Nicole Yunger Halpern

Nicole Yunger Halpern

Physicist at NIST, and Adjunct Assistant Professor of Physics, IPST, and UMIACS, University of Maryland

By David Zierler, Director of the Caltech Heritage Project

July 22, 2022

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Friday, July 22, 2022. I'm very happy to be here with Dr. Nicole Yunger Halpern. Nicole, it's great to be with you. Thank you so much for joining me today.

NICOLE YUNGER HALPERN: Thank you very much for the invitation.

ZIERLER: To start, would you please tell me, and I know this will be a complicated answer, your titles and institutional affiliations?

YUNGER HALPERN: First and foremost, I'm a theoretical physicist. I'm employed by the National Institute of Standards and Technology. At NIST, I am a Physicist. I'm also a QuICS fellow at QuICS, the Joint Center for Quantum Information and Computer Science that is shared by NIST and the University of Maryland. I also have some Adjunct Assistant Professor positions at the University of Maryland. I have one in the Physics Department, one in UMIACS, the University of Maryland Institute for Advanced Computer Studies, which is kind of behind QuICS, and one at IPST, the Institute for Physical Science and Technology, which is an interdisciplinary institute that has a great deal of thermodynamics and computation.

ZIERLER: I knew that was going to be a complicated answer. [Laugh]

YUNGER HALPERN: Oh, I do have an affiliate position at the Joint Quantum Institute, too. I think that's everything.

ZIERLER: Are all aspects of your research agenda and academic interests interconnected within those affiliations and partnerships, or is some of your research really compartmentalized with one institution or another?

YUNGER HALPERN: I very much love bridging subjects and organizations, so I see everything as interconnected.

ZIERLER: At Maryland, do you have traditional professorial responsibilities such as supervising graduate students, teaching, things like that?

YUNGER HALPERN: I do supervise graduate students and post-docs. I have the option of teaching. I have not taught yet. And I've been serving on PhD committees.

ZIERLER: Just a broad question about quantum information at NIST, what's exciting, what's happening more generally, and where does your research slot into those efforts?

YUNGER HALPERN: A great deal of NIST's work on quantum information is based on experiments and atomic, molecular, and optical physics. That work is happening in the Physical and Measurement Laboratory at NIST. There's also an Information Technology Laboratory that has supported a great deal of work on cryptographic applications and standards related to cryptography. I am one of two theorists in my group of PIs at NIST, where PIs are collected into groups of people who have similar interests. I am the only quantum thermodynamicist around. I do a combination of quantum information theory and quantum thermodynamics. Although my work is theoretical, I do work a lot with experimentalists. Since I arrived under a year ago, I have not yet started working with experimentalists at NIST. My collaborations are elsewhere, but I hope to start up such collaborations in the future. And for many years, I've had a number of interactions with theorists at the intersection of NIST and the University of Maryland.

ZIERLER: Just as a snapshot in time, what are you currently working on?

YUNGER HALPERN: I always have a number of projects. One main focus of my group now is a quantum twist on a very common, old thermodynamic problem, a twist that hadn't been realized until a few years ago. Across thermodynamics, we think about a small system interacting with a big bath, an environment. The two exchange things. If they exchange heat, the small system will thermalize to one state, if the two exchange heat and particles, the small system will thermalize to another state, and so on for many different things that can be changed, such as electric charge. When we do quantum statistical mechanics, we represent these things exchanged with operators that fail to commute each other. This assumption of commutation has been implicit in some really important thermodynamic arguments for many decades.

That was realized by a very small number of people up until a few years ago, and not very much was done about that assumption. But quantum theory is very interesting because it features operators that don't commute with each other. It's as though three times two is not the same as two times three, essentially, in quantum theory. Some of us in quantum thermodynamics realized that there is this implicit assumption, and it implies that these very common thermodynamic arguments, in a sense, are lacking something that's really interesting about quantum theory. My group has been asking what happens if you take a really common thermodynamic problem that is taught about in undergraduate statistical physics and remove this assumption that prevents the system from being, in a sense, fully quantum. What happens, what changes?

ZIERLER: What aspects of your research would you say are purely fundamental, just understanding the nature of quantum thermodynamics, and what aspects are geared more toward applications, such as actually building a scalable quantum computer?

YUNGER HALPERN: The answer has changed a great deal over the years. I earned my master's at the Perimeter Institute for Theoretical Physics. When I was there, I enjoyed hanging out with the foundations of quantum theory group. I loved eating lunch with them, I had advisors in that group, although I also belonged to the quantum information theory group. My work was very abstract and theoretical. Some of my work still is. For instance, within the last week, a friend and I posted on the arXiv a paper about closed time-like curves, so time travel put together with a quantum theory. That's a foundational work about the power of quantum theory. It was really at Caltech that I was pushed more toward thinking about physical realizations and experiments.

During my post-doc years, I started working with lots of experimentalists, and from there, I recently started thinking about applications of quantum thermodynamics to being useful. The theory of thermodynamics that was developed during the 1800s went hand-in-hand with the industrial revolution, which was eminently practical. The theory of quantum thermodynamics has been useful in the sense that it has shed light on fundamental matters and provided a better understanding of the nature of time, what is truly quantum in thermodynamics, and how general the laws of thermodynamics are. However, quantum thermodynamics hasn't been incredibly useful, although it seems like it should be able to have applications because, for instance, experimentalists, in preparing quantum computers, cool their systems down, and cooling is a thermodynamic process.

But quantum thermodynamics hasn't been very applied. I recently published a book, and it was writing the book that pushed me toward thinking about how quantum thermodynamics might become useful. I am working on a project with experimentalists at Chalmers University in Sweden, in which the idea is to realize a quantum thermodynamic refrigerator that can operate autonomously. We'd put this quantum refrigerator inside of a classical refrigerator that keeps a quantum computer cool. When the quantum computer–this one is made from superconducting qubits–finishes a computation, its qubits are used up, kind of dirty and high-temperature. We need to reset them by cooling them down even more. The idea is to hand these used-up qubits off to the quantum refrigerator so that it can, in its autonomous way, cool the qubits down more. That's a first step. I'm working with a grad student on the possibility of making useful applications of quantum thermodynamics into a research program for his PhD.

ZIERLER: A history of science question. Do you have a sense of the first or early use of the term quantum thermodynamics, or at least when it was considered a discrete subfield of physics?

