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Stephanie Wehner

Credit: Mark David

Stephanie Wehner

Stephanie Wehner

Antoni van Leeuwenhoek Professor of Quantum Information, Delft University of Technology

By David Zierler, Director of the Caltech Heritage Project

July 14, 2022

DAVID ZIERLER: This David Zierler, Director of the Caltech Heritage Project. It is Thursday, July 14th, 2022. I am delighted to be here with Dr. Stephanie Wehner. Stephanie, it is a pleasure to be with you. Thank you for joining me today.

STEPHANIE WEHNER: It is a pleasure, David.

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

STEPHANIE WEHNER: I am a Professor in Quantum Information at Delft University of Technology, and I am also affiliated with QuTech and the CS department at Delft University of Technology.

ZIERLER: Stephanie, a few broad questions about both your career and quantum information at Delft, first, what are you working on right now and what is exciting in your department?

WEHNER: I am working with my main focus towards the actual creation of real-world quantum networks. My background is in computer science and the problems that I am excited about right now are actually the problems that lie at the intersection between physics and computer science. I have worked a lot with quantum information during my time at IQIM as it's now called. What I find super exciting is to work with people in experimental physics and who really build the devices for quantum networks, since this inspires a lot of really new and exciting computer science questions. I like to work actually both on the super practical end—let me say on the confrontation with reality where we are really trying to make these networks happen—and at the more theoretical deep end, that is also inspired by this confrontation with reality.

ZIERLER: Stephanie, I want to ask specifically about the quantum internet. Do we need a quantum computer before achieving a quantum internet or does it work the other way around?

WEHNER: No. It is not necessary to have a quantum computer to have a quantum internet. Even though, of course, if you have one, you could hope to connect it to a quantum internet in order to do more advanced protocols that require such quantum computers.

ZIERLER: What does the quantum internet look like in the ways that we understand what the classical internet looks like?

WEHNER: On the very high level, it looks very similar to the classical internet actually, in the sense that you start from a network, that is from a small-scale network, connecting several devices—classically, for example, this might be your laptop or your phone. Small-scale networks are then connected into ever larger networks. The term internet actually is a technical term that refers to interconnected networks. In the quantum domain, on the very high level, one might imagine that this would work very similar in the sense that we connect things to the network on which applications are run. We like to call them end nodes. These may be very simple photonic devices, but they may also be much more sophisticated quantum processors. Of course we would like to connect them somewhat close by in, let me say, a metropolitan area network and also, interconnect these smaller networks to achieve both long-distance quantum communication, as well as of course enable quantum communication and interesting applications between users in all kinds of different areas.

ZIERLER: Stephanie, in the way there are so many debates about what a quantum computer will be good for, when we will achieve it, what are the materials necessary—how do those same debates apply to the quantum internet?

WEHNER: There is of course maybe the most famous application people think of, when it comes to quantum internet, which is secure communication using quantum key distribution. I should say in a sense, we have a position of luxury, in that we do have one real-world application, secure communication, where it is proven that quantum provides an advantage. That is not so much the case in quantum computing, even though of course, there are a lot of indications for that. Many other application ideas have already been developed, for example, in the domain of clock synchronization, for a client accessing a quantum processor in the cloud to perform secure delegated computations, or distributed sensing.

I can name a lot of them, but maybe one thing that one should realize also about quantum networks is that a lot of these things are what we like to call an application, meaning that there is a theoretically described project, a problem, where the origin is more academic. Like it is written in the computer science textbook that it is interesting to synchronize data into distributed systems and then people in quantum information make the protocol that solves this. I think what we are lacking both in quantum computing and the quantum network domain is a much more concrete bridge to a use case. There is a difference between some hypothetical academically interesting applications and what you want to use at home. So let me say what we really want is when the origin of the problem that one wants to solve is actually the user and not the textbook. However, I think actually a lot of indications exist - like in computing - from the textbook-type applications that there are quite many interesting real world user problems we could solve with such a network.

ZIERLER: In the way that, again, for quantum computers, there is something of a race between academic institutions, between industry, between national governments to see who will build it first, is there a similar competition in constructing the quantum internet that you can see?

