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Jeongwan Haah

Jeongwan Haah

Principal Researcher, Microsoft Research

By David Zierler, Director of the Caltech Heritage Project
January 24, 2022

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Monday, January 24th, 2022. I am delighted to be here with Dr. Jeongwan Haah. Jeongwan, it's great to be with you. Thank you so much for joining me.

JEONGWAN HAAH: Thanks for having me, too.

ZIERLER: Jeongwan, to start, would you tell me please your current title and institutional affiliation?

HAAH: I'm a Principal Researcher at Microsoft Quantum based in Redmond near Seattle.

ZIERLER: Tell me about Microsoft Quantum.

HAAH: It's an organization within Microsoft devoted to developing quantum solutions, ultimately having a scalable quantum computer. There are engineers, scientists, physicists, mathematicians, software engineers, all sorts of people working towards that.

ZIERLER: Are you operating at Microsoft more in a basic science environment or are there particular applications that are motivating the research?

HAAH: The large organization has a mission to develop a quantum machine, whatever form that may be. Well, ultimately, we would want to have a quantum computer that operates in a conceptual way that it was developed maybe 20 years ago. But the very work I do at Microsoft—well, it is motivated by that but it's slightly beyond that, I would say. I worked on some mathematical questions motivated by many-body physics that has overlap with an effort to create a quantum computer.

ZIERLER: In terms of your own research, it's such a big field, what are your interests and how do you see them contributing to that larger effort of creating a quantum computer?

HAAH: My contribution to the program is mainly on the error correction. Any quantum device that is developed and conceived suffers from various kinds of physical noise. And the noise level is significant so that naively you wouldn't be able to do any nontrivial calculation using those devices or elements. You have to suppress noise. There are some theoretical proposals to achieve that, and I am providing expertise on that area.

ZIERLER: What is so difficult about quantum error correction? What's the challenge?

HAAH: The classical devices that you use on your phone or a laptop computer, they hardly use any sort of error correction. They are astonishingly reliable. So the inherent noise level that is much higher than a classical device is an obvious challenge and the method to suppress errors that is applicable for classical devices is not applicable for quantum devices, so you have to fight back some noise using quantum mechanics only. And there are some limitations where such a protocol begins to work and experimental results are on the verge of making it viable. If you could go well beyond that barrier things would become much easier.

ZIERLER: Are there certain benchmarks that everyone in the community is looking for? In other words, is there a shared sense of what it's going to look like when a true quantum computer is achieved?

HAAH: I don't think so. There are various technologies—well, there are levels. You have a physical system, the true quantum system that gives you a qubit and on top of that you apply a layer of error correction, and there is another layer using those to compile quantum algorithms in terms of those [error corrected] operations. [As for] The qubit technology there are many things, each with advantages and disadvantages. That's why there are many platforms still out there. If one is so promising than the other, then everyone would be investing in that, only in one technology, not others. The very fact that there are many just reflects that the landscape in the future is not settled.

ZIERLER: To go back to your undergraduate days to get an understanding of your motivations to come to Caltech, at Seoul National University were you already interested in quantum information? Were you aware of the field and this was something that you might pursue?

HAAH: Oh, no. I would say no. Well, I had one book written by Nielsen and Chuang which was probably the first book everyone in this field would read. I had a copy of that book when I was an undergrad, maybe a sophomore, junior, somewhere around there. I certainly had no idea what it would be like as a graduate student or I didn't know if I would pursue a career in theoretical physics. But the book was well written, and I enjoyed reading it. Especially, I particularly liked the fact that they explained the quantum mechanics itself in a very clean manner when I was very mathematically inclined. Most quantum mechanics books start with—for historical reasons—now I appreciate for historical reasons people explain quantum mechanics as it is developed, so you first learn of classical mechanics and then you learn about problems with the classical mechanics, and you learn about the ideas that were introduced to correct those problems in the classical mechanics. And the course ends usually with somewhat ad hoc solutions rather than a coherent story that can be understood as a single read in a math book. For a physics major that is necessary because you need classical mechanics and intuitions from that in a larger context of physics.

