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Interview of Subir Sachdev by David Zierler on June 11, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Subir Sachdev, Herchel Smith Professor of Physics at Harvard University. Sachdev surveys his current research projects which includes a focus on Planckian metals and the Sachdev-Ye-Kitaev model, and he describes the interplay between theory and experiment on the topics he is following most closely. He describes the major advances in spin liquids research, and he recounts his childhood and Jesuit education in Bangalore. Sachdev discusses his undergraduate education at the Indian Institute of Technology and he explains the circumstances that led to his family’s emigration to the United States and his transfer to MIT where Dan Kleppner was a formative influence. He explains his decision to move to Harvard for graduate school, where David Nelson supervised his thesis research related to Nelson’s interests in developing the theory of the structure of metallic glasses. Sachdev describes his postdoctoral work on quantum spins and antiferromagnets at Bell Labs, and research advice he received from Bert Halperin. He explains his decision to join the faculty at Yale, he describes his key collaborations with Nick Read on quantum antiferromagnets and he narrates his increasing interest in cuprates. Sachdev discusses his decision to write Quantum Phase Transitions and he describes the origins of the SYK model and its relevance for black hole research. He discusses his involvement in string theory and his longstanding interests in Bose-Einstein condensation. Sachdev narrates his decision to transfer to Harvard and he describes his work in quantum chaos. He describes his professorship at the Tata Institute and the meaningfulness of being able to travel to and maintain contacts in India. At the end of the interview, Sachdev explains open issues in the theory of pseudo-gap in the high-temperature superconductors, how the SYK model may contribute to the development of a theory of quantum gravity, and he provides a long-range view of developments in the field of strange metals.
Okay, this is David Zierler, Oral Historian for the American Institute of Physics. It is June 11th, 2021. I am so happy to be here with Professor Subir Sachdev. Subir, it's great to see you. Thank you for joining me today.
Thank you for asking me to be a part of this. It's a real honor.
Subir, to start, would you please tell me your title and institutional affiliation?
I'm the Herchel Smith Professor of Physics at Harvard University.
Are you still Department Chair or that term has ended?
The term as the Chair ended about a year ago—
—on June 30 last year.
Well, congratulations are in order, I suppose.
Well, you have to ask others in the department, but I did my duty and I think I did okay.
Subir, who is or was Herchel Smith?
Well, I have never met Dr. Herchel Smith. He was a chemist, I believe who, according to Wikipedia, invented the first synthetic birth control pill. He donated substantial sums of money to Cambridge University and Harvard University. There are also several Herchel Smith Professors at Cambridge.
Subir, just a snapshot in time, what are you working on yourself? And what's interesting to you in the field more broadly right now?
I have many projects these days because I have a rather large group of students I am working with. There's one set of projects which are focused on what's now starting to be called Planckian Metals. These are unusual metals which conduct electricity, not by the motion of individual particles, but by some collective entangled quantum configuration of them. And they seem to be present in many interesting materials, especially the copper oxide materials which become superconducting at a high temperature, but also in the recently discovered twisted bilayer graphene compounds. I have had a long interest in this strange metal problem, and it's turning out to be connected in a very interesting way to something called Sachdev–Ye–Kitaev model, which I'll talk about more later, briefly called the SYK model. So that's a big focus of my research right now, the connections between Planckian metals and the SYK model, and other models of this widely observed phenomenon. They are also connections to all of this physics, remarkably, to the physics of black holes. And there's a lot of activity in the black hole field which I've been following, but not actively working on these days.
Another set of my interests have to do with states called spin liquids. These are states in which the electrons are entangled in a way that you get excitations which are anyons: these are new types of emergent particles that pick up strange phase factors when you move them around each other. This has, again, been a long-standing interest of mine, and there are now many interesting spin liquids being discovered. And a number of recent experiments are not only in condensed matter materials, but also in ultra-cold atomic configurations, which have interesting connections to quantum computing. And so I have several projects in that area also. So, these are my two broad interests right now, spin liquids and Planckian metals, to be short.
Subir, given your research, it's clear that you have, over the course of your career, always paid close attention to the interplay between experimentation and theory. Where is the state of the field right now in terms of advances and guidance?
Well, I think it's always a back and forth between theory and experiment. So for example, on the spin liquid field, I would say that theory is ahead of experiments at this point. There are a lot of very sophisticated theories of lots of exotic states with anyon excitations. And only the very simplest of them are now starting to be realized and understood in experiments. On the other hand, in Planckian Metals, these have been known for quite a while, and there are many experiments which still are not fully understood, including some new ones on something called a thermal Hall effect by Louis Taillefer from Sherbrooke, which we are also thinking about. And so here, I think there's no shortage of data, no shortage of materials, but theory is mostly working on not entirely realistic models of the real system. And I think this always goes back and forth.
If I had to pigeonhole you into exactly how you define yourself, what kind of a physicist are you at the end of the day?
That's a great question, because I get asked this question at parties and I don't have a good answer really—but to another physicist, I would say, a quantum-condensed matter physicist, but that only gets blank stares at parties. So maybe more briefly, I just say I am a quantum physicist. We are interested in the consequences of the strange principles of quantum mechanics to the macroscopic world. We're interested in how quantum theory, which was initially believed to apply to just a few microscopic particles inside the atom, also plays a fundamental role in the properties of macroscopic materials, and also on the even larger scale of black holes. So, more accurately, I am a physicist studying the macro-behavior of quantum systems, if you want an accurate description.
I'm always curious about the historical connections in the decision to transfer the nomenclature in the field from solid-state to condensed matter. I wonder with your emphasis on quantum condensed matter physicists, what unique perspective you might have about that transition?
Solid state physics, which is what my field was called, was the theory of electrons in metals and semiconductors. It was all very successful, and all of that theory had a tremendous impact on technology. But it was essentially a theory of independent electrons, free electrons moving around. I think the jump to condensed matter was made perhaps by Phil Anderson; he was a very influential figure, and he coined the term “More Is Different”—it's not always just individual electrons, it's what happens when you have millions of electrons. Then new phenomena can emerge from their collective properties, and so we are dealing with “condensed” matter. But unfortunately, this term invariably confuses the general public.
During the early days of condensed matter physics, many new phenomena, like the theory of phase Transitions, critical phenomena, were phrased in purely classical terms. So when condensed matter came into its own, the ultimate theory for the longest scales for observing experiments was basically classical, although the underlying ingredients are quantum. Once you got a sufficiently large system, you could treat its properties by some classical equations of motions for the emergent degrees of freedom. So “More Is Different,” but once you became “More,” things looked kind of classical at the longest scales.
I believe I played some role in pointing out that actually, that's not entirely true. Once you get to the collective behavior of various strongly interacting systems, the quantum survives for longer than you would think naively, especially near quantum critical points and in Planckian metals. So, there you have the uncertainty principle of quantum mechanics playing a crucial role, and indeed being crucial for fundamental properties that have been observed at the longest scales.
As to my own background, I started out as a classical statistical mechanics physicist when I was working with David R. Nelson for my PhD. Afterward, I moved into solid state physics, thinking about quantum topics. I suspect the transition brought a different perspective. I was well-versed in certain paradigms of classical statistical mechanics that suddenly were applicable to quantum problems. And that gave me a different perspective, a different way of looking at problems, and perhaps a real advantage in thinking about things.
Subir, from a technological perspective, what have been some of the major advances in your career that have allowed you to apply a quantum approach to condensed matter? I'm thinking of advances in instrumentation, computation, even material sciences, what stands out in your mind?
Certainly, there's been enormous advances in instrumentation, especially in nanoscience. Today, as a matter of routine, people using a scanning tunneling microscope can measure properties of individual atoms in the material -- including remarkable details of the spectrum of the electrons, how they vary from atom to atom in a crystal---that can all be studied, in a manner you could once only imagine. I mean, that was just a pipe dream when I first started out in this field. And that's really made a huge impact on the theory, understanding what's going on in materials, and the many kinds of interesting tests you can make.
There have also been enormous advances in the field of ultracold atoms. This came from atomic physics, where people cool a whole collection of atoms down to nano Kelvin temperatures. This led to the first observation of Bose-Einstein condensation of atoms by Carl Wieman and Eric Cornell and Wolfgang Ketterle, who won the Nobel Prize. Then attention shifted to putting the atoms into what are called optical lattices, or putting them in tweezers, and then coupling things together in interesting ways. A lot of the most exciting advances in seeing spin liquids, that I mentioned earlier, actually are now coming from all the amazing technology of ultracold atoms. And this ability to control individual atoms, and detect their quantum state, and then couple them together in interesting ways, of course, that's exactly what people in quantum computing want. So there are also connections, taking these spin liquids ideas, and applying them to quantum computing.
Again, these are things that when I was starting out, I couldn't have imagined. I wrote this paper on what's now called Z2 spin liquid in the year 1991. And at that time, we were just thinking about electrons in certain materials, and the kinds of interesting entangled states they could form. And we found what is now known to be the first example of a state that has anyons, but which doesn't break time reversal. In other words, you can take one particle around another particle clockwise or anti-clockwise, and it doesn't make a difference to the phase factor that it picks up. And this Z2 Spin Liquid has since grown into what's now called the toric code, invented by Kitaev. And the toric code is fundamentally the way in which people are now doing quantum error correction in quantum computers these days. This is one of the most amazing things that has happened, and not something that was on my mind while working on this in 1991.
Obviously, Subir, you operate very much in the world of basic science just trying to figure out how nature works. I'm curious if you've ever seen applications for your research, either that you can pursue yourself or how others have been inspired by your research to think about potential applications or even commercial viability of what you found?
I don't think so. I'm a fairly abstract theorist, but I do talk to people who are experimentalists, whose other interests or related interests have led to applications, I'd say. But not directly my own, to be fair. I would say the biggest application that has come out of the general field of magnetism and materials is been the giant magneto resistance, which is the key to computer memory without which the iPhone wouldn't be possible. I was not directly involved in any way in that but that's, broadly speaking, part of my field, of understanding quantum collective behavior of materials better from a fundamental point of view. These days, there's a lot of excitement in quantum computing, with many private companies and startup companies looking at quantum computing or at other applications of quantum entangled states. I don't think there's a killer app out there just yet, but certainly, my work is connected to a lot of the things that are at the frontier of the field of quantum computing.
Last of my current questions, Subir, it's one we're all dealing with right now, how has your science fared one way or another over the course of the pandemic and the mandates of remote work and physical isolation?