WEHNER: I would say that there definitely is. For example, there is a huge amount of quantum communication efforts in China. I would also classify this as competition. In academia there is collaboration as well as competition. Also in the US, there are a lot of efforts in the quantum network domain both in the academic domain and as well as more recently in industry. From the perspective of sitting in Europe, this is both an opportunity for collaboration and also competition. I think it is quite natural as things as move from the domain of research where people pursue more fundamental open science in which it is great to collaborate, to a more technology-oriented domain where also companies are involved that may not necessarily want to share with everyone.

ZIERLER: In what way is the Dutch government supporting this research as a national endeavor, something that's important to the country?

WEHNER: The Dutch actually have the Dutch National Quantum Agenda that supports quantum technologies as a whole. But of course, the Netherlands is a very small country embedded in a much larger collaboration of countries known as the EU. We actually, at least at QuTech, currently receive more support from the EU as a whole than from the Dutch Government, but activities are quite synergistic. A lot of our R&D activities are funded by the EU, but a lot of demonstration activities, like showcasing and deploying things on the test network are funded by national activities.

ZIERLER: To get a sense of your role as a mentor to graduate students and postdocs and given your dual interest in computer science and physics, what kinds of academic backgrounds are your students coming from when they want to come work with you?

WEHNER: I have students from various different disciplines actually. Most of my students are from computer science and physics, and sometimes also mathematics. This naturally reflects the nature of the problems that we want to solve because some problems are much more ‘physics-y' problems, where it is useful to also understand more about how quantum devices—realizing that quantum networks are actually built. While at the same time, actually, the task of building these networks creates all kinds of super exciting new computer science problems. These are a little bit more interesting or also more natural for students who originate in computer science. I have both types of students in my group and I think, at least I hope, they learn a lot from each other. I get the impression they do. I also enjoy collaborating with people from all kinds of disciplines. That is nicely reflected in my own group.

ZIERLER: In your work on quantum cryptography, does that get you involved at all in national security issues given that this is a major source of concern for intelligence and things like that?

WEHNER: To some extent, yes, even though I have to say that I am always very driven by some particular ambition. When I was still at Caltech, I worked mainly on quantum cryptography, and there was also a specific challenge that I always wanted to establish—I think not so important for this interview. But what I am trying to say is I managed to do it. I am very happy that this problem got solved and I learned what I wanted to learn. I am now actually much more interested in quantum networks, specifically also their real-world implementation. Yes, it is true that quantum networks enable quantum cryptography, but I have, myself, put the primary focus now on the realization of these quantum networks. I have decided to quit quantum cryptography.

ZIERLER: If you can comment because it is such an interesting source of concern for people in the National Security Agency and places like that, when quantum computing is achieved, what will that mean for internet security? What aspects will become less secure and what will become more secure?

WEHNER: Quantum technologies are both an opportunity as well as a threat when it comes to security. On the threat, it means that a lot of crypto algorithms known that are currently, really used to transmit confidential information are known to be broken if I would have a large-scale quantum computer. This is, of course, of concern, even if the computer exists far away in the future because you can go and record all the communication that you want to potentially decipher into the future today, and then when a quantum computer becomes available, you can hope to decipher it. Whether this is a practical threat to a specific situation on a network depends— are your secrets still valuable in 10 years? Your credit card information, probably not. Your credit expires in 10 years. But for secrets that are of high concern, like governmental secrets, this may be a concern. It is a real threat. On the other hand, quantum is of course also an opportunity because with quantum communication, one can generate and keep generating encryption keys to be used to secure the transmission of data in a way that cannot be later broken with a quantum computer.

ZIERLER: Let us go back now and establish some context before we get to your time as a postdoctoral scholar at Caltech. First of all, growing up, were you always interested in computers? Did you experiment with networking and hacking and things like that?

WEHNER: I should say that I love communication. I have to say what drives me is not so much the monolithic computer. I like computers. They are useful things, but I have to say that my fascination has always been with computer networks or actually communication. Maybe in some way, that is more challenging because you cannot control all the variables. In a computer to some extent, for example, if you make an algorithm for a computer, there are very little things you do not control. You program your code. You input your variable, say "factor this number for me," your algorithm runs, answers obtained, great. Of course building a real computer is still very complicated, but from that perspective, it is relatively simple.

