To work in quantum physics, you have to have an open mind with regard to what is possible and what is not possible. … You have to be open to the fact that the impossible may indeed be possible.
Physicists always try to simplify things as much as possible in order to understand the essence, and then we add the complexities.
—Pablo Jarillo-Herrero
Pablo Jarillo-Herrero is Mitsui Career Development Associate Professor of Physics, MIT, and Principal Investigator of the Jarillo-Herrero Group.
He received his master’s degree in physics from the University of Valencia, Spain, a second master’s degree from the University of California, San Diego, and a doctorate from the Delft University of Technology in the Netherlands. Subsequently he moved to Columbia University, New York, where he worked as a NanoResearch Initiative Fellow.
His research interests lie in the area of experimental condensed matter physics, in particular quantum electronic transport and optoelectronics in novel low-dimensional materials, such as graphene and topological insulators.
Among the awards he has received are the Spanish Royal Society Young Investigator Award (2006), an NSF Career Award (2008), and an Alfred P. Sloan Fellowship (2009).
Adolfo Plasencia:
Thank you, Pablo, for finding the time to speak to me again.
Pablo Jarillo-Herrero:
A pleasure.
A.P.:
Physics, I believe, is something of a vocation for you. You began studying physics in Spain, but a professor encouraged you to move on to discover new worlds. You took his advice and went on to study high-energy theory, the physics of particles, which is what you had originally been hoping to do. That’s why you left Spain for the University of California, San Diego, and then went on to the University of Delft, where you came across nanoscience. What was that journey like, from high-energy theoretical physics, on the cosmological scale, to the experimental nanoscience of the physics of condensed matter, on the miniscule scale? There are many worlds within physics, aren’t there?
P.J.-H.:
I wouldn’t call them worlds, but yes, there are many disciplines within science. When I was in Valencia, groups conducting research in condensed matter physics were not as important as the theory groups. I arrived in San Diego and began to go to seminars on other subjects. I realized that other disciplines interested me more, so gradually I became convinced that I had to take the step toward condensed matter physics. Once I had taken that step, I took another giant step, toward the experimental side rather than the theoretical side. That’s what took me to Delft. There I was carried away by experimental condensed matter physics and in particular nanoscience, because there was a research group, one of the best groups in the world, in that field. So yes, it was quite a roundabout route to arrive at the physics of nanoscience. But it was good for me, so I'm very happy.
A.P.:
I know that you use mathematics a lot. Moreover, I believe you like mathematics a lot but are more excited by physics than by mathematics. What does physics have that mathematics does not, in your opinion?
P.J.-H.:
Mathematics is a tool for understanding physics or to understand many other disciplines. Physics has a connection with reality and with the world. And sciences are sciences because of the scientific method, so that one can experiment and corroborate whether the world is as it is, or not. In mathematics, you can come up with a perfectly consistent theory but one that has nothing to do with reality, and what I like about science is that it is contrastable with reality. I mean, mathematics is fine and good, and without mathematics we could do almost nothing. It is the language that science uses to describe reality. In fact, many scientific theories, before making a discovery that later proved falsifiable, were perfectly coherent within their mathematical “apparatus.”
I believe that for physicists, mathematics is a very useful tool. There have been many advances in physics that have been purely due to mathematics. In particle physics there is a famous example in which a series of particles had been discovered and physicists saw that this corresponded to a certain type of mathematics. It turned out that mathematics itself required other particles to exist, so they were then sought and found. This was a triumph for mathematics. Mathematics is a great tool, but what I personally like about science, and about physics in particular, is the connection with experiment and reality.
A.P.:
One wonderful day for you—correct me if I am wrong—you came across quantum mechanics. I have heard you say that at the beginning of the twentieth century, a revolution broke out that was not only scientific but also conceptual and philosophical, the revolution of quantum mechanics. Can you explain why?
P.J.-H.:
That’s quite difficult to explain, but it is conceptual and philosophical. The most difficult thing about quantum mechanics was, at the beginning of the twentieth century, to convince people that the world and the universe were as we now believe they are, and that they also had these rare properties. Quantum mechanics tells you, for instance, that one thing can be in two places at the same time, and that notion conceptually is very difficult to assimilate, to understand.