If you're studying earth science, for example, you hardly need any quantum mechanics, but you need some old-fashioned way of doing classical mechanics which is problematic but it doesn't matter for that field of study. But as a person who would want to understand quantum mechanics as a single coherent story, that text was not satisfactory. But Nielsen and Chuang's book did a superb job. It's only one chapter but for a sophomore's mind it was enlightening very much.

ZIERLER: Now your degree in undergraduate was in physics?

HAAH: Physics and math, a double major.

ZIERLER: And did you specifically want to come to Caltech for graduate school? Did you apply more widely in the U.S?

HAAH: Well, I did what everyone else was doing, applied for, like, ten-ish grad schools, got some admissions from other places and chose one. Yeah, I think I knew that I wanted to do some theoretical study so it could be other theoretical physics in general or it could be something else as long as it's theoretical. Caltech was the only place where quantum information, I think it was explicitly mentioned that it is a field that is being studied there. I vaguely remember—my memory could be very wrong, but I think that was one of the main reasons that I decided to come to Caltech.

ZIERLER: Now, did you know about people like John Preskill and Alexei Kitaev before you got to Caltech? Were you aware of their work up to that point?

HAAH: No. [laugh] I did not know who Alexei Kitaev was. I think I knew John Preskill's name but that was pretty much it. I did not visit any part of the campus. Actually, I didn't even travel to the U.S. before my graduate entrance, so it was just a big bet, I guess.

ZIERLER: This was your first time in the United States when you got to Caltech?

HAAH: Yes.

ZIERLER: What were your impressions? What did you think when you first arrived?

HAAH: Oh, very sunny. It was the first week of September in Los Angeles, very brightly sunny. And people were speaking English, but I hardly could understand what they were saying. My mind was just full of survival. Going to the place where I could take the shuttle to the campus was a challenge. It took about an hour, I remember, from Los Angeles Airport to Pasadena. And reading the signs of the street names it was all challenging. Fortunately, I had a friend from my college who was attending Caltech Math Department back then so having him around me was a bit of relief. But yeah, everything was challenging.

ZIERLER: Did you involve yourself with the IQI right away?

HAAH: No. I spent my first year just taking courses. Many people were advising me when I applied for grad school that you should contact some professor that you want to work with potentially as an advisor. That seemed to be a standard course, but I didn't have any research output so to speak, so my ambition was probably solely based on my grades and not those standard things. I had no clue—people warned me it can be challenging even to choose an advisor and Caltech had very minimal course requirements so I thought I would do the course requirements on the first year and be done with that. And you could take a reading course. It's not a technical lecture course but it's an opportunity to be advised by a professor and I forgot with whom I did my reading course first. John Preskill certainly was not my first instructor in that reading course, and maybe it was the third semester or maybe the summer break after that I knocked on the door of John Preskill's office and asked if I could sit in that group meeting. John never pushed me away.

ZIERLER: Did you have a sense of what John was working on at that point?

HAAH: Yes. I attended the group meeting. He was running a pretty large group though I didn't know that it was big, so there were several postdocs and several students. Everyone sat in the room. It was a small room like maybe 20 feet by 20 feet, something like that. Everyone got together and had dinner that was delivered to that room. On the first round we were discussing what everyone was doing at that given week, so I hear words that I had no idea about. They were taking researchers and I was only an end of first year grad student, and I didn't expect to understand anything [laugh] but I could get some sense what was going on in that group. But that was the start.

ZIERLER: What were the big ideas? What was exciting to you as you were being exposed to this research?

HAAH: I don't think I was particularly attracted by one single big idea back then. I just liked the environment. I appreciated the breadth of the group. There was also someone who was speaking of some quantum algorithms and that discipline is usually connected with computer science very much and not so much into the physics. And there was another person who was deep in the physics side. So yeah, I enjoyed going back and forth about those different looking fields, that somehow there is a chain of relations that connect those two fields.