What I really love is problems between, say, different computers that do not agree on things and they do not send things at any time. They also can be broken and potentially, all the information is lost while they try to talk to each other. I find this much more exciting. I have been actually interested in computer networks for a very, very long time. I have been involved in some hacking activity in the past. I have also worked for a while actually in network security also on the offensive side for a company that offers services to investigate the security of networks. I have gone to university actually relatively late. Also, my fascination, maybe to explain a little bit of my trajectories for this quantum domain because maybe you are wondering how I get from this to what I am doing right now, while I was an undergrad student, I heard this very nice talk of John, actually, in Leiden about quantum information.

ZIERLER: What year would that have been roughly?

WEHNER: I have to say I do not remember. I think it was 2003, but maybe I am misremembering the date now.

ZIERLER: Maybe right after undergraduate for you.

WEHNER: Yeah. Right, it was still during my undergrad. I was super fascinated by this. If you like to think about what can communication do and the world is not fully classical, then it is very natural to say "Okay, what can I do with quantum communication? What does a quantum network mean as opposed to a classical network?" This ultimately, after many things, motivated me to actually become involved in quantum information. For a long time indeed, I worked on quite also theoretical things, also because at that time, quantum information was a much more theoretical subject, so you could either be an experimental physicist, like work in the lab, you built the thing; or you worked just purely on the theory side. In physics, they make a distinction between experimentalists, who are the people that work in the lab, and theorists who work on pen and paper and occasionally these two domains interact with each other.

I originally came from computer science. I then also decided to do all kinds of physics during my undergrad, even though technically my degree is in in computer science. In computer science, you also have more theoretical things. However, the terms "experiment" and "theory" do not really compute in the general domain of computer science. Given that I originally came from computer science, I then also did more theoretical quantum computer science basically because at that point in time, it was the only path to study interesting problems in quantum computer science. I became more and more fascinated by things that physicists were doing. I guess we don't need to talk about this now, but I sort of actually like it that to some extent, I am now actually a bit back to where I started at the very beginning. Because I have to make it professional in front of the video, I have to say that even though I love mathematics—and I love theoretical computer science, I love theoretical proofs, I really like doing it, it gives me pleasure—I really like the real world. I really like the confrontation in the real world. When I started actually starting computer science, coming from a very industry background on classical networking, I was very much driven by practical things. I was like, "All these theoretical computer science people, they make up all this mystical stuff. It is totally impractical. It is never going to work in the real world."

I find it very funny because later when I came to quantum information, where I am the theorist because I am not an experimentalist. I meet all these experimentalists and they tell me, "Oh, all these theories." [laugh] Now it's like—oh, it's very interesting. [laughs] In a way, even though I do super theoretical things, especially now actually, after a small dip in practical inspirations as we say, I really like it that now I can work on actually super practical things, like we are implementing things together with my colleague from experimental physics like in the lab, while also doing what physicists call theory.

ZIERLER: In what ways did your experience in hacking provide a skill set or a sense of perspective that was useful for your graduate work?

WEHNER: I have to say that I think hacking is actually science in maybe the possibly purest form. Nowadays hacking has become much more serious business. People do it for money and all kinds of directions, whether on the offense or defense. I think hacking is an investigation of how things work and actually also to push the boundary of how they might work, and to use that understanding of how things work to do something creative that was not there before. In the context of computer networks, this means asking "What can I do in this network that may not be within the prescribed specs? Can I somehow get out of this box?" "You, the user, have to be in there, and this is what you can do." "Can I go out there and do interesting things? What can I make this network do for me?" In science, it is very similar.

There is some understanding of the world that we currently have or the technology that we currently have, like there is this kind of box. And you constantly try to push the boundaries of that box. We try to figure out what is beyond the borders of this box. "Can I make it do something interesting for me? How can I understand it? Does it actually tell me more about how the box itself works?" In some sense, hacking is super pure science even without publications or anything like that, just driven by the discovery of new things that you can do with the computer system or new things that you can learn or do with the world.

ZIERLER: Did the University of Amsterdam embrace quantum information? Were there faculty working in that field when you were at graduate school?

WEHNER: Yes. I did a class during my master's on quantum computation. There are faculty at the University of Amsterdam and the CWI, Centrum for Wiskunde & Informatica. It is a research center for math and computer science. That is also where I did my PhD.

ZIERLER: Tell me about developing your PhD thesis. What did you work on?