Better than I would have thought at the beginning. I would say the biggest disadvantage is not being able to meet my students regularly in my office and having one-on-one discussions on the blackboard. In fact, I had those for the first time this week and I realized when I was having those discussions, how much I've missed this, and how much I think they would also have liked to have more of it. But David, apart from that, actually, it's been very good. If some talk in some conference somewhere interests me, I just go online and watch it. I don’t have to go to a different continent to listen to one person who I wanted to hear more from. The accessibility and availability of all kinds of interesting talks is tremendous. I hope we will continue with that.
That's been quite a revelation. Yesterday, I gave a seminar in India to a large group of faculty and students at an institute there. I've taught my course on quantum phase transitions and quantum phases of matter, and I put all the lectures on YouTube. And I keep hearing from students all over, mostly in India, who are watching those lectures. So, all of that's been really quite amazing, how we were really not, before the pandemic, taking advantage of these communication tools. I think those changes are going to stay.
In terms of my research there my group has made progress. There were several projects that were just getting off-ground with my students and post-docs, and we managed to continue on those. But yes, I think the time has come, if you want to start new projects and move in a different direction, that's challenging to do without the interaction.
Well, Subir, let's take it all the way back to—
So I’m ready to go back.
Let's take it all the way back to the beginning. Let's go back to Bangalore and let's start first with your parents. Tell me about them.
My father, Dharmendra Kumar Sachdev, is retired and used to work for the Indian Phone Company. He is an electrical engineer, and he worked in the R&D division of the phone company in India, and then a company called Intelsat which is based in Washington, DC, launching international satellites. This was in 1978, most international phone calls went through satellites that were controlled by Intelsat, which had been set up by the UN as an International Communications Satellite Company. When my father and my family moved to Washington, DC, I was in college, and I transferred to college here to be closer to them.
My father continued to work with Intelsat for a while in satellite communications. He's quite an eminent person in that area. He has had a consulting business after retirement which he's kept going until now; he’s eighty-six now.
My mother, Usha Sachdev, Usha Kapur before marriage, brought us up. She was a homemaker and a very important influence, and she made me the person I am. She died almost four years ago. I was born in New Delhi, and when my father’s work took him to Bangalore, I moved there at nine years old. Most of my memories are in Bangalore where I went to school.
Subir, I'm no expert on Indian surnames, but yours seems rather unique. Where does it come from?
Okay, so the last name, Sachdev, is actually quite common. It's a Punjabi name. It’s a name from Punjab, which is a state in Northwest India. All four of my grandparents used to live near Lahore, which is now part of Pakistan. The state of Punjab was partitioned in 1947 at the time of independence, but my family was entirely in Delhi by then. So, my father was born in Delhi, and I was also born in Delhi. My first name, Subir, is also a common name, but it's a common name in Eastern India, in Bengal. So, you will find many Subirs in Bengal, and you will find many Sachdevs in Punjab. But combination is quite unusual. I mean, I don't know why my parents picked a Bengali name. Might be they just liked the sound of it. I'm sure it's nearly unique, even though India has over a billion people, it's an unusual combination, but neither name is uncommon. Also, in terms of names you may have heard of in physics or technical fields, people in physics certainly tend to be either from South India, the Chennai area, or from Bengal. It is unusual for a Physicist to come from Punjab.
What languages were spoken in your house growing up?
Well, my grandparents from Punjab spoke Punjabi to my parents. But as I said, they had moved to Delhi, and in Delhi the major language in is Hindi, which is closely related, but not quite the same. When I was growing up, I could follow Punjabi, and my parents used to speak Punjabi with their parents, but they spoke Hindi with me, because that’s what I spoke with my friends. So it was a combination of Hindi and Punjabi, at least when we were in Delhi. And so I am very comfortable in Hindi, but not in Punjabi. And then, when I moved to Bangalore in South India, well, there it was English, because people there could not speak Hindi, apart from a very few. And there's a South Indian language called Kannada, which I picked up a little bit, but English was what was spoken at least outside the home. In home, it was Hindi for a while, but eventually it became English as time passed on.
Growing up, did you have an appreciation of your father's technical skills? And was that influential in you developing your interest in science?
Absolutely. I heard about all the great things he had accomplished, as a student and also as an engineer. He was Chief of Research and Development at the phone company in Bangalore. And it was quite unusual then to have somebody hired all the way from Bangalore to Washington. He was picked out to develop satellites at Intelsat. I very much was inspired by that, and I wanted to do something technical, and I started out, in fact, also as an engineer. And then, about the time when I came to U.S., I was pretty convinced I wanted to do physics, because basic science was what appealed to me. I suspect initially my father was not too happy about it, but he has always supported my choices.
Subir, was your family more religious or secular overall, would you say?
Oh, quite secular, I would say. We didn't go to any religious events except on special occasions.
What kind of schools did you go to growing up?
Well, I mostly went to what are called convent schools. These are schools started by Jesuits in India. I went to St. Joseph's Boys High School in Bangalore. This is quite an old school, established in 1858. At the time, it was exclusively for Europeans and was started by French Missionaries. By the time I got there, it was all Indian. There were a few teachers who are what we call Anglo-Indians, that is English people who are partly English, who didn't go back to England and stayed on in India after independence. There was my maths teacher, Mr. Yates, and another teacher, Mr. Luke D'Souza, they were both the pre-independence generation, and they were very good. There were quite a few Catholics in the school connected, originally to the Portuguese in Goa. So, I would say such people were maybe about a quarter of my school, but it's much smaller today, or so it seems when I have been back there. So it's a very good school, a lot of discipline, including caning, and toeing the line, and a lot of hard work. But also a lot of play, and I played cricket and table tennis.
Was it generally understood that Jesuit schools provided a better education?
Yes, I think at that time. Depending on where you are, the Jesuit schools were certainly among the best, but there were also other private schools, and also schools run by the central government. I was one year at Kendriya Vidyalaya, which is a central government school, which was also very good. Today, I would say that St. Joseph doesn't have the same elite status they did in my time. Now, it's just one of many schools that are very good, available to kids in Bangalore. But I'm still in touch with my classmates from those days.
And the plan for you when you got to the Indian Institute of Technology was to pursue a degree in engineering?
Yes. So the system in India was you sat for an entrance exam known as the Joint Entrance Exam. And if you were admitted, then you were asked to pick one of the many institutes, and also pick a major before you got there. You had to pick what you wanted to do, and there were a limited number of seats.
This is a vestige of the British system?
No, no, these institutes were set up after the British left. They didn’t exist before independence. Now, there's about twenty of them. At that time, there were five. The Indian Institutes of Technology in Mumbai, Chennai, Delhi, Kanpur, and Kharagpur. They had started in the 1960s, I think, and they didn't exist when my father went to college. They're still considered the premier places that families would want their children to go to in India. The competition was fierce, much harder than getting into Harvard as an undergraduate. You were required to pick a certain major on day one, or day minus-one really, before you even got there. And the person who was stood first, would pick first, and so on. There was a ranking, and I stood second in the country. It was just obvious that if you were in the top 50, you better do electrical engineering because that's what smart people do, that's where the money was. And only if you were far down, maybe you did physics. So I naturally went with the peer pressure, and I choose electrical engineering.
Now, what were the family considerations? Did your father go to the United States on a trial basis or was the idea that he went, and it was understood that the family would follow him?
The latter, I believe. I'm just trying to think back. He took a leave from a job in India with the understanding that after, I think, about five years that he could come; but he did have essentially, what we would call a permanent job at this company. I was already at IIT when he moved. So I finished that year at IIT and joined MIT as a sophomore.
What were your feelings about coming to the United States? Was this a welcome opportunity? Did you feel conflicted about this, about leaving home?
Yes, I did feel conflicted about leaving my home country, my friends at IIT, and many family and friends elsewhere. But my parents were in the U.S. The idea of going to U.S. as an undergraduate student wasn't something that many students in India thought about then. But Indian students did then consider going to the U.S for a graduate degree. So I was fairly sure that for my graduate work I would come to the U.S at least for a period, and I had never really thought of coming here as an undergraduate, it didn’t make sense given all that was available in India, and you had to leave your family. When I first came to MIT, I would say that at that time, I definitely thought I'd go back, and especially if my parents went back after five years. But you grow into a new place, and for my sister, who’s much younger than me, and grew up here in Washington – it would have been hard for her to go back to India at that age.
What were your impressions of MIT when you first arrived?
First of all, technically, in terms of the courses and all the labs, it was just amazing. It was really exciting and interesting. The hard part was the social environment. It was not easy for me at that age, especially seventeen, eighteen, certainly in a totally different environment, that was difficult.
Were there other international students or specifically Indian students that you fell in with?
There were, but there was essentially no one like me. There were graduate students, a few of them. Today, it's a totally different thing. There were also a very small number of second-generation Indian Americans whose parents that have been born here and come to MIT, but very, very few. And there was only one Indian restaurant in all of Boston – in 1979. It's still here. The social, normal part of life was difficult. It took me a few years to get adjusted, but the science was no problem at all. In fact, with my rigorous preparation for the IIT, and the tough courses I'd taken at IIT, the regular sophomore courses at MIT were not a problem. I was taking graduate courses, in fact, the next semester.
I'm not sure if you've talked to Cumrun Vafa about this, but he has a very similar story about the rigors of his education and then coming here and things were not particularly challenging on that front.
Well, but on the other hand, what was possible here at MIT, which was not possible in India, okay, you want to take a graduate course, go right ahead. In IIT, it is simply not allowed. There are the courses you take when you're a freshman, and courses for sophomores, it was very rigid and prescribed once you picked the major, with just a few electives. Whereas at MIT, everything was an elective apart from just a few requirements which I had already satisfied, so that was the amazing thing that you had all this freedom, and you could also start working in the lab.
I worked with Dan Kleppner. He was also another very important influence in the way I developed as a physicist. On the whole, it was a great, wonderful experience. I mean I missed my friends back at IIT, but I was happy.
What was the process of switching over to physics? Was it a professor? Was it a class?
Yeah, my sophomore year at MIT, I took electrical engineering courses, but I also took a lot of physics courses, still being like an EE major, and I just enjoyed them a lot more. I also took a class in computer science. It was all about artificial intelligence, and that was big in those days at MIT. This is the old artificial intelligence, and at that time, it was a new field and hadn't really had much payoff. It just didn't excite me as much. It seemed like in artificial intelligence you were subject to a human whim where somebody decided this was interesting, whereas in physics, it didn't matter what anybody thought, you had to describe the real world. So you had a better judge of the value of your work, I felt. Yes, this was the idealist in me, but it was nature that decided.
Who were some of the professors who you considered mentors or who exerted a real intellectual influence on you as an undergraduate?