ZIERLER: What do you mean? What does that mean, the "chain of relations"?

HAAH: There were a spectrum of people, more on the algorithm side, computer sciences stuff, and there are people who are doing the very device-related physics, for example, and somewhat abstract physics in a theoretical setting, and that theoretical setting can influence the algorithm side. So there are some relations on each of the topics although if you compare two extremes then hardly there is any overlap. I think I just liked the fact that I knew I would learn much by being exposed to that environment.

ZIERLER: What was the process by which John became your advisor? Was it informal? Did you present something to him?

HAAH: It was very informal. I was sitting in the group meeting for like two months, which means I was there in the room every week for about eight weeks or so. Then I asked him on one afternoon, I forgot the exact wording but, "Can I choose you as my advisor?" And he said, "Okay." And that was it.

ZIERLER: [laugh] At that point you knew you wanted to work with John on the basis that you would be focusing on quantum information, you had already made that decision?

HAAH: Yeah. That's right.

ZIERLER: What were the open opportunities? Where did you want to slot into that larger effort of everything that was going on at the IQI?

HAAH: I guess I wasn't too deeply into the possibility of making a quantum computer as I think I appreciated that some aspects of physics could be understood and asked from an information perspective. I think I was attracted more to that. Maybe it was before or after, I don't remember anymore, I asked him for a set of papers that I should start reading. Well, this kind of question could have been asked in any reading course, I guess, but that was the start. Those collections of papers, there were three or four, they were all on the quantum memory side which eventually became a topic of my thesis.

ZIERLER: What was the intellectual process like developing your thesis topic?

HAAH: A large portion was luck. The year I joined the group meeting and chose John as my advisor was probably in 2009, and then on the following January or February, somewhere around then, I was working on some very weird idea that—the collection of papers was all about quantum memory subject to some thermal noise. Could there be a natural mechanism to preserve a quantum qubit, a logical qubit in contact with the thermal environment? There was clearly a missing piece in three dimensions. One of the papers was, like, you cannot make anything reliable in contact with the thermal interaction in two spatial dimensions. There was another paper that argues that if you go to four spatial dimensions then clearly there is a model. This has nothing to do with the possibility that actually things can be realized in a real laboratory system but at least you could ask theoretical questions and with a dimension of three, it was missing. Not knowing how hard or how easy it could be, I just started thinking about that question. At one point maybe a few months later in that first round of group meetings where everyone was speaking on what they were working on, I presented something like a five-minute talk, basically, that I was working. The motivation was very clear: we want to devise a local Hamiltonian that provides a quantum memory and that is resilient to thermal noise, and the Hamiltonian should live in the three-dimensional lattice. The problem was clear. The problem whether it is good or not, that's far from clear, but at least there were some—

ZIERLER: You mean to this day, it's far from clear?

HAAH: Yeah. The mathematical question is more or less well posed but to test whether a given model satisfies all the criteria, that's a hard task. Even mathematically that's a hard task. But there are some criteria you should definitely avoid, and I would just try to work on that aspect. I thought I got a solution which made me excited—well, overly excited to the extent that the answer was completely wrong. It was a somewhat embarrassing experience. It was just a five-minute talk at the group meeting. I explained the model. Some of the people looked at me seeming very interested, but some other people pointed out that, "Oh, your model is wrong because of the following reason," which I didn't realize for a week. So my attempt just failed after one minute. [laugh] But somehow, I did not stop thinking along a similar line, so I somehow systemized the calculation for the next month or two and then I tried again at the group meeting—"Oh, here's another variant that avoids the previous criticism and here's an advanced calculation that shows, oh, it looks good." Then at some point John says, "Oh, that looks interesting." It's not a big word but from the, like, half a year experience of the group meeting, he hardly said something is interesting. [laugh]

ZIERLER: What happened next?