WEHNER: My PhD thesis was mainly about quantum cryptography, investigating also however all kinds of things which actually are at the core of why quantum cryptography actually works. This pertains to the understanding of quantum uncertainty, for example which measurements are more uncertain than others, and that plays an important role actually in quantum crypto. But also about entanglement, like what are the correlations that can be derived from entanglement, how can they be generated, what are the limits to such correlation and can we somehow compute what the limits to these correlations are. I worked on quite a few things, but the big theme is quantum crypto.

ZIERLER: From the first time you interacted with John Preskill, did that put a seed in your mind that you wanted to go to Caltech as a postdoc?

WEHNER: Well, it certainly occurred to me. I also actually have to say that I did not apply anywhere else so it is good that I could come to Caltech!

ZIERLER: Were you following developments at the IQI when you were in graduate school? Were you appreciating how it was in growth mode and all of the exciting things that were happening at the time?

WEHNER: I think from a graduate student's perspective, it is really impressive to see the papers that come out of this. IQI as an institute was not so much on my radar as a graduate student, more as, "Hey, people write cool papers there. Great things are being produced."

ZIERLER: Tell me about when you first arrived in Pasadena. What were your impressions?

WEHNER: Of Caltech or Pasadena?

ZIERLER: The whole experience.

WEHNER: I can give you the whole experience. I very much enjoyed Caltech, the group that was there. I really liked it that there was actually quite a large group of postdocs, and it was super nice to talk to everyone, to exchange and to find out what they were working on. It was really exciting for me. At CWI, where I was before, there were actually not so many people working in quantum information, certainly not so many postdocs that were also trying to investigate all kinds of topics, so that was super exciting. I love the campus of Caltech, so when I first arrived, it actually left a big impression on me. I really love the super thin palm trees in California, it is really nice.

ZIERLER: How did you slot in, in terms of your academic focus, coming from quantum cryptography and the things that you are interested in? How did that fit in overall with the things that were happening at the IQI at that point?

WEHNER: At that point, IQI was very diverse, so people worked in all kinds of specific directions. Another postdoc that arrived a little bit after me, Robert Koenig, but that was a bit after me, also was very interested in quantum crypto. I think one thing I actually found specifically interesting; it's not that everybody was doing quantum crypto. I think it is very dangerous, even though I was obsessed with a specific problem, to become too isolated and stay in your own bubble and never look at what are other nice things to do with quantum. I really enjoyed actually that there were people pursuing quite diverse ideas in quantum information.

ZIERLER: Getting outside of that bubble, what were some of the exciting new ideas that you were exposed to when you were at IQI?

WEHNER: I have to make sure I get the timing of these activities right.

ZIERLER: You were there from 2008 to 2010.

WEHNER: Yeah, I know when I was there, but I am not entirely sure what conversation I had with what person at what time. For example, just to name a random thing, this is what I enjoyed, that I met people also that came from other areas and also more ‘physics-y' than what I saw before. CWI is an institute for math and computer science, it is not a center of physics. So, I also quite enjoyed people who were sort of maybe on the intersection between quantum information and condensed matter, for example Lukasz Fidkowski. I learned some things from him about symmetries in condensed matter; I thought that was really fascinating.

ZIERLER: What was the interaction among postdocs with regard to both graduate students and faculty? In other words, was everybody all mixed together or did those groups more or less stay apart in their research?

WEHNER: I think in the beginning, things were a little bit more isolated, maybe also due to the physical location. When I arrived, we were still in the old building. All the postdocs were in one location. The graduate students, at least to me in the beginning when I arrived, were much harder to localize. When we moved to the new building, which I am sure you have been to many times, this changed a little bit because physical proximity was much higher. I did not interact much with the faculty while I was there, but I did work with some of the graduate students.

ZIERLER: What were your main areas of research? What did you expand on from your thesis? What were new projects entirely when you were at Caltech?

WEHNER: One avenue that was new to me while I was at Caltech is more the domain of quantum information theory itself. Basically, limits to how well you can send information over a communication channel, in this case a quantum communication channel. This is something that I started working on while I was at Caltech that I did not really work on before. Even though, David, I have to make a terrible confession—it was of course because I wanted to solve this crypto problem, but not it's necessarily very important that I learned these things.

ZIERLER: Was anybody working or talking about quantum networks or the quantum internet at the IQI when you were there?

WEHNER: No. At this point, not really. At Caltech, people were working on this, like Jeff Kimble worked on this, but it was not really a topic of discussion at the time.