I've mentioned Dan Kleppner. I probably met him somewhere towards the end of my sophomore year, and I worked under his mentorship for two years. He's an atomic experimentalist, and he took a liking to me, I suppose. He certainly let me do what I wanted, and took a risk with me and mentored me, so I'm very grateful to him. If it wasn't for him, I’m not certain I would be in physics today.
Initially, he let me work on some experiments, building things in the lab, and it quickly became clear I was not very good at it. Then he suggested theoretical problems for me to think about, related to experiments he was working on. This had to do with dissipation in metallic cavities with single atoms. It’s a very basic problem today, important in many fields, but at that time, there was not much work on it. So that was a tremendous problem that he gave me. I worked on it for a year or so. And my results got published in Physical Review B, my first paper. I won the Apker Award for it from the APS. And in fact, my undergraduate thesis is a chapter in the book on quantum optics by Scully and Zubairy. So, this is a point of some pride for me. And that all happened because Dan Kleppner just gave me the freedom to do what I wanted and suggested a very interesting problem to work on.
What was Dan working on in those years? What was his main research?
Well, he had two big projects. One was studying Highly Excited Rydberg States of atoms and putting them in electric fields. He took an anyon like sodium, and he put it in the n=50 principal quantum number. And these Rydberg states are now back again in my life, because they are used to make Spin Liquids. People in my colleague Misha Lukin’s lab can do things with these Rydberg atoms now that you couldn't imagine in those days. Dan was studying gases of these Rydberg atoms. But now, you can take a laser tweezer, and each tweezer has one Rydberg atom in it, and you can bring the tweezers together and have the Rydberg atoms interact in a controlled way. This is one of the platforms now for quantum computing and studying Spin Liquids and so on.
The other main project in Dan’s lab, which I was only peripherally involved in, was his study of the Bose-Einstein Condensation of Ultracold Atoms. He was trying to condense hydrogen. The thinking at that time was, hydrogen is the lightest atom, which is a Boson, and lighter atoms have a higher condensation temperature, so let's start with the lightest atom, it should be the easiest thing to do. That was one of Dan's unfortunate bets, because methods invented in Dan's lab, i.e. evaporative cooling, could be applied to heavier atoms, sodium and potassium and rubidium just down the periodic table. And, these methods were used by Ketterle, Weiman, and Cornell on heavier atoms, and they beat Dan to the Bose-Einstein condensate.
Dan was an amazing pioneer in that as well. Looking back, perhaps he should have started working on the other atoms a bit earlier on. Ketterle was partly mentored by Dan. After coming to MIT, Ketterle started working on the alkali atoms with somewhat different methods of cooling than Dan had been using. Dan Kleppner allowed Ketterle the freedom to pursue it without being a part of it. That was remarkable of him.
Subir, by the time you graduated MIT, how well defined were you in terms of pursuing either theory or experimentation in graduate school?
I was very much a theorist by then. I mean that's what I learned while working in Dan's lab, that I was certainly better at theory than experiment. I just hadn't quite decided what I would do in theory. I was interested both in condensed matter and particle physics, and I came to Harvard, and well, there were two factors that I’ll mention here. One, I really enjoyed working with David Nelson. I talked to him, and I thought he was working on really interesting problems. And there were some extremely smart students that I met there, and they were all doing particle physics, and I felt I couldn't possibly compete with them. So I said, I'll do something different. And I also remember a talk by Professor Glashow to all the graduate students. And he basically said, "Don't do particle physics, because I've solved everything. There's nothing left to do." I’m paraphrasing of course, but he was right about accelerator-based particle physics, and I am grateful to him today for his honesty.
Nobody was saying that in condensed matter, regardless of Glashow's ego?
Subir, what kind of cross-pollination was there between MIT and Harvard as an undergraduate? Would you go to Harvard for seminars? Did you know what was happening there?
When I was an undergraduate, no. There was already too much to do at MIT. But once I came to Harvard, occasionally I would go to MIT for a seminar, sure. At the graduate level, there's quite a bit of interaction, even today. During the pandemic it’s much easier, just go online, but it was not that hard, even before the pandemic. I just taught a course, and there were a fair number of students from MIT in my course, and my students take courses at MIT. There’s a lot going on between the two places.
Did you get advice specifically not to stay at MIT for graduate school, that it was better to move on?
I wouldn't say that. I had the opportunity to stay at MIT. It was not the easiest decision but I just wanted a change, and also at Harvard, they gave you some time to decide and you didn't have to commit on day one. That seems to be a theme in my life. You don't have to commit right away to a particular project, whereas MIT did at least assign you to a particular advisor right on day one. I wasn't sure at that point what I wanted to do. Having a year to explore all different fields, which is what Harvard offered, was what clinched my decision.
Did you have an idea before you started at Harvard who your advisor would eventually be?
No, I didn't. It could have been someone in particle physics like I mentioned, or in condensed matter, there were Nelson and Halperin were the two leaders, they're still here and I have learned a great deal from both of them.
How did you come to develop that relationship with Nelson?
Well, I just have to remember back. As I recall, he taught a course that I took my first semester, and I worked on a project during the first summer after getting there. It was a lot of fun; I made progress, and it led to my second paper. I continued working with him.
What was the intellectual process developing during your thesis research?
In those days, David was working on packing of spheres and a possible five-fold symmetry and the connection to metallic glasses. This was before the discovery of quasi crystals, which roughly can be understood as arrangement of spheres with five-fold symmetry. David was really ahead of his time, and he was developing the theory of the structure of metallic glasses. He gave me fairly well-defined problems to work on this area, and I made a lot of progress, and things moved on very quickly. I graduated in three years, which looking back, may have been too quick. I might have explored more, but the projects I've worked on were interesting.
Besides Nelson, who else was on your thesis committee?
Well, of course, Halperin, and I was a TA with Halperin on one of his courses, and that's when I really learnt quantum many body theory listening to him. This was his course on quantum condensed matter and quantum Hall Effect and things that Nelson was not working on. Halperin was, and still is, an imposing figure who seemed to know everything. And I learned a great deal from his courses, and his discussions, and all the questions he asked at seminars. Also, in my thesis, he asked some very tough questions, which I thought that meant I would surely fail, but fortunately that did not happen.
Anything memorable from the oral defense?
I thought I did terribly mainly because I couldn’t answer some of Halperin's questions very well. But I hope I took it the right way, I think his job was to show me what I needed to learn, and he did that very well and that’s how I took it.
What opportunities were available to you after Harvard? Where did you want to go next?
Well, I applied to post-docs and I had several offers, and I think the top two in the running were KITP, it was called the ITP then, the Institute for Theoretical Physics in Santa Barbara. That seemed like a very exciting place. And Bell Labs, which was still the “old” Bell Labs in those days with a very large theory group and lots of experiments going on. I don’t think people were surprised when I chose Bell Labs. At that time, ITP wasn't the big institute it is today. And once I got the Bell Labs offer, it was pretty clear I would go there.
Did you talk to Bert about Bell Labs? Did you hear about his experiences?
Well, yes, in those days, I couldn't talk to Bert as a friend like I can today. I was just a lowly student, but he gave me good advice on what it meant to be going to Bell Labs, and how I should approach it. And he felt that being a post-doc at Bell Labs was, in some ways, a lot of pressure. Because the system then was, you had a two-year post-doc, and at the end of the two year, they would decide whether you became a permanent staff member at Bell Labs. And that was considered the best job you could imagine, because you have the freedom of a great place to work in and also no teaching or funding obligations. You just worked on what interested you.
So that was the dream job that certainly I wanted, when I went as a post-doc to Bell Labs. Bert advised me that that's a lot of pressure, but don't worry about it; just do your work and the decision is not important. Even if they don't keep you, it doesn't mean that you're not a good physicist, because who can tell in two years whether you—there was an element of luck involved to doing something good in the next two years. If instead someone gives you ten years, and says, "Okay, do something interesting," and you eventually do something interesting, that's a better judge. In two years, who knows? It was good advice because they didn't keep me. But then soon after I left Bell Labs stopped being the old Bell Labs because they had a lot of financial pressures.
So, you were really part of the last day moment of the heyday years of Bell Labs? You did not get exposed to what would come?
Right, that's correct. Not the very last, maybe a year or two before. But even at that time, there was a lot of apprehension. People were starting to feel nervous. Bell Labs became part of Lucent, and Lucent was flying high for a while. And so Bell Labs was okay, but soon Lucent collapsed and then Bell labs collapsed. And the theory group simply doesn't exist today. It was very sad because it's an amazing place, and my students can never go and see what it was like.
What group did you join at Bell labs?
I was in the Theoretical Physics Group which is group number 11111, and Pierre Hohenberg was the chair of the department there. Later on, Pierre became Deputy Provost at Yale, so he was my boss twice in my life, once at Bell Labs and then again at Yale. Yeah, so you were given a desk and told, “Okay, do something.” That's the way it was. And so I spent the first month or two just finishing up things I'd been doing with David, while also walking around the labs there, talking to experimentalists, and trying to do something entirely different. And that's when I started working with Mikko Paalenen, who was doing experiments on phosphorus atoms in silicon, and Ravin Bhatt, a theorist who had also been working on this system.
I worked on those areas, made some progress in some of the things that Mikko was observing. For the first time in my life, I was doing research on a quantum problem with quantum spins and antiferromagnets, where the spins of different electrons like to anti-align. That turned out to be very fortuitous because in 1986, which was my second year of my post-doc, high-temperature superconductivity was discovered in the cuprates. There was the famous Woodstock Meeting in New York, which I went to as a post-doc. And many of the things that I started working on just by pure luck, many of those ideas found application in topics that became interesting because of the cuprates. Once the cuprates were discovered, I completely switched, and started working on problems inspired by the cuprates, and am still doing that today.
Did you understand immediately the significance of high-TC as it was still happening?
Probably not, because I was new to the field, and I didn't realize how unusual this excitement was. Everybody around me was tremendously excited. I was at Bell Labs and everyone was just, "Okay, stop everything. This is what we have to work on." And they were also saying “why wasn't it discovered at Bell Labs?” This was a point of some consternation there. How come somebody in Zurich discovered this, and it should really have been done here, because there were so many people working on related things. Anyway, they caught up very, very quickly, and a lot of ideas came out. But from people much more senior to me, including Phil Anderson and Chandra Varma were discussing these topics and would speak quite frequently. And I was listening in.
Did you talk with him? Did you talk with Phil during his visits?
Later in life, I did. I listened to his talks, and of course looked up to him tremendously. Once I started working in the field after I'd come to Yale, he was—I guess there is no other word for it, dismissive and said, "This is all nonsense," publicly in a way that would be totally not be permitted today.