YUNGER HALPERN: Quantum thermodynamics has a longer history than most people know about. I believe the earliest papers about it were written during the 1930s. Shortly after quantum theory was established, people began wondering whether it could explain, say, the second law of thermodynamics. In 1959, the first quantum engine was proposed. It consists of an atom that can serve as a maser. During the next few decades, there was some important work, for instance, on master equations that model a quantum system interacting with its environment. Those equations are still used today. During the 1980s, there were some pockets of activity. There were three theorists at MIT who were thinking about quantum thermodynamics in a kind of abstract sense. I know Yoram Alhassid was also thinking about quantum thermodynamics during this time. He's currently at Yale.

Also, Seth Lloyd published his PhD thesis, which has since become famous and the basis for recent work. Later on, Ilya Prigogine's school, I think it might've been called the Brussels school, had its own approach to quantum thermodynamics. However, there were only theses little sparks. And a few more I didn't mention. For instance, people were also working in Israel, in particular, one person, on quantum engines. But there were only these little sparks here and there until about a decade ago, when members of the quantum information theory community in Europe succeeded in obtaining a large grant, a COST grant, to fund quantum thermodynamics work, including conferences. A lot of people became interested and participated. I felt like I was the only or one of the very few Americans who participated in some of those activities.

I think the time was ripe for quantum thermodynamics then because quantum information science had matured during the early 2000s, provided a wonderful mathematical, conceptual, and experimental toolkit for analyzing quantum systems in terms of how they store and process information. These tools can now be used in quantum thermodynamics. The information theory was useful. Also, historically, quantum thermodynamics had been very theoretical. A lot of theory work is still going on now, and quantum thermodynamics is still, I think, primarily theoretical, but connections can now be made with real-world systems. That got people in some other fields interested. There was this huge boom in quantum thermodynamics during the past decade.

ZIERLER: Because of the obvious connections in quantum thermodynamics, both to classical Newtonian physics and quantum mechanics, do you see this field as a nexus or connecting point where we might see common ground between the debates from Niels Bohr and Albert Einstein to our attempts to complete the Standard Model or to make gravity fit with quantum mechanics?

YUNGER HALPERN: I certainly see quantum thermodynamics as a connecting point for a number of areas. It's also been delightful to see how many paradoxes can still be very compelling in quantum thermodynamics, even though thermodynamics has been around for so many decades, and quantum theory has been around for so many decades. A growing number of people in quantum thermodynamics, including me, and I think I was a bit on the early side, has been connecting quantum thermodynamics to other fields of science, including condensed matter, atomic, molecular, and optical physics, chemistry, and high-energy physics. We've sometimes used tools of quantum thermodynamics to answer questions in other fields, we've sometimes used quantum thermodynamic thinking to ask new questions about other fields, and personally, I've found it very useful to adopt new toolkits from other fields and use them to enhance my quantum thermodynamic thinking.

ZIERLER: An intellectual chicken-and-the-egg kind of question. Did you start thinking about quantum thermodynamic via information theory or the other way around?

YUNGER HALPERN: I started thinking about the foundations of quantum theory. I've always had a philosophical streak. When I was in high school, I had a wonderful philosophy teacher who was fascinated by the paradoxes of quantum theory and special relativity. He didn't understand them, he'd be the first to admit. But he was curious, and he transmitted his curiosity to me. Also pointed me to a few interesting pieces of material. In college, I wanted to study that more. I found it, in large part, in the physics department. But I didn't strictly major in physics; I had a major called physics modified. I took a number of physics courses and courses in math, philosophy, and history. I wanted a well-rounded view of physics. I was pulled toward the foundations of quantum theory because of their relationship with metaphysics, which has always been a particular interest of mine. I came to understand that quantum information theory shares a border with quantum foundations. On the one hand, from quantum information theory, we learn about the nature of information, space, time, and reality, but usefully, quantum information theory has practical applications. It was relatively easy for me to justify indulging in quantum information theory. I also loved linear algebra and its applications, so I enjoy the mathematics of quantum information theory.

Senior year of college, I spoke with a number of faculty members in quantum information theory who pointed me toward some professors around the world who had interests related to what I was expressing as interests of mine. I looked at their webpages and saw that a number of them engaged in quantum thermodynamics. I explored more deeply and found that a number of people who are drawn to quantum information theory are also drawn to thermodynamics. Both of these fields feature information, which is a very abstract concept, but also an extremely important fundamental concept and a concept that has lots of practical applications and entails operational tasks. One can reason about information very rigorously, even though information is some abstract thing. Relatedly, entropy features in both thermodynamics and quantum information theory. Both of these fields border the fundamental and the practical, so I was drawn to quantum thermodynamics upon discovering quantum information theory.

ZIERLER: Your book, Quantum Steampunk: The Physics of Yesterday's Tomorrow. Like all great, brilliant books, the argument seems so obvious, it's surprising that you're really the first to make these connections between the Victorian era and the modern world of particle physics. Where do you see the intellectual history in the germination of the idea that came from this book? What was your inspiration?

YUNGER HALPERN: I was driven to write the book because the idea of quantum steampunk I think of as very beautiful and something I really want to share with both scientists and people who wouldn't usually think of themselves as scientists. Starting in my childhood, I did encounter steampunk works, but I didn't realize there was a genre called steampunk. For instance, I read Diana Wynne Jones's The Chronicles of Chrestomanci, but I wasn't aware of steampunk until grad school. I don't remember the exact moment, but some time very early in my PhD, something clicked, and I realized what I was doing in my research was basically the essence of steampunk. Steampunk is this genre of literature, art, and film that features stories that take place in Victorian settings, like Victorian London, Paris, Meiji Japan, or the American wild West.

There are people in corsets, waistcoats, and top hats, but there are also futuristic technologies like time machines, dirigibles, and automata. There's this wonderful combination of the 1800s and futuristic technology, so the old and the new. This way of writing and creating art has a large following and a beautiful aesthetic. I realized that quantum thermodynamics consists of the thermodynamics of the Victorian era and the cutting-edge and futuristic technology of quantum information science. Fantasy really had become a reality in quantum thermodynamics. The first time I wrote about this notion of quantum steampunk, I wrote a blog post for Quantum Frontiers, the blog for the Institute for Quantum Information and Matter. After that, I toyed with the idea a little bit, I entitled my thesis Quantum Steampunk. Then during my post-doc, an editor from Johns Hopkins University Press reached out about whether I would want to write a book about quantum steampunk because I had just published an article for Scientific American called Quantum Steampunk.

