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Interview of Nicolas Gisin by Olival Freire on 2013 December 7,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/38326
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In this interview Nicolas Gisin discusses topics such as: his family and educational background; University of Geneva; Constantin Piron; Gerard Emch; University of Rochester; optics and quanum optics; Abner Shimony; Alphatronix; Guisan at the University of Geneva Group of Applied Physics (GAP); John Bell; Alain Aspect; Jean-Claude Zambrini; quantum cryptography; optical fibers.
We are in Geneva on December 7, 2013 with Nicolas Gisin for an interview.
So we can begin about your background. May you speak a little bit about your family background, education background and how you decided to go to a scientific area?
Yeah, sure. So my father is from the German side of Switzerland, from Basel, and he was also a scientist, but biology—actually ecology, but the word didn’t exist. He was a specialist in Collembola. I’m not sure how you say that in English, Collembola. Very small insects that you find in the earth. So he married my mother, who was a German. So she went through the war, which wasn’t easy, especially because she was…let me just remind…a quarter Jewish. This is not much, but sufficient to cause problems. But I was born in Geneva. I forgot to say my mother also got a PhD in biology that she actually did with my father. But she never really worked in that, I don't think. I think she did the PhD more for my father than for the biology. So I was born in Geneva in 1952. Actually, I started speaking German at home first until I was about four years old, and then I switched to French because of school and also because my brother, Patrick, had difficulties between French and German. He started confusing everything, so my parents decided at home to switch to French. Then of course there was this kind of scientific background, but not very…How shall I say? It was not very obvious, especially as my mother then started a kindergarten (not something especially scientific). So I always loved children and so on.
Well, I quickly realized that I was good at science and not very good at languages, so it was quite obvious already when I was a teenager that I would more go into science. But of course, I didn’t know what science. I had no idea what that even means. When I started college, I was also bright in physics, mathematics, especially mathematics at that time. I started reading books by Heisenberg, books about Galileo and Newton and these kinds of books. I don't remember all the books. On the other side, I didn’t read any of the books that were mandatory for school, so I was not very good at school in that sense. So it was clear that I was going to do something in that direction. Then after high school—The last two years of high school were…I really didn’t like school. So my first idea after high school was to go for a big trip to make a big break. At that time, all the youngsters used to go to Asia. It was a time when you could easily visit Iran, Afghanistan, and India. Because of my character, I guess, I decided to go the opposite way, so to go to America from north to south. So together with a friend, we went via Iceland. We actually lived in Iceland for a few…Is [the recorder] still working?
So I worked for I think—I don't remember—a couple of months there in construction building big houses and things like that. Then in America I started from Canada all the way down to, well, Brazil, for instance…
…country, and many more. I could tell a lot about this trip. It was a very important trip.
You crossed from north to south.
Yeah. It took altogether almost two years. Actually, before leaving I worked as a postman to make some money to pay for all that, and then during the trip, I really was pretending to be very poor, which of course is not really true because my family is certainly not poor. Actually, it is in Brazil—actually, it was even in your city in Salvador with another friend. I changed friend in the meantime. We rented a very small house, one single room house on a beach, and stayed there maybe a month. I read actually a lot because I found a French library there, and I read a lot of the books that were actually recommended in high school which I didn’t read as a high school student because of my bad character, I guess. But then I read them because it was not mandatory, and I enjoyed them very much, by the way.
So I did all that, and then I came back. After these two years of break, I came back to Geneva, and I knew that I wanted to study science. But since I didn’t really understand exactly what the different sciences are, I decided to do math and physics in parallel, which was not too difficult because many of the lectures were in common. Of course, it meant more exams, but exams have never been difficult for me. So I did all that. [Interruption]
Okay. Let’s resume.
Yeah. So I decided to do the two in parallel. While doing that, in the summer we had a longer holiday, so I was also traveling a lot. I still enjoyed that a lot. I was also reading books by Einstein and these guys. That convinced me to go to physics and not to mathematics. I realized that I wanted to know how the world functions, kind of a logical approach of how the world is built and so on. So I was really actually very much interested in theoretical physics and in foundations, I guess, from the beginning.
So let’s move to your doctoral studies. You came from a kind of long line in theoretical physics in Geneva because your supervisor was Constantin Piron. I think that he was a student of Jauch. So in a certain sense, interest in foundations of quantum mechanics may be obvious. But anyway, may you talk something about the choice of the subject for your doctoral dissertation and about your experience having Piron as an advisor.