Was that a motivator for you? Did you turn that into a positive that you could show Phil Anderson that he was incorrect?
Yes, hubris on my part, but it was, to a certain extent. I have been looking back recently on his ideas, and we understand those topics a lot better now, as there's been a lot of progress. I will admit that today I realize that he was more correct than I thought in those days. He initially stated that my approach was all nonsense. Today, I appreciate him more, and especially with the distance of not having direct personal interaction. He certainly was on the right track, but he seemed strangely unwilling to listen to ideas moving in a somewhat different direction. Not exactly what he had imagined, but somewhat different, in a way that seemed to me were dictated by what we were seeing in the experiments. He was not open to anything else. It had to be his way or nothing.
Maybe that's just the Phil Anderson I knew. People who knew him earlier say that's not the man he was. And maybe quite late in his life, he became a different person, much more vindictive and not open to even slight variations on what he was working on. I worked on something called Deconfined Criticality, which came out of a different set of ideas, but in the end, I think, connected very beautifully with some of Phil's ideas, not exactly what he had thought, but he was certainly smelling the right direction. But he just treated it as nonsense that shouldn't be mentioned and should be abolished from all conferences in the field, that was his attitude.
How did the opportunity at Yale come about for you?
Well, as a post-doc I interviewed for various jobs. I interviewed at many universities and Yale made me an offer very early on – December - before most other people had started interviewing, and told me I had just a few weeks to make up my mind. And I said, "Oh, fine." My wife Usha Pasi seemed quite happy with the idea. Also, I had come to Yale and I met R. Shankar, and I really enjoyed taking to him. He was a particle physicist, but I thought I could learn a lot from him, which I did, and so it was an easy decision. I was very happy to come to Yale and it turned out to be a very productive time for me.
What were you doing at that point with your research? What were you most interested in when you arrived at Yale?
That was '87 and just barely a few months after the Woodstock meeting. I'd been working on phosphorus-doped silicon with Mikko and Ravin Bhatt, and so I'd continued to work on that. But it was clear that I would move away from that and do something related to the cuprates. Once I got to Yale, I started working on the problem of antiferromagnetism; the parent compound of the cuprates is an antiferromagnet. One of the questions that immediately arose, partly from Phil Anderson's ideas was, you've got the antiferromagnet where the spins are aligned in a perfect orientation, what would happen when quantum mechanics became more important? A Spin Liquid would appear, I don't know who coined the term, perhaps Phil Anderson. The whole field of studying spin liquid phases of antiferromagnets seemed totally open.
In fact, while I was at Bell Labs, another person I talked to quite a bit was Duncan Haldane; he was at Bell Labs then. He also worked on antiferromagnetism, but in one spatial dimension. He started telling me things on the board in the tearoom, and in his office, about his ideas on how to extend this to two dimensions to the square lattice, and what would happen, and you get all these subtle Berry phases. So, a Berry phases is a phase factor that appears in many quantum systems, associated with the fact that the wave functions can have a certain geometric structure. That was a very new idea in physics, at least to me. It had been around for the previous five or six years, and it had started to be relevant in the physics of the quantum Hall effect. Haldane was really the first to start thinking about it for antiferromagnets in one dimension, and that's what led to his Nobel Prize winning work. And when you started applying it to two dimensions, Duncan would fill the blackboard with his ideas, and I would just sit there, and I could not understand what he was saying. But I got a little bit here, a little bit there, didn't understand the entirety of what he was saying.
When I got to Yale, I talked to Shankar about it and we said, "Okay, we've got to figure out what Haldane meant when he wrote all this stuff." And eventually, we figured it out in our own way, but we never published it. We said, "Well, Haldane knows this. So, there's no point in publishing this." And then, we found several papers, with many other people publishing the same idea.
Nick Read then arrived at Yale, which was another defining event for me. I started working with Nick on quantum antiferromagnets, and that turned out to be tremendously productive and generated ideas that even today are actively being pursued by many people.
What was so formative about Nick Read for you?
Well, he was an extremely brilliant young man. And he came from the fractional quantum Hall effect side of things, and he had done some very important work on Berry phase effects, and studies of anyons in the fractional quantum Hall effect. And as I just mentioned—it was quite clear to many people in the field that the same ideas should have some important applications in antiferromagnets, although it wasn't clear how. So there were, I would say, two papers that I wrote with Nick. We wrote many papers, probably a total of fifteen papers, I think, but two of them are very important, I think, if you look at the citations today.
One was on really understanding these Berry phases for antiferromagnets better. So antiferromagnets have certain defects called Hedgehogs: the spins all point in space-time, like a Hedgehog in all different directions. As the spins are fluctuating on the square lattice, they form a Hedgehog and go from one state to the other, fluctuating in time. And Haldane had pointed out that these Hedgehogs carry some strange Berry phase factors. What Nick and I realized was that the correct way to formulate this was in the language of gauge theory. Gauge theory until then mostly was a theory, like the gauge theory of electromagnetism or the gauge theory of the strong force-- of a fundamental field that existed in particle physics which mediated forces.
But in the studies of quantum Hall effect, people had started to appreciate that you can have what are called Emergent gauge fields. You could have just ordinary electrons, that don't interact via microscopic gauge forces, but because of the quantum mechanics of Berry phases and the presence of fractionalization, end up with an emergent gauge theoretic description. What Nick and I understood was how to do that for antiferromagnets, and how you could take all of the things Haldane was talking about, and then apply them very easily using gauge theory. And in the gauge theory approach, we had to consider Dirac monopoles. These were fictitious objects that Dirac dreamed could exist in the real world, and these appeared as tunneling events in antiferromagnets, and they carried certain Berry phases. But once you had the gauge theory, you're able to go much beyond what Haldane had said. We understood something about what's called valence-bond-solid-order, and we had the first example of what's now called a deconfined critical point. So that turned out to be a very productive avenue of research which has led to many, many developments, some by my former student, Senthil and many others. And that all started with work with Nick and previously with Shankar, trying to make sense of what Haldane was saying, but formulating in a different way, and that led to many, many new ideas and developments.
Subir, to go back to that earlier question currently about the interplay between theory and experiment, in those early days of high-Tc and you jumping in on cuprates, what was the interplay at that point?
As I mentioned, there was an insulating antiferromagnet, and then when you dope it with charged carriers, you lose the anti-ferromagnetism. That opened up the whole problem of quantum criticality or quantum phase transition where you lose magnetism. All of the experiments were in that domain of putting mobile charged carriers. What Nick and I and others focused on, we said, "Well, that's too hard a problem. Yes, we want to understand that, but before we can understand that, we have to understand what happens in an insulator." Indeed, that’s what I would advise young people today, "Take your inspiration from experiments, but you don't have to talk to experimentalists every day. Take a break and think about simpler problems which are related, and which maybe you can make more substantial progress, which will hopefully stand the test of time." And that's luckily what happened. So our work on valence-bond-solid-states and monopoles and Berry phases and deconfined criticality was not immediately applicable to any experiments. In fact, we were criticized, "Yes, that's all nice and good, but that doesn't apply to this experiment." Since then, it's found a lot of applications in all kinds of things, but it's taken twenty-five years for that to happen. Luckily, the tenure system gives you time to wait that long.
On that point, Subir, as a junior professor at Yale, what was your sense of the culture of promoting from within at Yale? Was tenure a realistic thing or you didn't see it that way?
You were certainly told repeatedly that it was not a realistic thing. Certainly, the chair basically said you should plan on just leaving in a few years, that should be the way you should view this, and that is the way I viewed it. Although in private, some of my closest colleagues would say, "You'll be fine, don't worry." It was very different then. I thought what would work to my advantage, I think, is there wasn't really an established quantum condensed matter theory group there. Nick Read, Shankar, myself, and Doug Stone—Shankar was more senior, he was in particle theory, but the three of us were junior. And so clearly, they wanted to form a group. So maybe one of the worries was, "They want to form a group, but they're not going to tenure all three of you." Eventually, all three of us got tenure, but it was a different time.
When did you start thinking about quasi particle excitations?
You mean anyon excitations?
Well, even the familiar electron can become a qausi particle, but an anyon is a particular type of qausi particle that has other unusual properties. Anyon first appear in my own work in a paper with Nick Read that's still making waves, my paper on the Z2 Spin Liquid. Our first paper was on just the square lattice antiferromagnet, and the role of these monopoles and Berry phases and quantum phase transitions which were inspired by the cuprates. Then, we said, "Okay, well, let's take these ideas and apply them to other lattices." Maybe someday somebody will discover another material on the triangular lattice or the Kagome lattice.
So, we started to think about more complicated lattices and found the situation was entirely different. You would get not a usual type of gauge field like that in electromagnetism, but what's called a Z2 gauge Field. So electromagnetism is associated with the phase factors that the electron wave function can acquire, and that phase factor is connected to the positions of other electrons which produce the gauge field. But in a Z2 gauge field, it's not a phase factor, it's just a factor of plus or minus 1. And, Z2 gauge theories had been studied earlier by Wegner, in a model with a microscopic gauge Field, and he showed it could have free excitations that were not confined, just like in electromagnetism.
What we realized is that here you get an emergent Z2 gauge Field in certain Spin Liquids, and that one of the implications of the emergent Z2 gauge Field was that there is any there is an anyon that has a strange phase factor. Now, anyons were not new at that point. They first appeared in the Laughlin Theory of the fractional quantum Hall effect, but the Fractional quantum Hall effect is done in high magnetic fields. One consequence is that they break time-reversal symmetry. So once you have a magnetic field, all the electrons are cycling counterclockwise just because of the force on them. So, the anyons also in the Fractional quantum Hall State pick up one phase factor, one when you go clockwise, and a different one when you go anti-clockwise. This structure of just going in circles in one direction seemed crucial to the way that whole theory of anyons was formulated. It's connected to Chern-Simons gauge theories that Witten had been working on, and those all had this feature of having handedness, things were chiral. They only went clockwise and not anticlockwise.
I think at that time, almost everyone believed that was needed to get anyons. There was a theory of anyon superconductivity by Halperin, Wilczek, Laughlin and Witten, who wrote papers on, "… the cuprate problem’s going to be solved by anyonic superconductivity with a chiral structure," because that's how you got anyons, and anyons could do interesting things like superconductivity. But you needed to break time-reversal, so you needed to have a chiral structure. That was certainly what people were saying in 1992.