I thought that the science is one of the most fascinating things in the world. I'm doing the science, so of course that's what I'd think. So I'd love to share the science with the rest of the world. But also, I think it's unusual for a scientific field to have an aesthetic the way that quantum thermodynamics has the aesthetic of steampunk. As I mentioned, I love bridging fields, I love bridging ways of thinking. Bridging hardcore physics to literature, film, and art is refreshing to me. Then, the opportunity to write in this refreshing way with these really crazy ideas about how physics is like novels will feed back into my physics and add some creativity to it.

ZIERLER: On that point, in writing a book that's accessible to a non-specialist audience, did you find there were ideas and arguments that were coming out that were enhancing some of the arguments that you were making in more technical or scholarly writing?

YUNGER HALPERN: In writing the book, I had to identify the basic physics in a lot of papers and arguments. Distilling the basic physics is tricky because we can't lean on the crutches of a whole bunch of equations that could confuse people into agreeing with us. I appreciated the need to distill out the basic physics. It absolutely improved my understanding of my own field. It also led to this research project I mentioned that centers on applying quantum refrigerators. The writing of the book was definitely helpful for my physics.

ZIERLER: Writing a popular book at an early stage in your career, what were some of the challenges and opportunities that presented for you?

YUNGER HALPERN: I've been grateful that my colleagues have expressed support for and excitement about the book. I was concerned that there has been a bias in physics against people who do outreach to and write books for the general public. Sometimes some of these people are cast as not real or not serious physicists. To tell the truth, until I announced the book, I told almost no physicists I was writing it. I wanted for my colleagues to see that I was writing papers as usual and being my normal productive self, so when I said, "I've written a book," everyone would have to say, "When did that happen? There was no gap in your research." Because I didn't want for anyone to see a gap in my research. I'm a physicist first and foremost. I've been very grateful that no one has even asked to see my publication list for this evidence that I made sure I had accumulated for myself. People have been very supportive. And as I mentioned, the book has been very useful for my physics. I was able to write the book so quickly in part because of the pandemic. I was not spending time waiting in airport lines, jet-lagged, and catching up on sleep because I arrived somewhere at 6 am local time after an all-night flight. To tell the truth, the pandemic was something of an opportunity.

ZIERLER: One of the values of the book is the way you explain quantum science to a population that might be vaguely aware that quantum information is this major field and that billions of dollars are being poured in the race to develop a quantum computer. I wonder if you see your work contributing to public perceptions of what a quantum computer might do and expectations of when we might see it.

YUNGER HALPERN: One of my goals was to explain about quantum information science. Recently, quantum computers have been in the news a great deal because quantum startups have debuted on the stock market, Google has performed its quantum supremacy/advantage experiments and published that within the past few years, so quantum is in the public consciousness. There haven't been recent books that I'm aware of about quantum information or quantum computing for the general public. I absolutely wanted to make sure I had a chapter dedicated to quantum information in the background section of the book, and I wanted to show how concepts and experiments from quantum information are useful in quantum thermodynamics.

ZIERLER: Before we go back and develop your personal narrative and how you got to Caltech, just a few contemporary questions based on your specific area of expertise. There's debate on this, and I get as many answers as there are people to ask them to. Do you believe now that we have a quantum computer?

YUNGER HALPERN: I believe we have small, noisy quantum computers.

ZIERLER: What do small and noisy mean in this context?

YUNGER HALPERN: I should back up. There are two types of quantum computers. There are quantum simulators and digital quantum computers. Quantum simulators are special-purpose machines that can solve problems within a relatively small set, but, that said, can be useful. Quantum simulators have been around for a number of years. They have up to a million particles, I believe, and they've been useful for, for instance, experimentally studying new phases of quantum matter. A digital quantum computer, when it's large enough and has all the expected capabilities built into it, will be universal. It can be programmed to solve any problem that a quantum computer can solve and then can be reprogrammed to solve any other such problem. Its versatility is much greater. We currently have digital quantum computers of tens of particles. They do not have large-scale error correction, so they can't perform large computations. Getting to full-scale error correction, so endowing digital quantum computers with their full powers, most of us believe, will take a good number of years.

ZIERLER: Do you see a specific niche that expertise in quantum thermodynamics must play in getting to these goals?

YUNGER HALPERN: To tell the truth, my personal goals don't include building a quantum computer, although I'm very happy that I have colleagues who are building quantum computers. And if my skills are of use and the building of a quantum computer is of interest to my group, then I'd be happy to collaborate. For instance, I mentioned the quantum refrigeration problem. That should be useful for quantum computing, and a goal of that project was to make quantum thermodynamics useful in a way. It's convenient to have uses of quantum thermodynamics for quantum computing. I think quantum thermodynamicists do have skills and knowledge that can be applied to quantum computing. Again, a personal goal of mine is to pursue research interests of mine, which are not necessarily building a quantum computer.

ZIERLER: Not only are there debates about whether we have quantum computers currently, there are forward-looking debates about what quantum computers will be good for whenever we get there. I wonder if you have any perspective on that, both from an applications perspective and the way that quantum computers, as you alluded, might be very useful for fundamental science.

YUNGER HALPERN: My impression is that full-scale quantum computers will be most useful for research and development, materials science, and chemistry. I suspect that a number of those are R&D applications that will go unnoticed by the general public. However, there are some intriguing opportunities. One example I find particularly compelling is the description by Microsoft's quantum team of how one could use a quantum computer to unlock the secrets of a molecule that could, if everything goes as planned, and that's a very big if, transform fertilizer production. We invest 3% of the world's entire energy output on producing fertilizer, because we produce fertilizer very inefficiently, using a technique from 1909, using nitrogen fixation. Bacteria fix nitrogen much more efficiently, but they fix nitrogen using a molecule that's too complicated for us to simulate it on classical computers. The molecule's quantum, so it's naturally suited to quantum computers. If we could figure out how that molecule works and use the information we glean, then fertilizer production could be changed, which could have implications for global energy use and food security.

ZIERLER: I can't help but point out in the way that you connect quantum science to the Victorian era, applying quantum computers to agriculture itself, it doesn't get any older than that for human civilization.