So I don't think I really chose a subject. The difficulty was to choose a professor. Of course a professor would then of course lead me to one topic or the other. Indeed, there is this long tradition in Geneva going also back to Stueckelberg…
…and Jauch and Piron. So it was this very strong fundamental research, and also all this axiomatic approach by Jauch and Piron. So Jauch passed away when I was, I guess, in second year study, but there was still Piron. I also had some lectures with Stueckelberg, but he was already very old and no longer an active professor. But there were quite some alternatives. I could have gone into high energy physics. In Geneva, there is CERN, the European Center for Nuclear Research, or high energy physics. But this I didn’t like too much because already at that time, that was kind of too big an organization, so I didn’t want that. But then there was also condensed matter, which is really the largest part of physics in Geneva. There were also some impressive figures there with Martin Peter. I knew also very clear that Martin Peter, for instance, and Piron didn’t go on well. They were actually saying quite nasty things about each other, so the atmosphere was quite bad. That’s also going to be important for my next thesis. So it was not clear whether to go to maybe condensed matter or if it would be in theory. I was kind of inclined to theory. Piron was probably the one doing the most interesting stuff, but on the other side, he was a horrible character. Let’s say his character was a quite difficult character, and it was not difficult to realize that he was not going to be an optimal choice in terms of social relations. Probably I didn’t fully appreciate how true that is.
Anyway, at the end I decided to go with Piron. It was partly fun because he also liked to be a showman, but it was also clear that he had…Well, I was going to say zero skill in communication, but I think it’s worse. He had negative skills in communication in the sense that the more he was talking, the more enemies he was making. He was really in conferences able to shout at people and to behave in an unacceptable way, and so clearly, each time he was going to a conference, his situation got worse. Then he got more and more frustrated also, and being more frustrated, he was shouting more. So it was a very negative spiral.
So anyway, the physics he did, at least he did with Jauch, was certainly good physics. But somehow I arrived too late for that good part, so I had to do my own stuff. I did my Ph.D. on statistical quantum mechanics, especially getting interested in nonlinear Schrödinger equations to describe possibly open quantum systems. But in the back of my mind, and I guess of Piron’s mind also, was the measurement problem.
Maybe a point that I should mention here is Piron made his Ph.D. in 1964. I think it was a very bright Ph.D. It was really a theorem which is very important in the axiomatic foundations of quantum physics. But 1964 was the same year as when Bell published his little paper on the Bell Inequality, which became enormously famous and is extremely important in today’s physics. And both in Geneva. So Piron got also very upset and frustrated that Bell got so much more success and attracted attention so much more than his own nice work, and for that reason in part, and probably only in part, and also because he’s axiomatic. The Jauch-Piron axiomatic approach to quantum mechanics completely failed to take into account entanglement or composite systems. For those kinds of reasons, I never really learned about entanglement.
So during my Ph.D., I started to discover entanglement. Actually, one summer I took holidays in the Maldive Islands where my brother was a professional diver—Patrick, the one who had difficulties in German and French. There I took along with me—Well, actually before going to Maldive, I went to Kerala [that correct], India, but that’s not important. I took along the review paper by Clauser and Shimony.
Okay. This is the 1978 famous review paper by—
’76 or ’78?
’78, I believe.
Yeah. It’s a nice review.
Yeah, it’s a nice review. It’s a Physics Report, I think.
So I took that along. Of course at that time, everything was on paper. I read that there on the beach, and that impressed me enormously because that was, for me, the discovery of entanglement and its power as a kind of causal resource to produce nonlocal correlations. Although that was not part of my Ph.D.—I was not even allowed to talk about these things to Piron—it impressed me a lot. I started thinking and working on the side on that kind of subject in parallel to my nonlinear quantum mechanics.
So you had kind of a double life doing your Ph.D. in foundations, two focuses of interest.
I guess…Well, I don't know how. So then of course at some point I got my Ph.D. I had good marks. The Ph.D. was recognized by the Louis de Broglie Fondation in Paris. I got a prize for this Ph.D., which is always nice. It’s a good start, let’s say. But then I also very quickly—and that came—I don't remember now how it went. Oh, yeah. No, sorry. Then it goes like that. I’m trying to remember. So after the Ph.D., everybody was at that time going to the U.S. for a post-doc, almost everybody. So this time I did what everybody did, so I went to the U.S. to Rochester, New York, and did a Post-doc actually with Professor Emch, who was a very mathematical-oriented—
Yeah, Gérard Emch.