HAAH: Well, naturally, I get excited. And thinking about the potential story around the solution I got, it made sense. Even to my premature mind, it made a lot of sense. Probably that was the spring or sometime, so as a grad student who has not much duty, anything else, was very into that calculation. I was doing the same calculation over and over again and was convinced that I was right. Then I was writing a note with all the details, stopped there. I went to Korea for summer break, came back after a month, continued, just to discover I was wrong. It wasn't a too subtle point in retrospect, but the calculation was somewhat complicated. I explained that my solution was wrong at one of the group meetings, and probably it was John who said, "Oh, last week it was good, now it's bad." [laugh] Well, naturally, I got a little disappointed—not just a little; deeply disappointed. Then some months passed, and I still was doing some random reading on the newest research papers and one day John came to my office to ask me a question, which is not a very frequent event. [laugh] But fortunately the solution came out pretty quickly.

ZIERLER: Well, what was the question?

HAAH: The question was somewhat technical, but I can say that on a restrictive kind of local Hamiltonian that specifically consists of commuting operators, can you have some classical memory in two dimensions? The solution was that it was almost a verbatim copy of a preexisting paper interpreting it a different way. In retrospect that's only my contribution. There is a complication between the correctability between a classical bit and quantum bit. If you could protect a classical bit too well then your ability to protect the quantum information in the same device degrades. So there is a sweet spot where the degree of protection for the quantum information is optimized and that optimal point you have to necessarily sacrifice some of the classical protection capability. That tradeoff is the physical ground to provide another evidence that you cannot make a reliable quantum memory subject to thermal noise in two spatial dimensions. The question was very much in the same spirit as my original failed problem I was working on, but the failed problem was in the three-dimensional lattice, and this is on the two-dimensional lattice, but the scope is slightly different. So we ended up writing a paper on that topic which kept me rolling from the disappointment.

I didn't completely give up on the three-dimensional project so at that point it marked about a year from the initial start to think about those problems. In that time, I decided that, well, I'm not going to be able to do it by hand so let me borrow the power of a computer to enable my search. I plugged in all the criteria I discovered by my manual calculation, and I designed a certain scope of search space, and I ran the program. It took a couple of days and I expected either no solution or a lot of solutions. The search space contained over a billion candidates and surprisingly there were only 17. Basically that finding became my thesis.

ZIERLER: What was the significance of this finding that allowed it to expand into what would become your thesis?

HAAH: It was a first kind—well, what's the English expression? It was the one-of-a-kind model, so it satisfied all my criteria, the native criteria that you have to avoid to have a reliable quantum memory subject to thermal noise in three dimensions. Once you've satisfied that criteria there are very standard questions that you have to answer. The first one is what are the operators that you have to operate by to manipulate the quantum information encoded in that system? It is a straightforward calculation but since it has a geometrical picture it is important to have an interpretation of the result of calculation. And typically that interpretation is given very easily in terms of geometry. In any previously studied models there are operators that you have to act by in order to manipulate the quantum information beneath. Technically it's called the logical operator. They come in a certain shape. Either they are in a line or a surface or any higher dimensional ones. But this one, it is not a regular shape.

Ah, I missed the important event. After I finished my computer search and verified the consequences of that search, Sergey Bravyi, who was and is at IBM now, visited Caltech as just an academic visit. I had a chance to present—before that, Matt Hastings, who is my colleague here, was passing by Pasadena and he was visiting the office, so I brought my calculation sheet. I knew his name from papers and so I thought it was a great opportunity to get my name known to him at least. I brought my calculation note and explained very briefly what the findings are like. He says, "Oh, I have not seen anything like this." Which is a great compliment [laughs] for a—was I second year or third year? Something like that. That propelled my excitement further. Then Sergey was visiting Caltech and he stayed for one week or something, so I borrowed an hour and explained all the details—well, not all the details but one example from that search. He got excited, too. Now I have three data points who I regard very highly in the field and got excited about this, along with other members in the group, postdocs and students. So yeah, I was excited and there was a full reason to pursue that.

ZIERLER: What were some of the conclusions of your thesis? What did you find?