ZIERLER: What is to explain for that? Was the theory not sufficiently developed or this was just not an area of interest locally at Caltech where it might have been elsewhere?

WEHNER: Let me say that I am not entirely sure. I think Jeff Kimble, for example, had this interest for a long time and still pursues it. But I think it was also a function of this being maybe experimentally much further away from realization. I think that was truer for all types of quantum technologies at the time as it is now. I am not sure if I have a good explanation for that.

ZIERLER: When we look nowadays at the billions of dollars that are being invested in quantum computing from industry and from governments, was your sense that that was already underway when you were a postdoc at Caltech or is that a more recent development?

WEHNER: I may not see all things here because one has a very different perspective, I guess on some of these things if one is not a postdoc anymore. But I think there has been quite a significant transition in the funding about four or five years after my postdoc. At least, that is my perception. Of course, IBM was already involved in quantum for a very long time and big companies were involved for a very long time. But I think that drive to focus on the new engineering aspects, to make a quantum computer, that was driven by IBM, and maybe a little bit earlier by things like D-Wave, where everybody was saying, "What they are doing is not a quantum computer." Nevertheless, they made a lot of noise. It was sort of towards the end of my postdoc, but unless this translated into some general momentum, at least in my perception, it took a bit longer than that. I think now things are very different than when I was a postdoc in the field. The field is just much larger. It is much more diverse. It has basically just grown up a bit from when I was a postdoc.

ZIERLER: Part of that story, of course, shortly after you left, the IQI expanded into the IQIM. It added "Matter." Were you aware of these developments? Were you aware or part of the discussions where experimentalists, engineers, condensed matter people started to bring literally the matter into quantum computing? Did you get a sense of that at all when you were a postdoc?

WEHNER: No, but I think it is a good idea.

ZIERLER: The theorists were really separate, or at least you were, in terms of understanding what was going on?

WEHNER: Yeah. Some people, for example Darrick Chang, worked much more closely with experimentalists. He also worked for a while in Jeff's lab. But I think most of us, say, theory postdocs, we were much more detached from the experimental implementations. I am actually not sure how things are now. I have not been for quite some time. But I think it is a good idea actually to bring these things closer together.

ZIERLER: To what extent, being detached from the experiments, is that a way of also saying that the theorists, the postdocs at Caltech who were theorists, were really not involved in applications? They were really doing fundamental research regardless of whether it had societal applications.

WEHNER: Yeah, I would say that, which is also a good thing. I think in the domain of quantum technologies, on the one hand, it is very important to bring together experiment engineering and as physicists call it, theory, in order to actually build the system. I think that is super important. At the same time, I think it is equally important not to lose that long-term trajectory doing fundamental research on things that may or may not—that's also okay—become relevant in 10 or 20 years into the future. We do not know yet what will become relevant. I think it is important to strike a balance between these two things and just be conscious on what is what.

ZIERLER: Coming out of Caltech, would you say that your main area of research remained quantum cryptography at that point, or had you already started to go on into different areas?

WEHNER: I had also, during my postdoc, started to go a bit more in something that I became very interested in, which is using quantum information to understand something about nature, so more in the domain of quantum foundations. That is something that I find super fascinating. I started that maybe in the last year of my postdoc. It was, by the way, always there a little bit because also, in my PhD thesis, I was very interested in uncertainty relations, for example, in what measurements are more uncertain, can we somehow characterize what are the most incompatible measurements that one can make in quantum mechanics, and these types of things. That, I think, evolved into a very general interest in the domain of quantum foundations.

ZIERLER: Was there a paper, a person, or a specific, idea that sparked this broader interest in quantum foundations for you?

WEHNER: Maybe one thing that quite motivated me was when I was, for some time, at the Kavli Institute in Santa Barbara. I was quite interested in relationships between uncertainty and locality at the time. I had a lot of discussion with Jonathan Oppenheim, who also visited the Kavli Institute at the time. I think it was actually just really nice to discuss these things with Jonathan. Let me say that my fellow postdocs at Caltech, I am sure they are, broadly speaking, interested in quantum foundations, but it was really nice actually to have some really in-depth discussions about all kinds of things with someone who is also super interested in it. I think that also sort of motivated me actually and caused me to actually spend a lot more time on this in the future.

ZIERLER: When it was time to move on from IQI, what opportunities were you looking at? What was most interesting to you in the field at that point?