Gérard Emch, yes. But at the same time, there in Rochester there were several things that happened. One was that there were a lot of optics there. There used to be Kodak. Kodak is probably still there but dying. But at that time, it was a big, enormous industry influencing all the area, and indeed putting optics in Rochester on a high, high level. So for the first time in my life, I saw optics labs and optics professors. There were no optics in Geneva at all, so I never actually studied optics. But there I met with people like Emil Wolf and Mandel, and visiting the labs of Mandel impressed me quite a lot because you could see on one table—bigger than the table here around which we are sitting. But it’s these optical tables, and there he could see now there is a photon, a heralded photon, and the single photon detectors. All that was completely new at that time, but I realized this is very fundamental. So that was very important to me because I realized that here you could do something fundamental, experimental, but at human scale. Fundamental in Geneva means CERN, high energy physics with teams of hundreds or thousands of people, so not human scale. So that was something that triggered my interest.
Another very important meeting that happened there—and that was, I have to say, thanks to Gérard Emch because he understood my interest in foundations. He shared them also. So he allowed me to organize or co-organize with him some series of lectures on foundations where we could invite some people. For instance, we invited Abner Shimony. I think it was the first time I met Abner there. So we started. I mean Abner is an extremely nice person, so easy to talk to. He was very famous and I was a young student, but still he managed to put me at ease. I mean he was kind of the opposite of Piron somehow. At least socially he was absolutely the opposite.
I know Abner in person, and I can corroborate your view. He is one of the kindest persons I have met in my life.
Yeah, indeed. Indeed. So I was also asking after a while to Abner how to combine my interests, and also of course I had no permanent position, and like every post-doc I was starting to think. By that time—I didn’t even say that—I got married in the meantime. I had my first baby. Actually arriving in the U.S., I had a baby four months old. So you start thinking differently. You think that at some point you should also be able to sustain, to support your family. Abner made a comment which also I registered but didn’t understand fully. He said, “Go into quantum optics. There you can do something practical and fundamental.” Okay. So of course for me, quantum optics was very abstract. Again, I had never any lecture on optics. On quantum on the other side, yes. But that reminder was implemented, let’s say, in my mind.
It was also there during this post-doc that I realized that these nonlinear equations that I was studying, if you combine it with entanglement, my second foundation life, that would lead to signaling, to the possibility of communicating without any physical support or communicating faster than the speed of light. So I did this, and I published after a while. But realizing that, I also realized that all these nonlinear equations—not only mine, but also many of the others; I was not the only one working on that. But all these equations would all lead to signaling, so we were all fundamentally…How do you say that? Rotten. So I was of course thinking about it.
Then I also realized that if you actually instead of having a deterministic equation, you would have a stochastic equation. I knew about stochastic differential equations because when I was in Geneva, a very good friend of mine, Jean-Claude Zambrini, was actually studying stochastic equations also for statistical mechanics, but quite differently from what I was doing and with a different professor. But anyway, that’s how I learned actually about these stochastic differential equations. When I could combine my nonlinear Schrödinger equation with the stochastic equation and come up with a stochastic equation which would avoid the signaling problem.
Okay, okay. A combination of the two. Okay.
Yeah. That is my first PRL, my first Physical Review Letter. That was ’84, if I remember well. I’m not exactly sure, but I think it was ’84. So that was very important. Also, I have to say when I showed that to Emch, he said, “No way. That’s never going to be published.” But I sent it nevertheless to PRL, and it was accepted.
I was very glad that I didn’t follow the advice of my advisor. Actually, that happened very often that I didn’t follow them, and more than once I was correct.
Let me come back to your doctoral dissertation because maybe I did not realize the clear direction of your first research because as far as I can understand, you were interested in your doctoral dissertation in nonlinear Schrödinger equation, but also open quantum systems. Almost at the same time, there was research in this second direction, but not necessarily linking it to nonlinear Schrödinger equation, for instance, in the works by Leggett. Were you aware of this, I would say the second approach at the time?
There was even a third approach. Will I be able to get a copy of your interview, of your file? That would be actually nice for me since I’m not saying so many things. A copy of what you are recording?