YUNGER HALPERN: That's a good point. [Laugh]

ZIERLER: Let's now go back to undergraduate at Dartmouth. Just a general question, was quantum information on your radar at all? Were you interested in those kinds of things? Were you even aware they existed when you were in college?

YUNGER HALPERN: I took my first quantum computation and information class as a senior from Sekhar Ramanathan, a brand-new faculty member. That course had been taught maybe every other year up until that point by Jay Lawrence, who was the department chair when I arrived. I had seen that course when looking through the course catalog, and I thought it looked really interesting. Granted, most courses I saw looked really interesting to me. But after I took a quantum physics course and then a philosophy of quantum physics course, I determined to take quantum computation and information as a sort of capstone and really enjoyed it.

ZIERLER: And you were a physics major?

YUNGER HALPERN: Physics modified. I very much belonged to the physics department, and that was my home base, but my major was kind of halfway between the physics major and a create-your-own major.

ZIERLER: At the Perimeter Institute, was that a terminal master's degree? Or could you have stayed on for a PhD if you wanted?

YUNGER HALPERN: That's a pure master's program. I've heard it called Perimeter's response to the University of Cambridge Mathematical Tripos. This is a one-year program called Perimeter Scholars International. It takes students from many different countries. My year, we had I believe 35 students. It's a very, very intense course that introduces students to modern theoretical physics.

ZIERLER: Was this purely academically oriented? Did you enter this program on the basis that it would provide a solid intellectual foundation for a PhD program? Or did you think about going into industry or anything else at that point?

YUNGER HALPERN: I was set on obtaining a PhD, and I saw this master's as a bridge to a PhD. After all, during undergrad, I had studied philosophy as well as German literature, and art history, and so on. I appreciated the extra year en route to my PhD.

ZIERLER: What were some of the exciting ideas you recall at the Perimeter Institute when you were there?

YUNGER HALPERN: There were so many. It was at Perimeter that I learned about resource theories. The resource theory framework is a mathematical and conceptual toolkit in quantum information theory. A resource theory is a simple model for any situation in which the systems an agent can access and the operations the agent can perform are restricted. The most famous research theory is the resource theory of pure bipartite entanglements. This resource theory models agents Alice and Bob, who are in different labs. They're poor, clumsy classical agents, so they can't control entanglement. They can perform only local operations and classical communications. However, occasionally, they might be gifted a bipartite entangled state. One asks, "How can this state be transformed under local operations and classical communications? Which states can this state turn into? Which states can it not turn into?" My master's Essay, which was like a thesis that turned into a paper, was about a resource theory for a particularly simple thermodynamics setting. I ended up using resource theories for many more years. In fact, I was editing a paper about a resource theory earlier this week.

ZIERLER: Did you have the opportunity to do original research, or was this paper more of a synthesis?

YUNGER HALPERN: It was original research. I worked with Markus P. Müller, who was a post-doc at Perimeter at the time, and Rob Spekkens, who was a faculty member there.

ZIERLER: What a great opportunity. As a master's student, you really got to collaborate with people senior to you.

YUNGER HALPERN: Yes, I was very grateful for the opportunity in general and very grateful for the many opportunities I had at Perimeter Scholars International.

ZIERLER: When it was time to start thinking of PhD programs, were you focused specifically on quantum information at that point?

YUNGER HALPERN: I was very focused on quantum information theory.

ZIERLER: What programs were you looking at?

YUNGER HALPERN: Caltech, MIT, Harvard, University of Maryland. I also applied to the University of Waterloo. Some Perimeter Scholars International students applied to the University of Waterloo to complete a PhD after finishing at Perimeter. The University of Waterloo is the university that partners with the Perimeter Institute, which is not a degree-granting institution, in order to award degrees.

ZIERLER: Did you have an initial visit at Caltech before you made your decision?

YUNGER HALPERN: Yes. I actually didn't attend the open house, but I received an email from John. I believe I was in the Perimeter library. John said, "I'm sorry you didn't have a chance to attend the open house, and I'm sorry I'm only getting to this now." The time for making decisions had, in large part, passed, but not entirely. He basically said, "I saw your application only recently, and I'd really like for you to visit before making a decision." I visited John's group, and I became convinced that Caltech was the right place to do my PhD.

ZIERLER: What convinced you? What was so compelling about your visit?

YUNGER HALPERN: The group kind of grabbed onto me and said, "You have to come here, and we'll make it worth your while." I got the impression that John and the group would be very supportive. And indeed, they were.

ZIERLER: What were some of the things the group was doing that made that obvious intellectual connection right from the start?

YUNGER HALPERN: I think John might've had a part in securing an IQIM graduate fellowship. I told John that I wanted to work at the intersection of quantum information theory and thermodynamics. I had a pretty solid idea of where I wanted to go, and he was very open to giving me a considerable amount of freedom. Also, the IQIM had a blog, which John and Spiros wrote for. I appreciated that they were open to an impulse to write and write about physics. I was also interested in quantum information theory of a particular flavor. It was quite abstract, theoretical, mathematical, and fundamental. I gained the impression after talking to a number of people and looking at a number of faculty interest pages that this flavor of quantum information theory was done mostly in Europe and Canada. However, there were a few hot spots in the United States that supported it. One was MIT, one was Caltech. My impression was that American quantum information was very much either closely tied to experimental platforms or drenched in computer science. I appreciated that Caltech supported some of the quantum information theory I wanted to do.

ZIERLER: When you got to Caltech, how well-formed were your ideas about the kind of research you wanted to do, even what ultimately would become your thesis?

YUNGER HALPERN: I knew I wanted to work at the intersection of quantum information theory and thermodynamics. I wanted to pursue the quantum information theory of this kind of flavor I just described. Also, after completing my master's, I came up with a new project that branched off from that. Between my master's program and my PhD, I visited Renato Renner's group in ETH Zurich, and I picked up a collaborator there, so we were working on a project together. I also had an invitation to work at Oxford as a visiting researcher in Vlatko Vedral's group supported by Jon Barrett in the fall, and I was planning to do a particular type of research with those groups. My research plans for the near future were fairly set.

ZIERLER: At Caltech, was anybody thinking along the lines you were about connecting thermodynamics and quantum information?

YUNGER HALPERN: Caltech had no quantum thermodynamicists, although since John has worked on most topics, he has certainly done a lot of quantum information, and he had experience with thermodynamics.