A.P.:
And to prove.
P.J.-H.:
Conceptually it’s difficult to prove; mathematically it’s easy. It’s also been shown experimentally, so there is no doubt—the world is like that. What happens is that our living experience does not correspond with quantum mechanics because, normally, its most pronounced phenomena occur in the microscopic world, or on a scale of energy very different from that which our natural and sensory perception responds to. It was also a philosophical revolution because it made clear that uncertainty is something very present.
A.P.:
The physicist Carlo Rovelli claims: “We are deeply unpredictable beings, like most macroscopic systems. There is no incompatibility between free will and microscopic determinism.”1
Quantum mechanics has introduced that uncertainty to us, and that is quite difficult for the rational mind to accept, is it not?
P.J.-H.:
The consequences of the uncertainty of quantum mechanics for the behavior of the more complex systems, like the human being, are still being investigated. It is not known whether human uncertainty has an ultimate origin that is provided by the uncertainties of quantum mechanics or if it comes from another kind of more complex behavior with which quantum mechanics is only indirectly related. However, it is certain that in quantum mechanics, for example, one cannot know with precision where an electron is and at the same time what speed it’s traveling at. We are accustomed to seeing a thing and saying “it is” in this place, and it is moving at this speed and in that “direction.” In quantum mechanics, that cannot be done.
And if you aren’t observing it, you cannot know.… In quantum mechanics the problem is that in order to know, one has to observe, but the moment you observe, you modify the behavior of the matter. All this is very difficult to understand and even difficult to imagine. But you have calculated it many times, you have tested it so many times that, in the end, you get used to it, and it no longer surprises you so much.
A.P.:
As quantum physicists, you are laying down a good challenge to philosophers. Should the philosophers be saying something in return?
P.J.-H.:
Philosophers can help provide a perspective on many things related to quantum mechanics, but fundamentally, quantum mechanics is a mathematical theory about mathematics and about what the universe and nature are like.
A.P.:
The science philosopher Javier Echeverria states in his book Entre cavernas (Among Caves) that what quantum mechanics physicists say fits in perfectly, as far as he is concerned.2
P.J.-H.:
Oh, very good!
A.P.:
Another science philosopher, Thomas Kuhn, explains in his book The Structure of Scientific Revolutions that science usually scorns its contradictions. He says, “Until the scientist learns to see nature differently in something new, it will not be a full and truly scientific fact.” Is it essential to work in quantum physics to see nature in a different way?
P.J.-H.:
You have to be more open-minded because nearly all quantum behaviors are almost nonintuitive. You cannot let your intuition guide you much because it usually leads to the wrong conclusions. So you mum,st have an open mind with regard to what is possible and what is not. I think that’s one of the fundamental principles; you have to be open to the fact that the impossible may be possible.
A.P.:
To considering the impossible possible.
P.J.-H.:
To what appears impossible being possible.
A.P.:
Let’s talk about a special case. The well-known physicist and Nobel Prize winner Richard Feynman said that nobody understands quantum physics because no matter how much people think about it, there’s no way of relating it to anything similar in the normal world. According to your MIT Web page, your interest in research focuses on “quantum electronic transport and optoelectronics in novel low-dimensional materials, such as graphene and topological insulators.” When I read that description of the specific field that you dedicate yourself to, I couldn’t help thinking that Feynman is right.
P.J.-H.:
Feynman was referring to something entirely different, but it isn’t a question of nobody understanding it because it is so complex, because the words are complex, or even because the mathematics is complex. It’s the concepts that are difficult to imagine. But what I actually do can be explained to novices.
A.P.:
Here you have a novice. …
P.J.-H.:
Here’s a simple example. When you take a battery and cable and connect it to a light bulb, the electrons circulate through the cable and lose energy in the bulb, which lights up for that reason. When transport by electrons is quantum, the transport takes place in a very different way.