HAAH: The previously studied models for quantum memory inherited one thing that it comes from some topological field theory and there the logical operators having a certain dimensionality and geometry is kind of at the level of assumption. It is embedded at the so fundamental level that people wouldn't want to give up that. There were no tools to address that. There was a belief that any lattice model would flow into such a framework in a very core sense, and my finding directly contrasts that intuition. It's a new kind of theories—well, theories in a sense of it's a model. It is more and more surprising for those who have exercised field theory and theoretical physics for a long time. It's really a new kind. My thesis appears to be about one example but the phenomenology and methodology to find it is broader than that. The papers that comprised my thesis, of course, is that the first paper that discovers this class of examples that allows you to have a—well, I keep saying reliable quantum memory subject to thermal noise, and you have to quantify it. Under what assumption is that? How much is it reliable, under what specific noise type? There's analysis towards that. Well, it turns out it is not as good as I originally expected but still good enough to get people interested. Again, my thesis is all about analysis around this.

Another calculation that is based on that model that is about the model is the so-called entanglement RG. In general, physicists do not want things to be complicated, so physicists want some guiding principle for all phenomena you see in the universe, and you would want that guiding principles to be as small as possible. In some sense, there is a solid framework to make that theme more concrete, and one of the themes is called the renormalization group flow. You look at the one system, you encode all the interactions in some microscopic model, and you ask questions that only make sense at a scale much larger than the individual building blocks. Then all the details somehow wash out and you are left with the essence of the system. The calculation method in that end is well established and I applied a variant of that calculation technique to my model. To our surprise, it behaves the opposite way. Instead of things becoming simplified, you get bifurcation. The model itself is reproducing at a longer and longer scale and indefinitely and that's certainly a new phenomena in this kind of model. That's another basis that you can say that this model is a really different kind.

ZIERLER: Jeongwan, what was John Preskill's style like as a mentor? How often would you meet with him? How much direction and advice would you get from him? How did that relationship work?

HAAH: John does not visit an individual's office too often, especially when it comes to students. I had one visit that resulted in a joint paper, like I mentioned before. But I tried to make use of much of his time by at least once a week. Preparing something to talk about in front of an advisor is a great way of summarizing what's important, what's not, so I tried to use that as much as possible. I think once a week was my standard cadence in addition to the group meeting that was running every week.

ZIERLER: What was the value of the group meeting for you when you look back in terms of not only sharing your ideas but hearing what your fellow students and postdocs were up to?

HAAH: Attending one group meeting doesn't mean much but attending a series of meetings in long periods of time, that I think helped to form my view, more especially when it comes to what's important or not. The words that I heard from the group meetings I think were crucial to form my own opinion, and how to read papers for example, to develop critical views towards others' ideas. The breadth of the group meeting, again, in retrospect it was very important. You don't just look at the very own problem you are looking at or your very closely related problems but you're also hearing a lot of other problems that look very different from your field. Those questions come later as a guidance when I chose my own problem, when I think about a problem that, "Oh, this problem is related to that area and these groups of people would be interested in these questions." That keeps my motivation going.

ZIERLER: Were you following the developments that ultimately led to the creation of the IQIM?

HAAH: Not closely. I was only a third- or fourth-year student and I had very little idea how the funding or Institute worked.

ZIERLER: What about, Jeongwan, in terms of the interactions, in the way that the adding of matter to the IQI, IQIM, did you notice how there were more people in condensed matter physics who were getting involved? In other words, was the IQI changing in a way that suggested the IQIM is a natural progression?

HAAH: I didn't particularly notice the transition as day-to-day life. People seemed to be excited about the transition and it involved a much broader group of people, but everyone involved was anyway someone who I could see on campus. [laugh] I think later on there were IQIM-wide activities, memorably the social ones. There I think I saw more participants. But other than that to my graduate student mind I think I was just excited because everyone else was excited.

ZIERLER: [laugh] Besides John, who else was on your committee?