ZIERLER: When you started presenting these ideas, how were they entertained? Did it sound crazy to people? Was it like, "Oh my goodness, why didn't I think of that?" What were some of the responses you were getting?

YUNGER HALPERN: One feature of John's group I love is, it contains a lot of very different people who have a lot of very different interests. Sometimes some interests are a lot more represented by others. When I arrived, the group was largely topological, and at the end of my PhD, the group was largely holographic. I was the quantum thermodynamicist, the person you would go to if you had a quantum thermodynamics question. People kind of accepted that quantum thermodynamics was some other type of quantum information theoretic science you could do, although it was more emerging, and not so many people did it.

ZIERLER: Tell me about the intellectual process of building up conceptually what would become your thesis topic.

YUNGER HALPERN: My thesis turned out not to include any of the work I did during my first couple years. After a couple of years, I had written a few papers in the style that I mentioned. I had a chat with John, and he said, "You've done good work. You could publish a good thesis about this. Now, I want for you to think bigger." I started getting involved in many-body physics. I developed an overlap between quantum thermodynamics and many-body physics. I worked on some of the topics that were circulating the halls of Caltech more broadly, such as many-body localization. I worked with Gil Refael on how one might apply many-body localization to a thermodynamic task, such as work extraction via an engine. I also got involved with out-of-time-order correlators; I proved a fluctuation-type relation for out-of-time-order correlators. A fluctuation relation is an equality that belongs to non-equilibrium statistical mechanics and quantum thermodynamics. Out-of-time-ordered correlators became extremely popular in 2015 because of Alexei Kitaev's introduction of them from condensed matter into the black hole information paradox. I did start thinking beyond the quantum information thermodynamics community. It's that broader thinking that ended up in my thesis.

ZIERLER: As a mentor and fellow physicist, what do you think the nature of John's advice was when he asked you to think bigger?

YUNGER HALPERN: From John, I developed an interest in questioning the impact of what I was doing, how it would change physics broadly. There are many opportunities a researcher can pursue. Which opportunities are really worth our time and effort?

ZIERLER: Just to orient ourselves chronologically, what years were you at Caltech for graduate school?

YUNGER HALPERN: I attended Caltech from fall of 2013 until spring 2018.

ZIERLER: The transformation into the IQIM was already fully mature at that point.

YUNGER HALPERN: Yes.

ZIERLER: Obviously you wouldn't be able to compare to before that time, but the addition of matter, experimentalists and condensed matter, was your sense that that changed things for the theorists from the previous IQI? Did it expand their vista, make them consider new avenues of research as a result?

YUNGER HALPERN: I can speak to my personal experience and stories I've heard from and about members of the IQI. There was a generation of the IQI that I like to think of as kind of the golden age of the IQI, consisting of people such as Michael Nielsen, who did quantum information theory that, in quantum information theory now, we refer to all the time. We know their names, and their results are extremely important across the world. I can't say what their experience was, but I got the impression they were doing quite abstract information theoretic work. As I've mentioned before, I personally love to interact with a wide range of disciplines and people, so I did appreciate interacting with the matter part of the IQIM. I don't know to what extent I naturally would've met and started interacting with these other people at IQIM had the IQIM not existed, but I did interact and work closely with people such as Gil and Oskar Painter, who were very much in the matter part of the IQIM.

ZIERLER: Intellectually, what impact do you think those interactions had on your research and what would become your thesis?

YUNGER HALPERN: They were very important. I can pass as a many-body physicist, I collaborate with many-body physicists, I had a post-doc fellowship at the Institute for Atomic, Molecular, and Optical Physics. But I think those interactions have helped me impact physics more broadly and have enabled me to think bigger.

ZIERLER: On the point of John's advice to think bigger, what were the main arguments of your thesis, and what, looking back, do you see as the larger contribution to physics that you were making?

YUNGER HALPERN: The full title of my thesis was, Quantum Steampunk: Quantum Information, Thermodynamics, Their Intersection, and Applications Thereof Across Physics. Some number of PhD theses during the past few years had been published with titles along the lines of, Quantum Information and Thermodynamics. I didn't want to produce just another of those theses. Much as I very much admire those theses, very much respect the authors, and very much have appreciated engaging with the results of those theses, I'm always a bit of a contrarian, so I always want to do something a little different. This thesis was not just about quantum information theoretic thermodynamics, it was about taking quantum information theoretic thermodynamics and bringing it out into the rest of physics, to atomic, molecular, and optical physics, high-energy physics, and condensed matter.

ZIERLER: By the end of your thesis, you mentioned there were no thermodynamicists at Caltech at the time. What did the broader international community in this subfield look like? What were the numbers? What were some of the bigger ideas you were contributing to?

YUNGER HALPERN: I should clarify, when I arrived at Caltech, I was the only quantum thermodynamicist. Later, Fernando Brandao joined. One of the fields on which he's had an impact is quantum thermodynamics. He later joined as a faculty member, and later, Philippe Faist joined as a post-doc, and he works on quantum information theory and thermodynamics. By the end, I did have some colleagues in quantum thermodynamics at Caltech. For the more international community, quantum thermodynamics was on the rise during the course of my PhD. I feel like I got in on approximately the ground floor. The interest has continued to grow. In maybe 2017 or so, I think the number of attendees of the annual conference of my field at the intersection of quantum information theory and thermodynamics was something like 80 to 90. Quantum thermodynamics has been practiced a lot more in Europe than in the US, but over the past few years, people from other fields have reached out about having a quantum thermodynamicist represent their field at a conference, collaborate on grants, and so on.

ZIERLER: Coming in at the ground floor, as in all academic specialties, there are foundational debates among the participants. What were some of those debates at the beginning, and what were you representing within them?

YUNGER HALPERN: One traditional debate in quantum thermodynamics is how to define heat and work. In classical thermodynamics, heat is the random energy of particles jiggling about. Heat is uncoordinated energy and not directly useful. Work is coordinated energy that is marked by a change in some control parameter, such as the height of a piston that is capping a gas, pushing against the piston to raise it. In quantum thermodynamics, we're interested in quantum systems that are not always in energy eigenstates, so they don't necessarily have well-defined energies. It's not clear how much energy we should attribute to the system, and it's even less clear how we should split up that energy into heat and work. If we measure the system's energy, then we disturb the system. We can change the energy. Even performing measurements in order to learn about how much work or heat is performed is dangerous. Many different definitions have been proposed for quantum heat and work. I have a file I keep that I call "Menagerie of definitions of quantum heat and work." Whenever I find a new set of definitions, I put it in this file. Personally, I'm of the opinion that different definitions make sense in different contexts. Which definition is useful depends on what kind of system we have in front of us, what it's doing, how we can poke it, how we can measure it.