WEHNER: You mean in terms of location?

ZIERLER: Jobs, collaborators, things to work on, institutes to be employed at, all of the above.

WEHNER: I have to say that maybe I did not spend so much time thinking about this. A specific opportunity arose at the Center of Quantum Technologies actually in Singapore at the National University of Singapore. I actually quite liked it because there were some critical mass of people doing different things in the area of quantum technologies and I kind of enjoyed that, in the sense that there are many different perspectives on quantum information and people being interested in various different topics. Also I very much enjoyed the idea that there were people there who also do experiments. Maybe that is because my original computer science background is actually super practical, and theoretical computer science has for me always been a means to an end to to quantum information coming from computer science. I am very driven, not just by understanding how things work, but also in actually making something, building something that people might, ultimately, actually use. I quite enjoyed that there were people there that worked in experiments of quantum crypto that one might collaborate with.

ZIERLER: Culturally, what was it like living in Singapore?

WEHNER: It was kind of interesting. I have to say that I liked it because actually I really like warm weather. I like outdoors sports. From a cultural perspective, my love for nice weather and outdoor sports allowed for me to make some interesting cultural connections in Singapore. I think in Singapore, culturally, on the surface, people always find it a very Western country, but that is actually, I would say, not true. It is like an extremely Asian country, which of course is perfectly fine. I think coming from the outside, it is sometimes very easy to become trapped, to only confine yourself inside an expat bubble. There are I think more than over 500,000 expats, chiefly from Australia, living in Singapore, and it is very easy to just move there and not see so much from the Singaporean culture. While I was in Singapore, I started competitive ocean canoeing and dragon boating. That was actually great because it allowed me to get to know all kinds of Singaporeans actually, who were on my team. I found that super interesting. I learned a lot of things about Singapore. I really enjoyed living in Singapore. It also made me see some things differently than I saw before, even though I actually enjoy living now more in Europe, because it is closer to everything than Singapore.

ZIERLER: As a PI for the Center of Quantum Technologies, was there a specific grant that you were responsible for or leading?

WEHNER: In Singapore, I applied first for a grant that was relatively accessible because it was internal to the Center of Quantum Technologies. We later wrote grants to the Ministry of Education, both with various of my theoretical and experimental colleagues on the general theme of random number generation or randomness in quantum crypto. I was one of the main writers of that grant. Later, together with Alex Ling, who works on satellite quantum communication, I worked on a grant on satellite quantum key distribution. I have actually resigned from these grants when I moved to Europe.

ZIERLER: Was this a dual appointment? In other words, were you also on the tenure track at the National University of Singapore (NUS) at the same time?

WEHNER: That is correct. Yeah. I was also tenured in Singapore.

ZIERLER: Was that considered a dual appointment or was it one overall research agenda split between the School of Computing and the Center for Quantum Technologies?

WEHNER: That would be considered a dual appointment. People from CQT can also be appointed at other institutions of Singapore, for example NTU, which is a different university in Singapore. My appointment was at NUS. This actually means that my research was fully located at CQT in the domain of quantum technologies, and my contributions to teaching were in the department at NUS.

ZIERLER: In the way that I asked you about the Dutch government's support of quantum science as a national endeavor or the EU, in what ways did Singapore, the government, the authorities want to promote quantum science locally?

WEHNER: In Singapore, they made quite a significant investment in the domain of quantum technologies. They tried to attract people to come to Singapore in order to actually execute research in Singapore itself. Singapore is a very strategic place actually in supporting specific efforts that often then come out of Singapore. After I left, there was still quite significant support also for, for example, engineering these quantum satellites that I just mentioned and actually building technology out of these QKD systems.

ZIERLER: In the way that quantum internet and quantum networking were not really a big focus at Caltech, was that not the same at Singapore? In other words, were those areas of research more of interest in Singapore?

WEHNER: I am not sure if I entirely understand your question. Are you asking whether I would like to go somewhere where there are more that are doing the same, or more—?

ZIERLER: In the way that these became such major areas of interest for you more recently, I am curious if the origins of that were in Singapore or was that a more recent development when you moved back to the Netherlands?

WEHNER: I think the origins of this, David, is because I have always liked communication networks. It is actually my starting ground.

ZIERLER: Was that a big focus in Singapore?

