success and you cannot even predict when that success will happen. However, if you conceive that topic is solvable and so on, so then, you must just give money for that, and people will do research, yeah, and that’s all, yeah? And then Ray Orbach, who is the head of basic energy sciences in the Department of Energy, so he said: I heard what you said, and I will think about that. And so he definitely has some positive thoughts about that.

So maybe soon we’ll have high temperature super conductors.

Alexei A. Abrikosov: Yeah, maybe. It was …

It depends whom I ask.

Alexei A. Abrikosov: I am sure that it is only a question of funding and a question of money. Because, you know, people work on these things which are well funded. For example, now it is nano science, it is called, nano science. Nano science, it is well funded, although not everybody really understands what exactly they have in mind under nano science. There exists miniaturisation, for example, which is necessary for electronics, or there exists granule materials which have new properties compared to bulk materials, and so on. So people really don’t understand, but if they can in their proposals write the word nano, they know the chances for funding increase.

Yes, the politics of science is far away from science. Well, do you want to comment on that?

Anthony J. Leggett: No particularly, no …

OK. So thank you very much for this interview. I’m happy that you took your time with us. Thank you.

Interview, December 2003

Interview with two of the 2003 Nobel Laureates in Physics, Anthony J. Leggett and Alexei Abrikosov, 9 December 2003. The interviewer is Joanna Rose, science writer.

The Laureates talk about inspiration, the necessity of experimenting (2:18), what makes people become discoverers (7:44), quantum mechanics (12:07), their views on quantum computers (16:02), and challenges for the future (19:01).

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