HAAH: Oh, my thesis committee. Alexei Kitaev, Gil Refael, and Eric Rains.

ZIERLER: What were some of the discussions that you had with other members of your committee? What was discussed at the defense, for example?

HAAH: Alexei was in proximity to my office and all the other group meetings, so I think he knew most of the content of my thesis already. Gil, as well. Gil was actually participating in the group meeting from time to time, and I think I had an opportunity to explain that, probably not directly one-on-one with him, but I had several talks within the Caltech campus about my work well before I was defending my thesis. Eric Rains, I think it was my idea to invite him into my committee largely because he worked on the quantum codes before, and I was using some of the mathematical tools to analyze my model, and so I thought he would be very quick to understand what's going on mathematically. I was in part proud of myself to introduce such a language into the study of this physical model, so I thought, "Oh, okay, well, let's get an opinion from a mathematician."

ZIERLER: After you defended, it was time to think about your next opportunity. Did you consider returning to South Korea or you knew you wanted to stay in the United States?

HAAH: I did not consider going back to South Korea. I didn't know where to live per se. [laugh] I think I just followed the general rule that you be exposed to a variety of environments as possible, that way your view will be more balanced and widened.

ZIERLER: Was there no one doing quantum information in South Korea? Was that part of the consideration?

HAAH: There are people, but the pool is smaller than America. If you specialize to what I was doing, the overlap is smaller.

ZIERLER: Why did you chose then MIT for your postdoc?

HAAH: MIT was kind of a natural choice. They had the largest group in terms of quantum information and they, I think, would understand what I was doing but at the same time I wanted to learn what they were doing. Of course, you can always learn through papers but it's a bit different from what it would be like to interact with them directly. The fellowship I earned at MIT was a department-wide one so I was not bound to a specific professor, so that was another plus.

ZIERLER: Was there an institute at MIT similar to the IQI? In other words, was there a particular center that you were affiliated with?

HAAH: No, it was just the Department.

ZIERLER: Who was working on quantum information at MIT at that point?

HAAH: The department distinction was not too important at MIT. There was Aram Harrow, Peter Shor, Scott Aaronson, Eddie Farhi, Seth Lloyd. Am I missing anyone? [laugh] Some of them were affiliated with Mathematics and some with the Computer Science Department but that hardly mattered, although I did not talk too much except for Aram and other condensed matter people at MIT, actually.

ZIERLER: Was there anyone in particular you worked with closely at MIT?

HAAH: I tried to be close to condensed matter people there. The condensed matter group at MIT was pretty strong. Somehow, although I had my degree in physics, I felt like I don't know the traditional language of physics, so I thought I would learn from them. At the same time—Aram is a hard-core information theorist, but the focus is quite different from what John was interested in. John is by training a physicist and all his motivation comes from physics whereas Aram's interest is—well, sometimes I find it more information theory, traditional computer-science-based information theory side. There were some fruitful results, so I thought that was the most interesting field, too. I tried to be in contact with both of the worlds, and luckily the office at MIT was sitting right at the center of those two places, and they were very, very close. [laugh]

ZIERLER: What aspects of your postdoc did you see as an opportunity to expand or refine on your graduate research and where was the opportunity to do new research, new projects?

HAAH: Despite my remote effort to talk to Aram as much, I only managed to write one paper in collaboration with him and that was a very different topic. It was on a quantum state tomography problem which happened by coincidence that I was at that moment working on some representation theory-related problem. I talked that, "Oh, I'm thinking about this problem." Some of the key words might have just clicked in his mind, and he told me of a problem that precisely needed the tool I was learning, so we ended up writing that one paper. But that was the end of the second year or the beginning of the third year or something like that. While talking to condensed matter people, I was able to put the findings of my thesis in the broader context. That was the time that we coined the term "fractons" which is now referred as a collective term of my model and others. Also, I met a very good postdoc there, Adam Nahum, who is now at ENS in Paris and he and I were talking about some quantum dynamics. It was quite before—that field is now very hot but when we were working on it, it was somewhat silent, no question. We were chatting about the entanglement growth, and I was providing some information and perspective and we ended up writing two papers, one when I was at MIT and then the other continued after I joined Microsoft.