A.P.:
But what’s all this about the quantum transport of electrons?
P.J.-H.:
It all rests on the way in which electrons circulate through a material. When it takes place in a quantum way, they may not dissipate energy. This is a phenomenon that occurs in quantum mechanics, where electrons may travel through the inside of a conductor without losing energy. These are very anti-intuitive things and almost incomprehensible. Moreover, we are not used to them.
A.P.:
And they do so in accordance with different equations, don’t they?
P.J.-H.:
Yes, they do so through the Schrödinger equation, which includes complex numbers in the mathematics, something that again people find difficulty in accepting, because almost everything to which they are accustomed is real number mathematics and the mathematics of quantum mechanics works with complex numbers, and those have their own specific reality, to put it one way.
A.P.:
But Pablo, why does this happen in the quantum transport of electrons? Is there any explanation that we can understand as to how the electrons comply with one equation in copper or silicon and another in graphene? Why do they do that with one and not the other?
P.J.-H.:
First of all, we have to accept that quantum physics also explains the traditional behavior of electrons; that is, it explains both quantum and classical behavior. To explain the classical behavior, a classical theory, let’s say, is sufficient; it’s easy. But in the quantum transport of electrons, the characteristic effects of quantum mechanics are the most unusual, for example, the fact that electrons can circulate through the inside of a material without colliding with the atoms inside that material, without crashing into anything.
A.P.:
But the effects of quantum mechanics have to do with a huge number of things. The technologies that they induce, that are created by those effects, we use in our daily lives, almost everyone, at home, all the time, and we don’t even notice, do we?
P.J.-H.:
Exactly. People think that quantum mechanics is something esoteric, but mobile phones, laptop computers, lasers, GPS, and chips are all possible thanks to the technology of quantum physics. People use quantum physics every day but don’t notice it.
A.P.:
Pablo, I know that you have a serious and personal relationship with carbon and its nanometric forms. Tell me what you do with carbon and its more exotic incarnations.
P.J.-H.:
Carbon is a very special element. It’s not only for making diamonds, which are beautiful, it is also the main chemical element responsible for life, along with hydrogen and oxygen. And carbon, in terms of solid-state physics, is a very special element because it is the component that makes graphene, with which, in turn, carbon nanotubes, graphite, and fullerenes can be made. I did my doctoral research on carbon nanotubes, and when I finished, graphene was discovered. So, as I was already in love with carbon nanotubes, and graphene is their second cousin, I moved over to investigating graphene.
A.P.:
Graphene, which occupies most of your research time, is a material that has gone in a very few years from being completely unknown to being perhaps the best known of the new materials, given that in 2010, the Nobel Prize in Physics was awarded to the scientists Andre Geim and Konstantin Novoselov for their revolutionary discoveries about this material. As to its definition, what I like most is what I heard you say: “Graphene is the finest material that has ever existed, exists and will exist.”
Why is graphene so important and why has it captured so much interest, including your interest as a researcher?
P.J.-H.:
Well, let me say that since our first meeting, other materials have been discovered that are just as thin as graphene. So, although graphene is the thinnest and finest that has ever existed, exists and will exist. …
A.P.:
Because it only has one layer of atoms, right?
P.J.-H.:
Yes, but it does have “cousins” that are also as thin and therefore share that distinction—for example, hexagonal boron nitride.
A.P.:
So, that definition “perfect” that you used of a perfect material—we can keep it or not?
P.J.-H.:
We can keep it, but now it is no longer the only material that exists with that definition; there are others also. Graphene attracted a lot of physicists’ attention at first and then engineers’ attention because the electrons in it behave as ultrarelativistic particles, and that is something extremely unusual.
A.P.:
Ultrarelativistic?
P.J.-H.:
Ultrarelativistic, yes. You see, you have to join quantum physics with Einstein’s theory of relativity in order to explain the electrons in graphene. This, before, was unnecessary for other materials, such as silicon, copper, iron, aluminum. There was no need to join the theory of quantum mechanics with the theory of special relativity in order to make a theory of relativistic quantum mechanics. It was necessary for other things—for the physics of particles—but not to study normal materials.