WEHNER: No, but like I said, I think actually one thing that kind of, for me at least in my mind, brought me back to the domain of networking, which I previously pursued in a quite practical sense in the classical space, was the progress on building quantum hardware. In Singapore, with a lot of people that build quantum key distribution, they build quantum key distribution devices that enable point-to-point quantum communication over short distances. That is very nice, but it does not really allow one to realize a network in any sort of sense, meaning that both the distances might be larger as well as the connectivity between the nodes might be a little more interesting than having them all on one. I think one thing that motivated me quite a lot is interactions both with people in Delft, as well as people elsewhere in Europe who are in experimental physics and who work on, say, the quantum hardware of quantum networks. In Delft, with my colleague Ronald Hanson, but also in other areas in Europe—for example, in Spain or in Innsbruck—I am quite motivated to work with them now on the advances that exist in the domain of quantum networking that really motivated me to, let me say, come back to networking, but this time in a quantum space.

ZIERLER: Tell me about the circumstances of you returning to the Netherlands in 2014. Originally, had you thought you were going to build a longer-term career in Singapore?

WEHNER: I do not know, David. You assume now that I spend a lot of time thinking about these things, which I appreciate that you have so much confidence in my long-term planning. Let me put it this way. I found it super exciting to come to Delft, maybe for a similar reason that I was first interested in Singapore, namely because I really like proximity to people who build quantum devices. In Delft, there are a lot of people in experimental physics. They spend all their day building quantum devices. I really like to work with them. For this reason, I found Delft super exciting. I was actually missing this a little bit in Singapore. Singapore was also very far away from things, I have to say. Sometimes people think that I came back to the Netherlands because I studied in the Netherlands, but that is actually an accident. It's a total accident. I just happened to find this opportunity in Delft and like I said, I found that super exciting and I still find it very exciting.

ZIERLER: What was happening at Delft circa 2014 that was exciting to you? What was specifically compelling about returning, not just specifically to the Netherlands, but what was happening at the program at the time?

WEHNER: One thing actually that was super exciting back then was this loophole-free Bell test implemented using nitrogen-vacancy centers in diamonds and by the group of my experimental colleague, Ronald Hanson. I guess I already mentioned to you that I was interested in non-local correlations, and I found it really exciting that one might now be able to do such an experiment in a loophole-free fashion. Maybe it is not important what this technically means, but it means basically the gold standard of the convincing verification that nature is not classical, but that there is something else going on, for example quantum mechanics, and that one needs in order to describe such correlations. When I moved here, Ronald was already working very hard on building this experiment. I then later also became involved in this, specifically in trying to understand how with an extremely slow rate of entanglement generation, one could still do any useful statistical conclusions from any such test.

ZIERLER: For the last part of the talk, I have three questions, one about outreach and science communication, one looking back at Caltech, and then we will end looking to the future. On the first, with all of the ways that your science has been covered in national media, in the popular press, and all of the outreach that you have done, giving talks to people who are not specialists in the field, what are the public most curious about? What have you learned about communicating the science and also about managing expectations about what is achievable in the near term and the short and the long term?

WEHNER: These are all very good questions. I think the things people are most interested in, before they ask anything else, is "What can you do with these things?" I think this is a very important question. It is also the one that you asked a little while ago. I find it also quite interesting because I think it is important for us to communicate what we do, but I also see it as an opportunity to grow, let me say, the quantum network community. For example, if you released the Quantum Network Explorer now, any person in the world can explore how they would program quantum network applications and what they can do with it. I like outreach to be interactive and I like these questions because they can motivate people to participate in this. I believe the most interesting applications of quantum networks are not going to be invented in academia, but by people who have access to the technology and can program it themselves to do interesting things.

Maybe now about the managing expectations. I think in quantum technologies, in some domains, maybe not so much networking but especially with computing, maybe things have grown a little far. I think there is a very fine line. On the one hand, it is important to inform the public on what we are doing and to explain to people, and it is also important for the funding, let us be frank, of these super costly efforts in order to motivate funders or the public or whoever needs to justify giving money to these efforts that this is worth pursuing. One should realize that this is a marathon and not a sprint. It is very easy to err on the other side where one is, let me say, overpromising too quickly. I guess it is if you're with children in the car and you say "Are we there yet? Are we there yet?" This is sort of what the public wants to know: "When are we there? When is the quantum computer ready? When is the quantum internet ready? When can I use these things?" I think it is very important to not say "Yeah, yeah, we will be there very soon now" but to manage expectations because it is a marathon, both in computing and in networking. I think it is important to engage with the public. I think also that all of these technologies have immense potential, but there's sort of a boundary. When I read things that quantum computing will cure cancer, I'm like, "Oh my gosh." I go, "Am I involved in this field?"