ZIERLER: As you mentioned, it was an editor at Hopkins Press that approached you, but just with a thesis title as provocative, even playful, as Quantum Steampunk, was there a kernel in your mind that this was something that might reach a broader audience than the specific physics sub-discipline you were speaking to?

YUNGER HALPERN: I was mainly thinking about blogging. I also appreciated being contacted by an editor at Scientific American about writing an article about quantum steampunk. I really appreciated that opportunity, and sometimes one opportunity can lead to another. That was the case with this book, though I hadn't expected to write a non-fiction book.

ZIERLER: Between you and discussions with John, when did you know you had enough to defend, that you had something that represented a complete intellectual work?

YUNGER HALPERN: As a student in John's group, I felt like I was encouraged to simply write a lot of excellent papers as a PhD student. Then, if I had enough material after five years, which was pretty standard–and I did have enough material–then staple papers together, as they say, and add an introduction.

ZIERLER: Of those discrete papers, what were some of the intellectual through-lines that were obvious to you?

YUNGER HALPERN: The papers we chose for my thesis did exhibit a broader thinking that tied quantum thermodynamics together with other spheres of physics and showed how quantum thermodynamics could be useful in other spheres.

ZIERLER: Besides John, who else was on your thesis committee?

YUNGER HALPERN: Fernando Brandao, the faculty member who joined who has contributed a great deal to quantum thermodynamics. Manuel Endres served as the experimentalist. He was also a new faculty member at the time. So was Xie Chen. That might've been everybody. How many people are on a PhD committee? It's been a little while.

ZIERLER: Anything memorable from the defense?

YUNGER HALPERN: Oh, quite a bit. I received a lot of questions from Manuel and John. Manuel apologized afterwards, saying there was some combination of his already being irritable that day and curious that led to his many questions. But I was grateful for reports from multiple attendees that I handled the questions well.

ZIERLER: Was yours the first dissertation in thermodynamics that had taken a specific look from the quantum perspective to win the Ilya Prigogine Prize?

YUNGER HALPERN: I don't recall whether any of the earlier winners had treated quantum aspects of thermodynamics, although I got the impression that none of the earlier winners of this prize belonged to what I think of as my core community at the intersection of quantum information theory and thermodynamics.

ZIERLER: After you defended, what opportunities were you looking for? Did you specifically want to pursue a post-doc at that time? Or were you looking at faculty appointments as well?

YUNGER HALPERN: I was going to take a post-doc fellowship.

ZIERLER: Where were you considering?

YUNGER HALPERN: I was very grateful to receive a few offers. I took the ITAMP post-doc fellowship at the Harvard Smithsonian Institute for Atomic, Molecular, and Optical Physics. I was also very grateful to receive an offer of a Miller Fellowship from Berkeley and an offer of a Hartree Post-Doctoral Fellowship from the University of Maryland. After receiving those three, I pulled my remaining applications because I was incredibly delighted and very grateful, and I would've been very happy with any of those.

ZIERLER: Tell me about the opportunity, being an ITAMP post-doc at Harvard. First, what was happening at the time at Harvard? What were some of the exciting ideas?

YUNGER HALPERN: Misha Lukin's group had its 51-Rydberg-atom quantum simulator. They published a now-famous paper based on the first experimentalist performed with that simulator. That paper led to a surge of interest in the topic of quantum many-body scars, which are features of a quantum system there are surprisingly out of equilibrium. ITAMP had hosted the first quantum thermodynamics conference or workshop on US soil, to my knowledge, the previous year. I was grateful that the director, Hossein Sadeghpour, was supportive of quantum thermodynamics, even though quantum thermodynamics didn't have very much of a foothold in the United States. I'm grateful he was supportive of me personally.

ZIERLER: I asked earlier about your interface over the course of your career with applications and experimentation. To narrow that question from the transition at Caltech to Harvard, what specifically did that change for you in terms of how your interface has changed over your career?

YUNGER HALPERN: During my post-doc years, I came to be a theorist who works with experimentalists. I published my first experimental collaboration with Kater Murch's lab. Kater is a professor at Washington University in St. Louis. His lab has superconducting qubits. Kater studies quantum information theory, the foundations of quantum theory, and quantum thermodynamics. Over the course of the past couple years, I've worked with five experimental labs. I've been very grateful for the insights I've been given by experimentalists, who do think very differently from theorists. I think I was able to make the jump, in large part, because of two factors at Caltech. First, I did work with Gil Refael, and he thought about whether the theory he proposed could be tested with the experimental capabilities of the day. He did order-of-magnitude estimates and was thinking, "How would this system be realized in, say, a system of ultra-cold atoms?"

Second, I had a lot of conversations with Oskar Painter and his group. Oskar was starting up a superconducting-qubit effort in his lab. I came to learn which questions are the right questions to ask, which are the time scales to ask about, which are the numbers that are more important, such as ratios of qubit lifetimes to gate operation times, and which numbers are less important, such as certain absolute numbers rather than ratios. Relatedly, I had the opportunity during my PhD to collaborate with Justin Dressel, faculty member at Chatman University nearby. He's a theorist who works with experimentalists. He has modeled experimental systems very closely, and yet he has also thought broadly and deeply about the foundation of quantum theory. I started to learn from him how to maintain this balance. I was very grateful to have opportunities to put those experiences to use in starting to collaborate with experimentalists during my post-doc years.

ZIERLER: If you could help me visualize the scene, from a theorist's eyes, what do the experimental labs look like? What was happening in them, and where did you see your theoretical perspective enhancing the experiment and vice versa, how the experimentation was enhancing how you were thinking about these issues?

YUNGER HALPERN: I visited Kater's lab once at the beginning of our project. I recently published a paper in Physical Review Letters with Aephraim Steinberg's group on a photonic experiment. I saw the photonic setup only within a few months of the paper's publication. I still have not seen the lab in Sweden that I mentioned. I don't have a great deal of experience with actually being in the lab, although it's always fun to have a lab tour given by an experimental group I am collaborating with so that I can see what they've been up to and spending so many hours on between our meetings. I'm used to collaborating remotely. I heard from a number of physicists that this switch to remote collaboration during the pandemic was rough for them. During the first year of my PhD, I collaborated only with people outside the country. Then, I collaborated with someone across the country. I've always felt that remote collaboration offers freedom. I'm not limited by who's right nearby.