Electrons in graphene behave as if they were particles that have no mass and travel at a similar speed to the speed of light. It is not the speed of light, it’s less, but they behave in a similar way to light particles, the photons. They “travel” like neutrinos, or, let’s say, like particles without mass, and that is very weird. It’s very unusual from the mathematical perspective and from the perspective of the consequences that it has for quantum transport and for devices or technologies that can be made, for example, using graphene.
A.P.:
And has that been shown experimentally?
P.J.-H.:
Yes, it has already been shown.
A.P.:
Because physicists would never say that a thing is as it is, if there were no physical experiment to prove it.
P.J.-H.:
If it is not shown experimentally, of course not. But this has been shown in many experiments, and let’s say that the incredible properties that graphene has, from the electronic and optical perspective, are the direct result of those ultrarelativistic properties. That is why engineers, who normally do not think about ultrarelativistic physics, now have to think about it. Anyway, graphene is the best conductor that exists and is so because the electrons do not crash into obstacles within it. It is partly for that reason. And they don’t crash in it because ultrarelativistic particles do not collide with obstacles. It’s very strange behavior that is not shared by non-ultrarelativistic particles.
A.P.:
Information technology has so far consisted of hardware and electronics based on silicon and its miniaturization follows the rate of development pointed out in Moore’s law more than fifty years ago. It’s clear that soon it will be saturated, according to the logic of physics and geometry. Intel already manufactures chips with technology of 22 nanometers and has announced that by the end of this decade its chips will be 8 nanometers. It seems impossible that chip electronics can continue to produce smaller and smaller things for much longer.
Do you think that graphene chips would be a feasible alternative to the present electronics? Or should we not be thinking about making the same things that we make today with it?
P.J.-H.:
I think that graphene chips, as people currently think of graphene, will not be the alternative to the present silicon chips. Silicon is very good for what it is used for at the moment. It’s being perfected and investment is going to remain there. If they arrive at 8 nanometers, I don’t think that graphene will replace silicon. I believe the main applications for graphene are still to be discovered. It is a very unusual material that engineers are still fighting over because its characteristics are so different from those of all the other materials they have had before. They still do not clearly know how best to use it. And although now there are some very small applications for very specific things, let’s say a “niche” market, the new properties are so extraordinary that I am sure they will be used in another way; a technology will be invented that makes better use of the extraordinary properties of graphene.
A.P.:
Could one ‘niche’ be carbon nanotube batteries?
P.J.-H.:
Carbon nanotubes are a material basically consisting of rolled up graphene.
A.P.:
Graphene is now being manufactured. Everyone is boasting of it, is that not true?
P.J.-H.:
Graphene can be used, for example, in batteries because in batteries it is essential to have a large surface in relation to the volume and, as graphene only is only one atom thick, a graphene surface has the largest surface with respect to the possible volume that you can imagine. This is very important in batteries and nanomechanical composites. But all these composites only use relatively basic properties of graphene. Graphene has much better advanced properties than those, and it will take time to discover and invent the applications that can make use of its best properties.
A.P.:
That would be a far-ranging revolution, much further than people are imagining now, wouldn’t it?
P.J.-H.:
I believe so. Besides, graphene is not the only material. Over the last four or five years, we have learned that there are many other materials that are similar to graphene in that they are two-dimensional, ultrafine, flexible, with optical and electronic properties very different from those of three-dimensional materials. Graphene is just the flagship of a new generation of materials, but I believe that engineers are going to make much more use of these materials because they are easier to understand.
A.P.:
And those materials were there just waiting to be discovered, weren’t they?
P.J.-H.:
Yes, they were there just waiting to be discovered. Someone just had to be brave enough to try.
A.P.:
To take the risk of investigating them.
P.J.-H.:
Exactly.
A.P.:
That is to say, when something appears, a new challenge in research, it is usually because there are researchers who are less conservative and more willing to take risks, right?
P.J.-H.:
That’s it.