We realized that was not needed. You could get anyons without having a chiral structure, and this was the first example. The same structure has since shown up in so many other places, although it's not often acknowledged, including Kitaev's toric code, in many of the developments in mathematics, and in the study of topological phases. The theory is now extremely sophisticated. But the zeroth-order example, the baby kindergarten example of that was our paper, and it was the first one.
More broadly, Subir, in the 1990s, what were some of the advances in your contributions on quantum Hall effect research?
I didn't do very much on the quantum Hall Effect per se partly because— well, there was enough to work on the cuprates, and that's what Nick was focusing on. So I thought, "That’s what Nick’s doing, and I should work more on the antiferromagnetic side”. It was great when we collaborated, but there was an unspoken agreement that, well, we just focus on different areas, and don't want to be on each other's toes. But at the same time, we discussed our work a lot. I certainly learned a tremendous amount from him.
I wasn’t working specifically on the quantum Hall effect. I was looking at quantum phase transitions and quantum criticality, and I wrote a few papers on the nature of critical points in the quantum Hall problem. So, what happens when you go from one quantum Hall state to other as you change the magnetic field? There were experiments on that, and they were rather mysterious. I think I played a role in helping understand some of that. It's still not fully understood, but that was my main contribution to quantum Hall physics: more on the study of the phase transitions between different quantum Hall states, not the states themselves.
What were your motivations in writing the book, Quantum Phase Transitions, both in terms of seeing that there was a gap in the literature versus using this as an opportunity to help clarify your own thinking in the field?
I would agree with both those statements. I taught a course based on these ideas to students including Senthil, whom I’ve mentioned, Kedar Damle, and Ilya Gruzberg, and also benefitted from good post-docs there at that time and teaching them, and hearing their questions was really helpful. But what I started working on around that time, as I mentioned earlier, is the theory of quantum critical points after the important paper by Chakravarty, Halperin, and Nelson in 1989.
I started investigating this further, especially in the context of the work on antiferromagnetism and spin liquids. What I think I played a role in understanding then, was that at critical points you had the breakdown of the qausi particle idea. All of these states that I have mentioned, the Fractional quantum Hall states - everything is dominated by qausi particles. The qausi-particles could just be plain old electrons, but they could also be Laughlin’s qausi particle with fractional charge. They could be these visons, that’s what we call what you get in Z2 Spin Liquid, phonons, spin-waves, these are all qausi particles. Condensed matter Physics is now dominated by the Theory of qausi particles. And the thing you have to do to become famous was invent a quasi-particle.
What I played a role in developing the theory of was, well, at certain quantum phase transitions, there are no quasi particles, there's nothing. It's just some kind of incoherent soup. If you try to create any excitation you want, it just becomes like jelly and flows away. Well, that sounds very boring, but the reason there were no quasi-particles had everything to do with quantum entanglement and the long-range structure of quantum entanglement.
This led to a new type of universality, nowadays called Planckian universality, that you find in things that become chaotic and soup-like and equilibrate locally in a fundamental time that is just Planck's constant divided by absolute temperature. And the importance of this fundamental time for all kinds of processes was something I investigated a lot. I wrote many papers in the nineties on this. I felt this was a new way of thinking about these phases, and that it'd be best to put it all together in one book, and that's what I did. Although one of the interesting things, also in '93, I developed what's now called the SYK model, a solvable model of a system without quasi-particles. There you could directly see that they will be no quasi-particles. But it was a very strange model. It appeared to have no direct connection to anything in the real world-- it was an antiferromagnet with lots of random spin coupling, but with every spin was coupled to every other spin.
Subir, why do you say at the time that SYK was artificial, and then how does it become more real over time?
Yes, that's the story I'm going to tell in a minute. Anyway, to finish the original story, the SYK model does have this Planckian behavior without quasi-particles. One of the curious things is that I didn't even mention SYK in the first edition of my book on quantum phase transitions. The SYK work had been done a long time earlier, and I am addressing that in my next book. In the first edition, my book doesn't even mention SYK mainly because it seemed then like a sidelight. Although my colleague Antoine Georges in Paris was more sanguine than me, and he worked on the SYK model some more; but that happened when my book was almost done in '99. Then Antoine, his student Olivier Parcollet, and I started applying the SYK model to somewhat more realistic models, and I was understanding the connections to some work that Antoine had done on dynamical mean-field theory. It started to become clear that, it wasn’t that artificial. The kinds of equations that you were solving apply to a very wide class of models, some of which were reasonably realistic. We wrote a few papers saying some of this, and there was essentially zero reaction from the community.
So, another piece of advice I give to young people is, "Don't let that bother you. You just continue if you think it's interesting." I was busy with other interesting things, and so we didn’t work on it that much. I came to Harvard in 2005, at that point, I was looking for other solvable models of Planckian dynamics—there was the SYK model, but nobody seemed to care for it. So, I was looking for other models, too. I was talking to particle physicists and not finding any interesting leads. One day I saw this paper by Dam Son on the viscosity of the quark-gluon plasma and quantum gravity and AdS/CFT correspondence and I said, "Wow, this is exactly what I'm looking for." Some solvable model, from string theory, that can have this kind of universality that we have been finding in condensed matter with non-qausi-particle Dynamics.
I contacted Dam Son, and I asked him to teach me some of this technology and apply to other systems that were more interested in condensed matter or connected to condensed matter. And what is sometimes called, the AdS/CMT connection between string theory and condensed matter Theory, that was shown in the paper I wrote with Dam Son, and others, and later in papers with Sean Hartnoll who was a post-doc at Harvard at that time.
At the same time, I was learning about black holes and black hole dynamics and their connections to super Yang-Mills and conformal field theories, and their connection to quantum critical points and condensed matter. I had learned about a particular type of black hole that I'd never heard of before from Hong Liu and John McGreevy at MIT. And I looked at their solution of this black hole with its Planckian dynamics and said, this is exactly like the SYK model. And so then, I wrote a paper on that.
In what ways, Subir? What was the connection to SYK as you saw it?
There was this black hole solution that I learned of from John McGreevy and Hong Liu. They started studying a string theory model of a metal which lost quasi-particles. But in the end, at low energy theory had no spatial structure. It just became a different theory of a point. So, it was a point with one-time direction. In holography it became one space and one time, because there's an emergent spatial direction. And that emergent spatial structure was called AdS2.
So first, it had only time-dependent correlations, without any spatial correlations. That's exactly what you get with the SYK model. The characteristic time for relaxation was a Planckian time, that's also what you get in the SYK model. Then, it also had the features of the entropy, the Bekenstein-Hawking entropy of the black hole went to a constant at zero temperature. It seemed to violate the third law of thermodynamics. It doesn't really, but it seems like there was a lot of entropy down to zero temperature. And that was also the case in the SYK model. That was something that Olivier, Antoine, and I discovered in 2001. And when we found this Entropy, we said, "This is the strangest possible thing. No real system is ever going to look like this." And that's partly why we didn't continue working on that. The entropy just bothered us. And suddenly, I saw this other system that seemed very realistic from a string theory point of view in the study of black holes, these are actual black hole solutions which seemed totally stable, they had the same entropy. And finally, if you looked at the correlation function of the fermions, they obeyed a certain conformal structure with a certain particle-hole asymmetry and a certain functional form of how time correlations decay with time and temperature. And that functional form was identical, the same function, the same ratio of gamma functions and so on, highly non-trivial functions, was the same as in the SYK model.
I wrote a paper in Physical Review Letters in 2010, and I even gave a talk at the Strings Meeting in Sweden on this. And luckily, in the abstract of the paper, I said the SYK model—I didn't call it as SYK because K wasn't on the scene then; I said, “Certain random quantum spin systems realize the physics of extremal charged black holes.” That's the sentence. I'm proud of that sentence because it is true today that that particular fact, and all of its implications, is a significant part of current research in string theory.
Subir, what connections were you seeing between black hole information and quantum information?
I wasn't thinking in terms of information then in 2010. It was the Ryu-Takayanagi formula that gave this connection between entanglement entropy and black hole entropy, this had been proposed in 2006, and many of its implications had not yet sunk in. Certainly, I hadn't fully appreciated it in 2010. Hawking’s black hole quantum information problem is encapsulated in something called the Page curve on entanglement. The first examples of how black holes realize the Page curve came out of the SYK model. And now, they've, of course, moved well past the SYK model, and worked with many other models. I was going to read this sentence from my 2010 paper, here it is precisely: "And this correspondence implied that certain mean-field Gapless Spin Liquids,” that's the SYK model, “are states of matter at non-zero density realizing the near horizon physics of Reisner-Nordstrom black holes.” That’s what I said in the abstract, but this paper fell on deaf ears. Nobody reacted to it because—
Is that because to some degree in the strings community, you were an outsider? You think that was part of it?
Well, they treated me pretty well. The string community invited me, and I think they took me seriously. There are parts of the paper which are written for the condensed matter theorists which were incomprehensible to the string theorists, and there were parts of it written that were for string theorists which were incomprehensible to the condensed matter theorists. I think that's kind of what happened.
Subir, to go back earlier in the decade, I wonder if you can talk a little bit about how your treatment of quantum criticality attempted to get at deeper questions in quantum mechanics?
Well, I guess I would say what I was trying to get at was deeper questions in, let's say, quantum dissipation. How does quantum mechanics lead to dissipation and how is it important? For example, if you have electrons moving in a metal, any metal has a finite resistance. And to compute that resistance, you have to do a quantum mechanical calculation. It's all related to how a particle which is moving in a given direction can scatter into this infinite set of faraway states, and so those electrons quantum mechanically tunnels their way to some other region and their energy gets dissipated. It never comes back. So, it seemed quantum mechanics was important, was involved in dissipation, but it also seemed as if dissipation was some extraneous effect, having to do with impurities, had to do with other things that weren't really that quantum mechanical. And if you could just clean up a system enough, it should have no dissipation and everything will be nice, quantum, and completely coherent with quantum mechanics forever.
But when you study quantum critical points, you realize that that simply wasn't the case. I've been studying quantum dissipation starting my undergraduate work with Dan Kleppner in his lab, and I brought that same perspective to quantum critical points and realized that actually there was this Planckian regime where dissipation was certainly not a distraction, and indeed had a universal value. It was very intrinsic to quantum mechanics, and it was crucial to lots of dissipative phenomena. And this was a consequence of the entanglement, the complicated scale-invariant entanglement of the wave function. I think that was a feature of quantum mechanics was something I helped uncover.
And around the same time, how much were you paying attention to advances in Bose–Einstein condensate research?
I paid attention to it all the way through.
You did. You did. When was it most valuable for your research? When was your most intensive paying attention to Bose-Einstein?