It's okay with me that I don't spend much time in the lab, but I do ask a lot of questions about how measurements are being performed. I ask my experimental collaborators to walk me through some of their processes. As I mentioned, from experimentalists and theorists who work closely with experimentalists, I've learned what useful questions to ask about a platform are. If someone tells me about the lab they've just set up or a setup they want to try out on a new question—they're looking for an interesting question—then I can draw on these experiences to ask questions about their lab and setup to figure out which ideas of mine they might be able to connect to. I have seen my role as a theorist as, in large part, to come up with interesting questions for experimentalists to answer and to convince the experimentalists these questions are worth answering. I'll also work closely with experimentalists on figuring out if an idea is feasible, what we should measure, how we should interpret the data, why the data don't look the way we expect, what we should do instead, and the overall significance of what we've been doing.

ZIERLER: I can't help but ask, but imagining a Victorian world of thermodynamics, factories and trains, it's a very physical world. Being exposed to these laboratory experimentalists, did that introduce a level of physicality into your theoretical framework that enhanced your ideas and ultimately made the book more impactful or even more possible?

YUNGER HALPERN: For starters, this more physical way of thinking has certainly impacted and improved my papers. I'm often, even when writing abstract theory, thinking about potential physical realizations, and I now feel somewhat comfortable with pointing to promising platforms and gathering evidence suggesting a proposal could be worth pursuing experimentally. I do find that when speaking to someone who is not in my field, including a member of the general public, it's useful to speak more concretely rather than abstractly. I do try to speak in terms of atoms, electrons, circuits, and so on.

ZIERLER: A question about the job market when it was time to move on from Harvard and think about faculty or research appointments. It's amazing to think that graduate students and post-docs at Caltech in the early 2000s had trouble finding jobs. Incredible people like Daniel Gottesman. It's amazing to think. They were just so far ahead, there weren't jobs in the fields. They were creating the fields in real time. For you, in terms of the excitement in industry, government, academia, what was available, what was most compelling, and why did you ultimately make the choice you did?

YUNGER HALPERN: I'm very grateful to those earlier generations for establishing the field of quantum information so that members of my generation could have job opportunities. I do tell members of those generations, "Thank you for doing that."

ZIERLER: "Thank you for your service." [Laugh]

YUNGER HALPERN: Yes. When I was in the second year of my post-doc fellowship, the job market looked very good. There were openings in quantum information theory. I applied for a few, the ones I was really interested in or encouraged to apply for by people I knew. The pandemic happened. A number of positions closed. I knew one person who was offered a position and then had the offer retracted, which was heartbreaking. Fortunately, there was a happy ending after a while. She got the offer again. I actually had one of the first virtual faculty job interviews because of the pandemic. I was very grateful to receive a number of very good offers from academia. I had not expected to solicit a job from NIST. I actually didn't solicit a job from NIST. There was no application for NIST. I had a friend and colleague at NIST who said, "Why don't you send an application to the University of Maryland? They have an opening in computer science. You're not a computer scientist, but just get your application into the system."

I'd had quite a bit of contact with the University of Maryland over the years. There are thermodynamicists and quantum information theorists there with whom I have close connections and collaborations, and I just love the quantum information community at the University of Maryland. It's like Caltech's, although maybe not quite as old. Communities such as the University of Maryland, Caltech, MIT, and the University of Waterloo began investing in quantum information science before there was such a thing as quantum information science, so they now have strong, rich communities. The Physics Department at the University of Maryland was going to try to create a position for me. The pandemic happened, hiring froze, and NIST quickly came up with a position. I had not thought I would end up in government. I very much did not think I would.

Whenever, over the years, in grad school or my post-doc years, someone would ask me where I wanted to go in my career, I'd say, "I want to do extraordinary science and keep doing what I'm doing. I've been looking most at academia, but if there's an offer I can't refuse from elsewhere, I'm not going to ignore it just because it's not academia." I thought I would end up in academia because I thought there would be no other position that would interest me as much. But I was very grateful that NIST made an offer that I couldn't refuse. The position I was offered is very much like a faculty position. I have complete control over my research agenda, I hire students and post-docs, I lead a group, but I don't have to teach, and I don't have to apply for grants so much because research funding comes every year from the government. I hate writing applications, I love doing research, and I love mentoring. I was very grateful for the opportunity.

ZIERLER: This is more a question about the state of the field than your particular motivations, but in industry, the way that Microsoft, Honeywell, IBM, and others are pouring tremendous resources into quantum information, for your specific area of expertise, do you have a sense whether they're supporting fundamental science in quantum thermodynamics, and was that something you had considered at all?

YUNGER HALPERN: I had looked at industry, in large part because John's group has connections all across the world. I had visited the group at IBM, for example, I had friends who went to Microsoft, and I heard from them about their environments. IBM does have an extremely impactful quantum information theorist, who could also be seen as a thermodynamicist, Charlie Bennett. IBM also supported Rolf Landauer, who, although he didn't believe in the promise of quantum computing, was very important in helping solve a key paradox in quantum thermodynamics, and his marks are all over quantum thermodynamics. I was aware of these positions. There isn't a strong tradition of supporting quantum thermodynamics in industry. A lot of the quantum thermodynamics I saw was based outside the United States, as I mentioned. The United States has had a lot less quantum thermodynamics than, for example, Europe. I wasn't thinking very much about industry, but I knew about its opportunities, I had friends who went into industry, and whenever I spoke with others about possible opportunities, they might not be aware of that might be good fits for them, I pointed toward industry because I think it's very healthy for the scientific community to have members doing good work in academia, industry, and government.

ZIERLER: To bring our conversation right up to the present, in this new stage of your career, serving as a mentor to post-docs and graduate students, if you'd like to have the opportunity to teach undergraduates–at the right time, I assume that's something that might be exciting to you–what is your sense generationally of the kinds of graduate students and post-docs who are attracted to work with you? In other words, is the field of quantum thermodynamics sufficiently mature that students already come in with this specific interest? Or are you drawing from a more generalized scholarly population?