A.P.:
They run the risk of their colleagues saying, “Where are you going?” or “You're crazy.” They stick their necks out.
P.J.-H.:
That’s true. In research as in other fields, the greater the risk, the greater the danger, but there is also a greater possibility of reward.
A.P.:
In another conversation in this book, I was speaking to the physicist Ignacio Cirac about the letter that Albert Einstein wrote to Max Born in 1926 in which Einstein said, “Quantum mechanics is very serious, but something inside me says that this is not the way.” Ignacio replied that if Einstein were alive today and had been accustomed, as you say, he would no longer be so surprised that things of nature are as they are, that they are also quantum.3
Do you agree with Ignacio Cirac on this?
P.J.-H.:
Basically, yes. As I said earlier, I think we are nowadays brought up to study physics from the quantum physics principles and calculate quantum properties using its equations. Moreover, the development of computers and present visualization techniques offer many new ways of seeing quantum mechanics, of seeing the effects they have, ways that did not exist in Einstein’s time. And perhaps, if he were living today, he would be one of the physicists occupied with the fundamentals of quantum mechanics.
A.P.:
But if Einstein were to appear through this door and you said to him that relativistic quantum physics now exists, he would probably laugh, wouldn’t he?
P.J.-H.:
Relativistic quantum physics was already there in Einstein’s time. It was discovered by the physicist Paul Dirac. But then it was only thought of as belonging to the field of high-energy particles, not to describing the electrons in a material that we all use, such as pencil lead.
A.P.:
But he would laugh, don’t you think?
P.J.-H.:
Yes, I do. Einstein would have been delighted with the discovery of graphene. It would have appealed to him.
A.P.:
Returning to personal matters, Pablo, I think that for you physics—something so difficult to learn and master for many pupils and youngsters—has to do with emotion, passion. Normally a scientist contemplates things almost strictly from a rational plane. In your experience, do you think that one has to combine both halves of the brain, the more rational and the more emotional, in order to make a good nanophysicist? Is there a mystery? Is there emotion in the physics of condensed matter? How do you manage to devote that endless passion to something so pragmatic?
P.J.-H.:
Of course, there is mystery in the physics of condensed matter. That’s where the fun lies, in not knowing what you are going to find when you undertake an experiment. There are many surprises, and clearly there is emotion in that. Because of the mathematics of quantum mechanics that we employ, we are obliged to make simplifications or approximations; one cannot completely study mathematically such a complicated system as a piece of silicon or graphene. So mathematics alone does not easily lead you to predict how an object will behave from the electronic, optical, or atomic perspective. We are in a discipline where there are continual surprises, where we discover things that we don’t expect, and where mathematics only helps you to understand a posteriori. Surprises? Yes, every day.
A.P.:
But finally the rational half wins, doesn’t it?
P.J.-H.:
Yes, in the final analysis. But don’t forget that in the process of discovery, in research, intuition also plays a fundamental role. And not only intuition but also such simple things as the search for aesthetics or beauty. Normally we physicists like simple things.
A.P.:
Simple and elegant.…
P.J.-H.:
Yes, simple and elegant. The search for simplicity and beauty is clearly a two-edged sword. Sometimes nature is not so simple and elegant and we have to accept that, but it is also true that many times the search for that simplicity or elegance, or even the beauty of behavior, leads us to discover something that really is so.
A.P.:
It can’t always be minimal in quantum physics as in the minimalist painters, is that not right?
P.J.-H.:
Minimalists, no. There are many things that are complicated, and you try to understand them as best as you can. But we physicists tend to try to separate a little, to simplify things so that we know how to capture the essence of things. We remove noise and distractions from the essence. So the essence is usually something that can be explained with mathematics, that can be understood in a quantitative way, and later it turns out—
A.P.:
That it may be complicated.
P.J.-H.:
Yes, later it may turn out to be complicated. You can always add the complexities. Physicists always try to simplify things as much as possible in order to understand the essence, and then we add the complexities.
A.P.:
Many thanks, Pablo, for talking to us.
P.J.-H.:
My pleasure.