Well, it's continued to today. Initially, it was Bose–Einstein condensation that I was paying attention to, but in the first few years, they were simply confirming the theories that have been developed in condensed matter a long time ago, but in a very different arena, in a very different parameter regime. But a lot of the condensed matter technology that had been developed to understand, say, liquid Helium transferred amazingly well to ultracold atoms.
So that was great as an experimental achievement, but as a theorist, okay, there's nothing for me to do here. But then, I would say, Immanuel Bloch changed that with his experiments on optical lattices where he saw the Superfluid-insulator transition with a quantum critical point between the Superfluid phase and an Insulator Phase, the likes of which had never been seen in condensed matter, not certainly in such a clean system with this kind of controllability.
And so at that point, from that day on, once I had heard about Immanuel Bloch’s experiments, I've been paying close attention to what people have been doing in cold atoms, especially in optical lattices. A more recent development has been pioneered by my colleague, Mikhail Lukin, and also Antoine Browaeys in Paris: they take optical tweezers and put Rydberg atoms in them, and then also cause them to undergo quantum phase transitions and form spin liquids. That’s something I'm very much thinking about today.
Around this time, also, at the turn of the century, you're working a lot on superconductors. What were some of your contributions more broadly in the field at that point?
I would say that the theory of the superconductor-insulator transition, and of transport near the superconductor-insulator critical point, that's been fairly influential, and that's probably my most important contribution to that subject. We also thought a lot about superconducting pairing of electrons at quantum critical points. In the usual BCS theory, it is phonons that mediate the attraction between electrons, leading them to form Cooper pairs that undergo Bose-Einstein condensation. Could there be some general attraction between two electrons to form Cooper pairs near quantum critical points? I've written a paper on that with Senthil and one of my more recent students, Max Metlitski and one of Senthil’s students, David Mross. So that, I think, has been a fairly impactful paper on superconductivity near critical points.
And back on the administrative side, what were some of the considerations leading you to join the faculty at Harvard?
Well, that's a good question. I had been at Yale for eighteen years. And of course, I had gotten my PhD at Harvard, and I had very good memories of my time there, and all that I had learned from my fellow students and from the faculty. I loved the idea of being back in Boston in a big city where there'd be also more opportunities for my wife, Usha, who was working at Yale. Usha and I had gotten married just after I finished my degree and before I was going to Bell Labs, and she's been very supportive and important and given me the freedom to pursue all these interests over the years. Both our daughters, Monisha and Menaka, were born in New Haven, and they seemed excited, at least at some point, about a possible move to Boston. Anyway, so why did I move to Harvard?
Well, I felt Yale had been great, especially the collaborations I'd had with Nick Read and Shankar. I had learned a great deal from both of them. I'd moved my research in a whole different direction, and I felt maybe the time was right now to do something new again, to talk to other people, and be stimulated by other experiments and other theories, and do something different. I had written my book on quantum phase transitions, and that whole line of thinking had reached a certain mature status. I felt I was still young enough to try something different. In a way that happened, all this work on connections to black holes, connection to holography, that all happened at Harvard. So, some of it planned, but mostly through serendipitous interactions. And that could have happened at Yale, you can never tell, but I just felt it was good to have a change of scene, and it worked out well.
Who were some of the new collaborators that you were able to work with as a result of making the move to Harvard?
Well, I ended up working mostly with my own students, I had a much larger group of students and post-docs.
Did any of them come with you from Yale?
One of my students did. Stephen Powell stayed at Yale, and Adrian del Maestro moved with me to Harvard. But I’ve had many new students here. And I've collaborated with several experimentalists here, that's Mikhail Lukin, Amir Yacoby, Philip Kim, Markus Greiner, Jenny Hoffman, all of them are experimentalists here at Harvard, and they've all stimulated me in different ways to think about different problems.
Culturally, what were some of the differences? I mean, obviously, you've known Harvard as a graduate student, but you see things very differently as a faculty member. What were some of the big differences that you saw with this new vantage point when you got to Harvard?
Well, they're similar. I think Harvard is a bigger department, and it had more strength in experimental atomic physics and also experimental condensed matter physics. At Yale, those fields were rather small. The culture as well, I think Harvard, it's more of an individualistic culture, in that each professor is a little bit of an empire to themselves. Yale was less so that way. There are many more students in science generally at Harvard, and also physics, and I think it's more lively at Harvard than Yale. Yale is smaller. I would say, for me, I got the best of both places. At Yale, I was left alone for five, six years to do whatever I wanted. I didn't have to worry about too many things: I didn’t have to worry about students finishing, getting student jobs, and all these other things. Just do what I wanted, and develop my own set of ideas, without too many worries on running an empire. And I think that's good at a younger stage in your career, because you have to develop your own view on things, and to think deeply about something for a while without all these pressures.
Whereas at Harvard, you don't have that much time to sit down and write a book for example, which you might try, but it's much harder because it's a bigger place, and there are many people. Someone's constantly in your office, one of my students is constantly telling me this and that, and I have to keep up with them, and keep many different projects going. Which is fine if you had a lot of ideas to build on at a younger age, which I had from my time at Yale. So sometimes I feel sorry for some of the junior faculty at Harvard, where they feel these pressures immediately, and I say, "Well, I would not have survived that. You need to take some time to develop your own view on science.”
Subir, when did you see that graphene would be such an exciting area to work in?
Actually, fairly early. I had mentioned this theory of quantum critical points. When graphene came on the scene, you have these massless Dirac fermions, and I immediately remembered some earlier work I'd done with my former student, Jinwu Ye on Coulomb interactions with Dirac fermions. We'd written that paper even though the Graphene didn't exist then, and there weren't any other Dirac fermions known in nature of that type then. When Graphene came on the scene, there were many papers in the field treating these electrons as free electrons with Dirac dispersion with disorder, using some Landauer-like ideas, or quantum coherent transport ideas, which had been very successful in metals. To me, that immediately seemed wrong, because it seemed clear that Graphene should also have some kind of Planckian behavior with this universal quantum dissipation, just given by Planck’s constant divided by absolute temperature. And then you’ve got to think about transport in graphene in a very different way.
So, we wrote a few papers, with Markus Muller and Lars Fritz who were my post-docs then. Again, there was not much attention paid to them. And eventually, there were connections to the work with Sean Hartnoll on holography. It was interesting how this all happened: we realized that you could see some of these holographic connections by looking for hydrodynamic behavior in graphene. Graphene should be hydrodynamic, we said this in 2008 long before anybody. And around, when was that, in 2014, '15, when experiments started seeing what we had predicted, and our research built on that. Today, studying hydrodynamics in Graphene is a big field. But that's just one small part of graphene research. That was my impact with the help of these collaborators on graphene research. These days, all the excitement is not so much on graphene but on twisted bilayers of graphene. That was a complete surprise to me, for sure, and that's been also fun to follow.
More broadly, what do you see as some of your contributions to the field of quantum chaos?
Many-body quantum chaos was an amazing new idea that Lenny Susskind, Steve Shenker and Douglas Stanford brought into the field. Earlier, I talked about Planckian Dissipation, and how there was a certain universality to resistivity or some quantity like that, that you could measure in the lab. And the underlying physics in my approach was the absence of quasi-particle excitations. But Steve Shenker and others were motivated by different considerations related to black holes, and they introduced a new way of looking at the Planckian universality. They focused attention on a measure of chaos, these out-of-time-order correlation functions, which I'd never heard of. At that time, you couldn't conceive of an experiment to measure such correlations, although people have made some progress since then. But what they pointed out was that this Planckian behavior was even more universal, when you look at the chaos aspects of it. I had been thinking about Planckian universality for years, but suddenly, they brought in a fresh new idea into it, which allowed nearly rigorous progress. One of my students, Aavishkar Patel took that idea and combined it with some of our older work, and he tried to understand quantum chaos in a more condensed matter system. And actually, we're still working on some of that.
To go back to your comments from 2010 at the strings conference about what was comprehensible and what was incomprehensible, in light of your later interests in Hawking radiation and entropy, when did things start to click where it seemed more feasible that there really was an intellectual connection between these fields?
Well, that connection, I think had already been accepted by 2010. I mentioned my work with Dam Son and Sean Hartnoll and Markus Muller and others. And there was a lot of work in that direction, by Hong Liu and John McGreevy. So, it was clear that there was a connection between black holes and quantum critical points and Planckian dissipation, strange metals, these are all really closely connected problems.
What was not clear before 2010 was the connection to this strange SYK model. The SYK model had random couplings, and this was definitely an unacceptable idea in black hole physics that time. The couplings were just random numbers. And I think that was just too far for any String Theorist. They couldn't relate to that at all. It didn't seem clear, why should you study a model random coupling? My 2010 talk was mostly about the more general holography ideas, but I had at least fifteen minutes on Random Couplings where I got deadly silence. I gave a couple of other talks, and it just never went anywhere. It’s amazing today to see that many of them are now working on random systems, random conformal field theories, realizing that averaging over theories simplifies life, that's a big trend today in black hole research.
Again, a lot of these computations of Page curves and so on, they are for essentially random matrix models or random SYK models. I think people now understand this. A very beautiful understanding of that is due to Steve Shenker and others: when you're dealing with a system that's chaotic, it doesn't matter so much whether the chaos came from a random coupling in your Hamiltonian, or was because of the nature of many-particle quantum mechanics itself. It doesn't matter that much. It just so happens that if you have random couplings in your Hamiltonian, that makes it easy to treat the chaotic dynamics because you can easily average over the possible Hamiltonians. If you want to deal with a chaotic system which has no intrinsic randomness, then you have to do an average over some sets of initial conditions, and because you don't know the initial conditions and you have to do the average in a subtle way, and that's much harder. This is an understanding that's emerged from the connection to quantum chaos, I should say.
Subir, I'm curious about your professorship at the Tata Institute of Fundamental Research and how Americanized you were at that point to the extent where you felt like you were coming home or not?
That's a sensitive point. I think I do feel like I'm coming home. I mean I have been visiting the Tata Institute often. I will be going in the fall, pandemic permitting, for a couple of months to teach a course there. Earlier, I have presented seminars, but I haven’t taught a course at the Tata Institute. Oh, I go back regularly to India once a year, except for the pandemic, at least, and I have close contacts with people at the Tata Institute and the Institute of Science in Bangalore, and also many of my classmates from high school in Bangalore.
Oh, so it's a bit of a reunion?
Yes, especially when I go to Bangalore my classmates are very nice. They organize a get-together someplace or the other for one evening, and people I haven't seen for thirty years sometimes show up.