YUNGER HALPERN: I receive a lot of inquiries from students and prospective post-docs who would like to do quantum thermodynamics with me. That might be in part because I'm more visible than a lot of other physicists in that I blog, I've written articles, I'm on podcasts, I've written a book, I give a lot of academic talks, I publish papers that receive press, and so on and so forth. I know that a large part of the international community doesn't know about quantum thermodynamics. For instance, this past June, I lectured at a summer school in Sweden, and a number of people there hadn't heard of quantum thermodynamics. But I was invited to lecture at the summer school, so someone thought the topic was interesting. I have received so much interest from students and prospective post-docs. They're excited about this field, about its possibilities, about its fundamental flavor. I've been disappointed that I've had to say no to so many people because I just can't take any more students, since I have so many. I hope the students we have now will become PIs so they can take on more quantum thermodynamics students, and the field can grow even more.

ZIERLER: Even though your research group is new and still in building mode, I wonder if you can share a sense of the kinds of topics that your students are interested in that might provide a window into where the field is headed next.

YUNGER HALPERN: A lot of our work currently is on the topic I described earlier, in which we take a problem from basic undergraduate thermodynamics class and add this quantum twist of operators that fail to commute with each other. We're pursuing that topic from multiple different angles: many-body physics, fluctuation relations, quasi-probability distributions, using many tools and trying to find many applications. I also have a student working on that quantum refrigerator project, and we're going to see if we can turn his PhD research program into a program of seeing how we can make quantum thermodynamics useful.

ZIERLER: Now that we've reached the present, for the last part of our talk, I'd like to ask a few retrospective questions, then we'll end looking to the future. I'd like to ask you about the concept of taking intellectual risks in scholarship. The idea that you were willing to pursue very new, and as you said, some crazy ideas in a book in an early stage of your career, what are some of the lessons to be derived in your experience about following your intellectual interests and not being too focused on where they might take you?

YUNGER HALPERN: I definitely drew from Caltech the spirit of taking intellectual risks. For instance, John encouraged me to work on Matthew Fisher's quantum cognition idea. Matthew Fisher is a condensed-matter theorist at the University of California, Santa Barbara. He's very well-established and has done excellent work for many decades. A few years ago, he became interested in the notion that, according to most physicists, quantum phenomena probably do not have much of an effect on cognition or other biological phenomena that we could point to as large effects. He thought, "What if quantum phenomena did have an impact on cognition? How could that possibly happen?" He put together a story about how ultimately, the phosphorous atoms in a certain molecule could potentially lead to, via entanglement, coordinated neuron firing.

With a post-doc in John's group, Elizabeth Crosson, I translated this biochemical story into the language of quantum information and computation and asked, "If Matthew's right," and we recognized that he very well may not be, "what would be the quantum information processing capabilities of this biochemical system?" Matthew's story sounds like a crazy idea. Some people say it is crazy, some say it's brilliant. But a venerable scientist spent a lot of time thinking to put together this story, and we thought it was worth analyzing with the tools we had at our disposal, which are the tools of quantum information and computation. That project was, in a sense, a bit crazy, definitely daring. I hadn't thought it would be a good idea for a young student to take on such a topic, but John did encourage me to look at it, and I don't regret it.

ZIERLER: Now that you're at NIST, looking to the future, whenever and however we get to scalable quantum computing, what do you see specifically as NIST, and more generally, the governmental contribution to this effort, that's unique from an academic perspective?

YUNGER HALPERN: I'm not the most qualified to answer that question because I started at NIST quite recently, and a lot of my activities are at the University of Maryland. I don't have an office at NIST because NIST does partner with the University of Maryland on QuICS, the Joint Center for Quantum Information and Computer Science, as well as the JQI, the Joint Quantum Institute. My office, and my students' and post-docs' offices, are in the QuICS space on the University of Maryland campus. I spend my time there, and I don't directly interact with what I would think of as government infrastructure. I interact with government scientists in matters that involve science or a few logistical matters, such as travel, paying for post-docs, and so on. It's difficult for me to say, although I do hope that government funding will remain in place for quantum information science, even if or when industry runs out of interest because of the short-term thinking of investors.

ZIERLER: And because the government, by definition, has more patience and is not profit-motivated in these things.

YUNGER HALPERN: Yes.

ZIERLER: You highlighted Caltech in teaching you or giving you perspective about risk-taking. What about in collaboration, in the way that quantum information theory, building quantum computers, by definition requires a multidisciplinary approach in collaborating with people who come beyond your specific area of expertise? What did you learn from Caltech in that regard?

YUNGER HALPERN: I was grateful that my interdisciplinary way of interacting was welcomed at Caltech. I heard a number of times, "The IQIM consists of a number of different research groups and disciplines. We want to connect these groups as much as possible." I was grateful that people would welcome me to their lunch tables. There was the condensed-matter theory lunch table, there was the quantum-information theory lunch table, there was the ultra-cold atoms lunch table. I appreciated the opportunity to talk with many different people. There was a period of time when I was given a desk in Bridge so I could hang out with the condensed-matter theorists. They adopted me, even though my home base was always with John's group in Annenberg in the quantum-information theory group. I really appreciated that this side of me was embraced and encouraged. It's definitely proven useful. I've found other people in the physics world who don't encourage interdisciplinarity so much. But during the faculty hiring process, I was surprised and delighted to find that some institutions actually created room for me because they saw me as a connector, and they wanted connectors.

ZIERLER: For my last question, looking to the future, I'll use the concept of a second book just as an intellectual stand-in, whether you decide to write it or not, as a way to encapsulate where you see your research agenda headed. On that point, if there is a second book, what aspects do you see that would be a sequel to what you've already done, and what aspects are you looking in totally new areas because you always want to push the boundaries?

YUNGER HALPERN: I definitely always want to push the boundaries. I think the research I'll do will hopefully continue to impact quantum thermodynamics, and quantum information, and their interactions with the rest of science. I'm definitely always looking out for what can be new and impactful, and as a matter of fact, the next book I write, I think, has to be a fiction novel. Completely different. But I'm always a physicist first and foremost.

ZIERLER: On that note, it has been a great pleasure spending this time with you. I'm so appreciative of taking time out of your busy schedule to share your perspective with me. Thank you so much.

YUNGER HALPERN: Thanks very much for the conversation.

[END]