ZIERLER: What do you see as your key contributions during your postdoc?

HAAH: My key contribution for postdoc? I am proud of one of my papers that I wrote in second year of my postdoc which is a solo project. Actually, Aram was asking one question, how would you detect—this is getting into a technicality—it was a question about the complexity for many-body states, and showing that something is easy is always easier, because you just provide a solution how to make it easy. But showing that something is hard is harder because you have to defend against unknown strategies. This question was about such hardness tailored to the physical situation, of course. At one of the group meetings Aram mentioned that, "Oh, this problem appears not to be solved." I came up with a solution that is pretty widely applicable, and it is refining what it means to have a charge and other notions. Later, maybe a year later, Alexei told me that a similar idea has appeared in a very old book by Haag. There is a mathematical physicist who has written a book for a similar idea. But I did not know the existence of the book, but I ended up managing—it's not exactly the same thing but a similar flavor. Of course, putting the model in my thesis in the broader context of fractons, that's in collaboration with Sagar Vijay, who is now at UCSB, and Liang Fu, faculty at MIT, had some people to appreciate these kinds of models more because it's putting it in a right context.

ZIERLER: What professional opportunities were you pursuing when you finished your postdoc? Were you looking both in industry and academia?

HAAH: Yes, that's right. I did not consider this Microsoft position that I'm currently holding as a pure industry position. It aligned with my impression back then I can write very mathematical and theoretical papers, that is somewhat distant from making a quantum computer per se. So I regard my position as a research institution.

ZIERLER: Jeongwan, for the last part of our talk, we've already worked up to the present from the very beginning of our discussion. Looking to the future, I'm very interested in getting perspective on if and when the quantum computer is achieved, what do you see institutionally as how Caltech will have contributed to that achievement?

HAAH: Well, it should not be neglected the contribution by Alexei. It's quite profound. Some of the work that resulted in the current situation was done before Alexei was at Caltech but nonetheless his influence still persists until today. The very basic error correcting code that looks promising is due to him and much of the questions that form the foundation of error correction was done at IQI beginning with a 2002 paper, Topological Quantum Memory. Caltech will have contributed to a scalable quantum computer. A very large fraction of leading researchers in the field have been at Caltech at some point. That much I think many people would agree. Another big idea is probably the influence of the tensor networks towards the general understanding of physics. That has a big overlap with the Caltech people, the IQI people. It appears very many places. In a sense it is one of the now—a conceptual foundation level I think I would say.

ZIERLER: Jeongwan, I'll leave you with a question that might be easier or harder and let's just see where it goes. For you personally, how do you see your own contributions up to this point and the trajectory that you're on, how do you see your own contributions as they lead to a scalable quantum computer? Is that an easier question or a harder question than the Caltech contributions? [laugh]

HAAH: My thesis is an example of quantum code and analyzing that, we have developed some universal decoding algorithm, but those codes are not thought to be practically used in the quantum computer. [laugh] Maybe some variants of the coding algorithm may be used. My work on the magic state distillation, which is somewhat orthogonal to any other physics papers I wrote, some ideas in those papers—those papers contain some practically useful ones, small gadgets and circuitries and codes and others. There I have written papers that tabulate codes and some of them look useful still so that might be used. [laugh] Well, it will certainly feel very good if any part of my papers is actually implemented in the real machine. But it's never the case that I work for the purpose of being actually used. As a researcher I just explore a lot of options and propose useful-looking many candidates for the future technology and I'd be satisfied with that.

ZIERLER: What you're saying then is that there's more than enough pleasure to be arrived simply in fundamental research wherever it goes?

HAAH: Right.

ZIERLER: Well, Jeongwan, on that note, I'm so glad we connected and that you were able to do this. I'd like to thank you so much.

HAAH: Thank you.