On the question of honors and your identity, both in India and the United States, I wonder if you could contrast how it felt to be elected both to the U.S. National Academy and the Indian Academy of Sciences?
Both are really very meaningful to me. The U.S. came first and that was quite a surprise. I was really thrilled. And The Indian Academy came a year or two later. I was very flattered that they remembered me, but I've been going there quite often, but still even so, the Indian scientific community recognized me, I went there for the ceremony. It’s a huge honor and I want to contribute more to India, since that's where I have spent my formative years. And I try to go whenever I can and contribute to science there.
Tell me about some of the work that your graduate students have been doing in recent years, perhaps as a window on to long-range interests in the field.
I've had quite a number of successful students. I don't know if I can name all of them, I will forget a few. Well, okay, they've all done their own things in different ways. Many have gone on and worked at private companies in Amazon and Google and so on. But let me name those who are faculty in the U.S. I have mentioned Jinwu Ye, who is now a professor at the University of Mississippi. Senthil is now a professor at MIT: he has pioneered many beautiful things, including the theory of symmetry-protected topological phases. These are quantum phases where you combine anyons with symmetry in a very interesting way. Max Metlitski, who is assistant professor at MIT, and he has worked on related things, and also recently on various quantum impurity problems. Andrew Lucas is a Professor at the University of Colorado, and he's a leader in some of these things we talked about earlier, on hydrodynamics, chaos, quantum mechanics, holography, how they all connect together, and how are they all connected to experiments in materials like graphene and other systems. He played a key role in connecting hydrodynamic behavior in graphene to experiments by Philip Kim at Harvard and has taken that field to the next level as a junior faculty member.
Currently, one of my former students, Alex Thompson, is a post-doc at Caltech and will be a faculty member at UC Davis soon. She's played an important role in various things with twisted bilayer Graphene. I should also mention Debanjan Chowdhury, a professor at Cornell; Adrian del Maestro, at the University of Tennessee; Rudro Biswas at Purdue; Anatoli Polkovnikov, who is at BU, and working on various problems related to quantum chaos and many-body localization. They’ve gone on in many different directions.
And it's always great to see when they developed some whole different area, and perhaps they learned a few things from you.
Subir, to bring our conversation up to the present, I'm curious what are some of the major open questions remaining in antiferromagnetism?
Well, I would say the biggest open question is the theory of pseudo-gap in the high-temperature superconductors. This goes back to the very beginning of the field. I mentioned that you take an antiferromagnet and you put charged carriers in it. If you put enough of the charged carriers in it, it loses memory of the anti-ferromagnetism mostly and becomes the high temperature superconductor. But before we get there, there's this intermediate “pseudo-gap” region where we still don't quite understand what's going on, and that's really crucial to understand because, really, the underlying structure of superconductors very much relies on the anti-ferromagnetism. And so this pseudo-gap regime is probably deeply connected to spin liquid physics. I certainly believe that. All the experiments seem to support that idea. But we still have some ways to go towards understanding which spin liquid precisely, and how can you really pin down the spin liquid characteristic, and how can you relate that to some set of experiments? To me, that's a big open question and something that several of my students are working on right now, in fact. So that's certainly a very big one.
Another one is why is it the SYK model, which has had quite a bit of success in explaining not only black hole type physics, but also certain physics on measurements of strange metals, why can this be captured by SYK, despite it being an artificial model. So can you extend and understand that better, and ultimately—where you'll be able to say someday, “Okay, well, this material that you showed me will display SYK-like physics and that material won't.” We have some microscopic understanding of this, but there remain several open questions.
What about the major open questions in Ising gauge theory? What do you see as major work that needs to happen in this area?
Well, the basic Ising gauge theory is pretty well understood. But you can imagine taking Ising gauge theories and perturbing them. You can perturb them with disorder, you can perturb them with magnetic fields, or you can perturb them with fermions. Presumably, some of that is important for the pseudo-gap physics also. So, I wouldn't put Ising gauge theory on its own. It's connected to deeper problems in the theory of spin liquid phases, especially when the spin liquid phases are metallic and not insulators.
I will say that of spin liquids that are insulators, the theory of that is now fairly complete, especially after the mathematicians got involved with it, with ideas from category theory, of all things, it kind of blows my mind. So, at a deep level, they're understood, but we still need progress at the computational level, how do you connect this particular exotic spin liquid to this particular experiment? That’s still very hard.
To think really expansively with regard to the way that the SYK model has been valuable for black holes, do you see this research being valuable also for developing a theory of quantum gravity?
Well, it's hard to predict the future. When you say quantum gravity, what I imagine you mean is you take the equations of gravity which are classical, as written by Einstein, and you quantize them. Now I think today, it's pretty well understood that that's not a well-defined task. You can't just quantize gravity on its own. You have to have additional degrees of freedom at very short distances at the Planck Scale, like string theory or something.
So string theory is not, per se, a treatment of gravity. It's a theory which puts in lots of new stuff that doesn't even exist in Einstein's Theory at very high energies, or as we say in the UV. So at very high energy, there's lots of new stuff, which in the end, helps us quantize gravity even at low energies. The low energy theory looks kind of like Einstein's equations with a little bit of quantum put in.
What the SYK model has shown is, in some cases, you don't need to go all the way up in energy. You can put something much, much, much simpler, and still quantize gravity in a self-consistent way and get answers that are insensitive to what's going on at the very high energy, i.e. UV, scale. Indeed, for many familiar things, you don't need to know the microscopic details. For example, when you're doing the theory of fluids, you don't need to know that the fluid is made up of atoms which have nuclei, which are made up of quarks. They're just totally irrelevant. Similarly, if you want a theory of black holes in quantum gravity, do you really need to know that at some incredibly high energy scale, it may be made up of strings? Maybe not.
And so, the SYK model provided what we call a UV completion of some very simple theories of gravity. In particular for charged black holes at low temperatures, there was an entropy that was computed by Bekenstein and Hawking- sorry, Gibbons and Hawking in 1977, and they did this without knowing anything about string theory. They just said there's a quantum entropy in a black hole because that's the way it has to be for everything to hang together. And we understand from string theory mostly, that of course, their answer was correct. And today we appreciate that one reason it's correct is because that particular quantity is insensitive to what really happens at high energies.
Indeed, the SYK model has shown is that it's not just the Bekenstein-Hawking Entropy, it's the correction to the Bekenstein-Hawking Entropy, it is the Page curve, many other things are insensitive to the UV, because you can compute them using the SYK model in a way that the detailed SYK character is not important. You can see that. The whole calculation is so explicit, you can see that the detailed SYK structure is not important for the quantum gravity. I'm not saying that actual world is made up of SYK models, and black holes are made of SYK, they're not. I'm saying that for many of the most interesting quantities, we can get by without knowing what black holes are actually made of.
If you want to understand quantum gravity on its own terms, especially its low energy structure and many properties of that low energy structure, which is really what people have tried to understand, ultimately, sometimes you don't need something as complicated as string theory. And I think that has changed the culture of the field these days—there's a string theory conference later this month and there's so many talks about black hole entropy and information, and all done using just theories of gravity, where they have now a much better understanding how you can work with gravity on its own terms, without having to put in strings at very high energies.
If you can survey the state of play with the “strange metals,” are things more or less the same as when you started to think about strange metals or what has changed over the years?
Well, okay, a lot has changed both on the experimental and theoretical front. On the experimental side, I would say there's some very beautiful experiments by Louis Taillefer especially where he's made a very convincing case that there is this Planckian universality, that there's a time of order Planck’s constant divided by temperature, is present in many different experiments in many different systems. I should also add the name of Andy Mackenzie here, for his earlier observations. More recently, Louis has measured something called some thermoelectric effects: the Planckian metals seem to have large thermoelectric effects, larger than are present in ordinary Fermi liquids.
So, on the theory side. Okay, I'm biased here, but I think there's been a quite a bit of progress in the last two or three years where we have taken a lot of the work on the understanding of the SYK model that came from string theorists, Alexei Kitaev, Juan Maldacena, Douglas Stanford, and Stephen Shenker.. We have taken that technology, and now brought it to condensed matter, and applied it to more realistic models like the t-J model and the Hubbard model. And as a result, we've made a lot of progress in understanding the connection between SYK physics and realistic strange metals. We're not completely there yet, but I think we're getting quite close. But I should say that's not a completely neutral statement because I'm very much involved in this work.
Subir, if you can contrast that original excitement with high-Tc at the Woodstock meeting, right, at the APS, where is the field today? Where is the excitement? Where is the sense that there's that fundamental breakthrough or not from your vantage point?
Well, I don't think there's been anything at that scale, but with some remarkable regularity, there's been something exciting every few years that causes everybody to pause, and say okay, maybe I should work on this. So, in the past few years, I would say, well, there was the discovery of Graphene, there was the discovery of Topological Insulators after the prediction by Kane and Mele. There were also the iron-based superconductors that also have quite High-Tc’s, but have a more complicated lattice structure. Twisted bilayer graphene is the most recent area with a lot of excitement. And I would say, also, most recently, there's been slow and steady progress in the work from quantum computing, for example, Google's quantum computer. From my point of view, it's a wonderful system for studying quantum phase transitions and spin liquids, which is what they're doing with it. So anyway, the ability to control qubits in systems of 100, 200 qubits, so you can control each one individually and have them interact to yield interesting many-body physics, I think that's a really exciting development in the last two or three years, and happening as we speak. There are two remarkable papers that are being refereed right now that I know of.
And often, I think it's hard to identify revolutions as they are happening, when you may be, in some way, a part of them. It takes a few years to look back and say, oh, that was a tremendous development. So I think, it feels to me, with my limited experience, that all these advances, which were motivated by the dream of quantum computing, which I think remains a dream, are having a huge impact on experimental studies of macro-quantum properties like spin liquids, quantum critical points, quantum chaos, I think there's a revolution going on with all of that right now.
Subir, it's been so interesting hearing so many different physicists from so many different fields delineate where they see the reality and the true application versus the hype in quantum computing. Where do you see those things, the reality as it exists now and the hype and specifically on those existential questions of, do we even know what quantum computing is good for? And if not, how do we even know how to get there?
First of all, let me say that I'm a great admirer of all the work that's been done. It's just amazing what can be done now, and how much it's teaching us about many-body quantum systems. What perturbs me is: I guess you asked about hype, I heard a speech by the chairman of a major computer company saying that within five years, quantum computing will be some ten percent, twenty percent of their revenue stream. That is simply not going to happen. We should not be in such a position, the science community shouldn't allow that kind of incorrect statement to be made by the leader of a major company. And I realize I am using strong words, but I think we scientists have failed in that regard.