ZIERLER: Looking back at your time at Caltech, either in the approach to the science or the collaborations or just the community of scholars that you were a part of, what has stayed with you?

WEHNER: I have to say, I really loved my time at Caltech. It was super nice. Like I said, I loved the palm trees. I found it, intellectually, super stimulating. I think quite a few things have stayed with me, even though they have sort of evolved. This is going to be a microscopic thing, but for example, at CWI, we never had group meetings, like just somehow did not happen. Then when I came to Caltech, John always had his group meetings. I found this very interesting. I, actually for a while after I left, I basically replicated John's group meeting. Actually, now I do it a bit differently. I think a lot of things that relate to exchange in a group really stayed with me from Caltech. I know this sounds weird, but at CWI, there was no such thing. People randomly met maybe occasionally, sometimes, possibly, under some circumstances. But this group concept, as weird as it may sound, was actually a bit new to me when I came to Caltech. This is something that much stayed with me. I really enjoyed my time at Caltech, and I was also, by the way, always super impressed like how John can juggle all these balls and make this Institute happen, while at the same time doing brilliant science. This I find very fascinating and impressive.

ZIERLER: It is amazing.

WEHNER: I am not entirely sure how he does all these things.

ZIERLER: Finally, looking to the future, in however long you define your research agenda, looking out 5 years or 10 years, what is the frontier for you? What are the big unanswered questions that will be most important for you to focus on?

WEHNER: I have to say that now with the Quantum Internet Alliance, being that we are actually now continuing as collaboration in Europe funded by the EU, this is widely European, is we have set ourselves quite an ambitious goal. It is a bit of a moonshot goal, I have to say, in the sense that the technical requirements are super high, but we have decided together to build a specific prototype network, a quantum network prototype, by the end of 2029. I really like the fact that this sort of goal actually focuses the collaboration very strongly because this is what we decided to make together, like we decided to make this thing.

Now the question is, "How can we all make this happen together?" I very much enjoy this because I really like collaborating with other people, and its actually a different type of challenge just to work with so many diverse people, eo enable collaboration to on our moonshot goal. From a scientific perspective, one of the things I find with my own hand as a researcher actually the most exciting is that from this I learn about all kinds of computer science problems that I did not even know existed before, which are actually super challenging and important. We do not really know how to solve them, but we should solve them if this should happen. For example just to mention a simple example that we ran into, I mentioned to you that I work a lot with my colleague Ronald Hanson and we have actually built a test network in Delft. On this network, we also even now run small scale programs. I did something super practical, computer science, because we literally implemented the first ever quantum network operating system that runs on these nodes and you can program them and do things. From this, one quickly learns actually that there are some interesting computer science problems to tackle on how to, just to give you a simple example, schedule applications and instructions on these processors. On a cartoon level, each processor, you can think of a little quantum computer, I can either decide to do local quantum gates or I can do a network operation, produce entanglement. An operation to produce entanglement actually necessitates tight timing synchronization and precision with the other node. This means that this is a schedule that I cannot make by myself. It has to be done in the network or in cooperation with the other nodes. At the same time, I also want to do local gates. But I cannot do both at the same time. This, actually, one can formalize this into some computer science problems. That is something, for example, that I learned from this implementation that we are doing now. These are the challenges that we have to solve, even for this prototype network. That is just one example.

ZIERLER: Even in five years, things are going to look pretty different. It is quite exciting.

WEHNER: I find it super exciting. Yeah, I should say that this goal that we set ourselves with the network, every time I think about it, sweat arrives on me. [laughs] Because we said, "Yeah, we will connect these networks or some repeater link exceeding 500 kilometers." If you think about the requirement about what quantum memories need to do in order to achieve that, like sweat breaks out on my face. [laughs]

ZIERLER: Nothing like a good deadline to make magic happen though, right?

WEHNER: Yeah. But I am quite excited actually because I think together, we might actually do this.

ZIERLER: This has been a wonderful conversation. I am so glad we were able to do this. Thank you so much.

WEHNER: Thanks, David.