I once happened to be at a conference that Sergey Brin of Google was at. Google is also funding quantum work of various types. And there was a talk by one of the scientists, Lenny Susskind, just saying that this is wild, out-there work, and we have no idea what it’s going to yield, and we have no idea if it's going to work. So, as long as the companies understand that's what they're funding: they're funding really exploratory research because of its own intrinsic interest, and we can't predict when it's going to really pay off with any money for the company. And with that understanding, if the company is still willing to invest and say, "We'll take a chance, and this is interesting," if that's the understanding, then I'm fine with it. That's how it should be done. It's a moon shot. And will it work out for the quantum computing applications they envision? I don't know. I'm a little skeptical, myself.
But what I'm sure of that, much more sure, that something interesting will come up. Who it will make money from it, and what the application will be, I don't know. But just given the history, every time you make progress in an emerging area of science, even physical science, it's led to so many remarkable things that you didn't imagine at the outset: the transistor, the laser, the semiconductor circuit, or in the iPhone, all these amazing memories made out of magnetic materials. That’s an incredible track record of benefit from understanding quantum mechanics better. And if that's the point of view, then yes, we should definitely invest in quantum computing. The government should definitely do it. Should a company do it because it's going to make them money in five years, my answer on that is no.
Perhaps a question that's a little less far afield, what are your thoughts on AI and Machine Learning and the extent to which your research will contribute to advances in those fields and vice-versa? How AI and Machine Learning might advance the questions that you're most interested in?
Well, I think that's still early days. One set of ideas is that you just give some machine a whole bunch of data and ask the machine to classify it, which is how machine learning works. So maybe you can give a whole bunch of data from experiments of some cold atomic system, and you keep measuring, and measuring, and you get gigabytes of data, and you just feed it into a machine; so there's quite a few people that are trying to develop that idea. Let's see where it goes. Some of my students actually are quite excited about it and are working on it. I don't know enough machine learning to really contribute, but, and I think it could pay off eventually. But so far, it's a promising idea; that is all I think I can say at this point, but I'll be converted if they finally find something new that somebody hadn't thought of by other methods before. I don't think there's any example of that yet, but there could be pretty soon. Let’s see.
Subir, now that you had graduated from being department chair, I'm curious if you reflected at all about the opportunities you had to set a tone, not just within the department, but just by virtue of it being Harvard’s department of physics, where what happens there resonates far beyond Cambridge. I wonder if you've ever thought about some of the changes that you saw needed to happen, and what your role might have been in that. And that, of course, has academic, it has social, it has political elements to it.
Well, certainly something we worked a lot on as chair, and which I thought was extremely important, was “climate issues.” Really setting a tone that the department is there to support the graduate students and educate them, and that that is a responsibility of the faculty. And so every student is given every opportunity to develop their talents completely. That sounds obvious to say, but if I think back to my time at graduate school, although I had a great time and I have nothing but high respect for all of my professors, that wasn't the attitude. The attitude was, okay, you've come to Harvard, here are these professors and you sink or swim. And if you can learn something, well then good for you, but don't expect them to go out of their way to help you, and you're lucky to be here. Just work hard and maybe some of them might pay attention to you. And if you're good enough, you'll do well.
So there definitely was that culture, which I think has basically faded away at Harvard. I think we've worked very hard to make sure that that's not the culture, and that's not the way students should be treated. And there should be a real mentoring relationship, where it's the responsibility of the professors to mentor their students, and make sure they grow to develop their talents and not just say, "Well, you haven't come here well enough prepared. I can’t work with you." We don't explicitly allow a professor to say that anymore.
And then what I've learned is that students come to you with many different backgrounds and preparation, and especially female students, all of whom have had a different experience in their previous education. You have to account for that in the way you mentor them. Earlier in my career, I wasn't good at it. I've gotten better. I've seen how, with the right mentoring, the student you think is not quite ready. In the old days you'd have said, "Well, you're just not going to cut it and you should leave." With the right mentoring, such a student has done extremely well, not suddenly, but eventually developed into somebody who, with the old thinking, you couldn't have imagined. I think that's changed the way I deal with my students, and several of them have really done well, it's been very gratifying to have seen them more than exceed my expectations.
Given your longer-range appreciation for Harvard, and given the fact that there has been, over the past year, so much emphasis on the importance of diversity and inclusivity in STEM, how have you seen particularly in light of your vantage point as chair, how have you seen Harvard physics respond to some of these larger societal challenges?
I was personally involved in some of that. So, I can just tell you about some of the things we did, especially, in the #ShutDownSTEM movement that happened while I was chair last year. I gave a department address when we had several long discussions and follow-ups on what we should do which is continuing. But what I emphasized was that Harvard has presented, and American Education in particular, has presented incredible opportunities for someone like me coming from a different country. There are so many ways that we can get here. And once we get here, the whole world has opened up.
What's disturbing to see is that the same opportunity is not available for many minority people, or people of color, who are Americans who are born here. I like to take the example of the ICTP, International Center for Theoretical Physics, which was set up by up Abdul Salam for the express purpose of giving a place for people from countries like India and Pakistan, although I would say many of these countries don't need it anymore, but there are many countries in Africa who definitely need that kind of place to be exposed to modern science. And it serves that purpose extremely well. I really enjoy meeting people from all over the world at ICTS, who then go back to their countries, or stay in the West, and become part of the community of scientists. And there isn't any similar kind of place or similar kind of mentoring place available for people within America, for example.
So that's something I see that we should definitely do more of. I think our department is working on, I think, some kind of training program where students from universities that don't have cutting-edge science ; who can come to Harvard and have a summer where they take some courses to come up to speed. We're working on some of that, but I don't know the details of that now since it's in the hands of the next chair.
Subir, now that we've worked right up to the present, for the last part of our talk, I'd like to ask two broadly, retrospective questions about your career, and then we'll end looking at the future. So the first is, if you can survey all of the research you've done, all of the collaborations you've been a part of, all of the questions that you've asked, what sticks out in your memory as being most intellectually satisfying, either because something really clicked for your understanding or you felt like you made a really significant contribution that moved physics forward in a positive way, what sticks out in your mind?
That's an easy question to answer. That's when I wrote the original Sachdev-Ye paper in end of 1992, early '93. People had been thinking a bit about methods of randomness in Spin Liquids and then I decided to look at this. And I'd worked with my student, Jinwu Ye on a different model, where he had made quite substantial contributions, but in the end, that model didn't seem quite right to me. And so I came up with this other model, which is now called the SYK model, it's a variant of it. And when I looked at it, I'd worked out some equations and found a solution, at least at low frequencies, which just seemed totally strange. First, I was convinced that there's no way this could be right. Just didn't make any sense, and I almost just threw it aside.
And then, I decided to do some numerical work on it, and it seemed to check out, and things seem to be hanging together, Jinwu also did some additional numerics. But I just felt probably this is totally unstable, and it is never going to make any sense. I probably sat on it for six months, but I felt there was something really interesting here. And so, I published it after some difficulty. But then it's been amazingly satisfying to see this crazy idea of mine now at the center of so many things going on in studies of black holes and strange metals. I barely knew what a black hole was at that time, but I learnt more after 2007, and first pointed out the connection of SYK to black holes in 2010.
There's a lesson of the importance of intuition in physics where even if you're really not sure if something works, if you have a funny feeling, you should go ahead and publish it?
Absolutely, absolutely. I was lucky, but I was also, I would say, ready to explore the unexpected and just kind of ignore the skeptics. Because I was doing other stuff that the skeptics liked and that they gave me a little time to do other things too. I said, “I can afford to do something crazy too.”
Subir, to flip that question on its head, of all of the research you've been involved with, what's been most difficult where you thought that you were on the cusp of a breakthrough or some significant understanding and no matter what you do, you hit a wall, a theoretical wall, or the experimentation and verification is not there. What sticks out in your memory as the intractable problems in your field and what you've worked on?
Well, usually what happens when you get stuck, it's not that uncommon you get stuck on something, well, you just do something else for a while. Then you come back to it a few years later. So, by that criterion, which is the problem that I keep coming back to over the years, because each time I look at it slightly differently, it definitely has to be the strange metal problem. That drove the introduction of SYK in 1993. It drove my interest in quantum criticality, it drove holography, it drove a lot of numerical work that we've been doing recently. I will say that I don't think we've fully understood it yet. Right now, we have a new set of ideas that I think will finally make answer the open questions, but let's see.
It does beg the question, when you hit a wall, when you know to keep pushing and when you notice which course. What have you learned in that regard?
I have learned that you should switch course quickly. You’ve had some idea, you hear about some experiment, and you work on something, and you realize it's not going to work. I think you learn fairly early on that if you're just going to sit in a chair, and keep banging your head on this, nothing's going to happen. You’ve got to get out of it, talk to people, do something different. And then maybe a year down the road, you'll hear about some other experiment, or somebody will say something, that will spark something in your mind and then, you can come back to it. So when you get stuck, just forget about it, completely just give up for a while. I would say that the effort you spent was not wasted. That's happened to me many times in my life where I just say, "Well, this doesn't—not going to make sense. Let me put it aside." And then five years down the road, I say, "Where are my notes? Now, I know what to do."
Subir, finally, last question, to go back to that funny comment earlier, in the fields that you work on, obviously, there's no Shelly Glashow who's going to come along and say, "We figured it all out." These are fields where major questions will remain for decades, for even generations. So, on that point for you personally, what are you most curious about yourself, and where are you most optimistic that your research will continue to have a big impact in the field?
Right. Well, I guess what I'm most curious about is really the whole issue of quantum computing and whether it's possible to have fault-tolerant quantum computing. I think the most promising direction is using spin liquids. So can we really make that work? It seems very, very hard even today, but will that really work in the lab and not just as a theoretical idea? So I would say, I'm very curious about that, and I'm glad so many people are working on that question, just for curiosity reasons. I'm not enough of a proponent to tell everybody to spend their millions of dollars on it. Okay. And then, what are you most optimistic about, is that what you said?
Right. I'm optimistic about, I would say— the problem that I've always been optimistic about, the strange metal problem, but also problems in quantum Gravity. It's stunning the progress that has been made, and let's hope that some of this fusion of ideas from quantum information, condensed matter physics, and quantum gravity relate to some bigger conception of how quantum mechanics works on the large scale. I think I'm optimistic that there is still some deeper set of ideas to be discovered there which will change the way quantum physics looks.
Subir, it's been a great pleasure spending this time with you. I'm so glad we were able to do this. Thank you so much.
Thank you, David. It's a real pleasure and honor.