Okay. So to come back to open quantum systems, indeed I was—My approach was of course extremely small. I was not the only one doing that, but it was a very small group. Leggett was doing something, Caldeira and Leggett, but I didn’t know about them. But there was a third approach which I knew quite well, and that was kind of the more traditional and the mainstream one, which really consisted of coupling your system to a heat bath made out of a harmonic oscillator, or some spin-½s. We didn’t say qubit at that time, so they were spin-½s. It was all linear unitary evolution, and then you made some approximation, and that’s where you turned the unitary deterministic evolution into a density matrix evolution. For me, it was very clear that density matrices mean mixing and in this approximation that you were doing, you’re actually sneaking in the essence that allowed you suddenly to go from pure state to mixed states or from reversible to irreversible. I thought that was kind of cheating, all these approximations. Fundamentally, it is actually linear and deterministic. So my hope was to come with an alternative which would be from the beginning nonlinear and stochastic. For instance, you see in the collapse of the wave function a reduction of the state vector, which has its normalization which clearly introduces some nonlinearity. So I wanted to do something like that. Clearly, also this stochastic equation I published in PRL was having a lot of this flavor. It was indeed a nonlinear stochastic differential equation. It was relative renormalization, so it was also the norm, and it was driving the system to some eigenstates. That was all quite…The flavor was good, let’s say. Yeah, so I was aware of that. I was not so much aware—actually, no. I think at that time I was not aware at all about Caldeira and Leggett.
Okay. Now I may understand. So let’s follow ahead. So you stayed in Rochester two years?
No. Actually, I stayed only one year, but it was a very important year and a very busy year. The reason I stayed only one year is that during that year, right at the beginning, my wife got pregnant again. So we are going to have a second baby, and of course this is possible. I could have stayed. Gérard Emch was offering to stay a second year there. But for my wife, it would have been very difficult. I mean her English was poor, and having two children, she really didn’t socialize well. In Geneva we had the family who would help…
Very different, yeah.
…with the children, the babies, and all that. So for my wife, it was a kind of choice either to say my priority is my career or to combine that with the wish and the life of my wife. It was actually interesting because my friend Jean-Claude Zambrini, he was a kind of similar—okay, without children. But he had his girlfriend together with him at Princeton, and they were also hesitating. He made the opposite decision. But I decided at this point that I should go back to Geneva. I was hoping that Piron would take me in his group at least to help me to find connections in Geneva or in the area to get an Academy post-doc position or to get into industry. I was open to go into the industry because I was realizing that the kind of physics I was doing was not mainstream, and it would be difficult to get a position, especially if I was not flexible to move around over all the world. But Piron has really not been very helpful at all because he realized that I was talking about entanglement. I don't know exactly why. He was such a strange character. So he was probably doing a lot of harm to himself with this attitude. But at the end when I came back to Geneva, it didn’t take long to find actually a position in a spin-off company from the University that was working on optical fibers. So I said okay, optics, optical fibers. I had not really an idea of what that is, but optics, I understood the word and said, “Okay. Maybe I should go into that direction.” So I applied, but the position was actually as a software person. I did some software already when I was at high school, but you know, that was still at the time when you had the…How do you call that in English? The cards with…
The punch cards.
Punch cards. Exactly. It was the punch cards, so it was of course completely outdated. I had another brother Bernard, a younger brother who is a computer science guy, and so I went to him for the weekend and he explained to me what actually modern programming is. That was a time when you had the first kind of personal computer. It was not IBM at that time. Well, you had HP, but you also had—I don't remember all the names. But it was very early. I remember that was in ’84, I guess. So essentially I had to learn in a weekend to say enough key words and understanding that I can pass my interview on Monday, and it went on well. At that time, I guess physicists were the only guys able to program anyway, so being a physicist was not bad at that time at all.
So I got hired by this company, Alphatronix [correct], and there was one thing which I really liked enormously in this company. It was a small company—I don't know, 20 people. Maybe we started at ten and at some point went up maybe to 25 or 30 people. It was that really everybody was working together. It was a superb atmosphere. It was really a team spirit. When I compare that with this horrible atmosphere that I lived during my Ph.D.—
When you were a graduate student.
Yeah, with Piron and the other professors shouting at each other and saying nasty things about each other to the students—I mean unacceptable things—I liked it very much. I have always been a team player, and I really like the idea to work together. The hours were long. I mean we were working really like hell. It happened very often that we were working until midnight or like that. It was really very long hours, but very nice atmosphere and really working together and trying to find out. So I was doing software and people were doing electronics and optics, but at the end it didn’t work together. Then everybody could say, “Okay, it’s the fault of the others.” No one was saying that in an aggressive way, but the difficulty was to say, “Okay, my software seems to be working. Yeah, my electronics work. My optics work. Why doesn't it work together?” There were all kinds of small details why this didn’t work together. Here, I guess also because of my background, I quickly understood enough of the optics—pretty simple optics, but still. I found out optics, enough of the software, and I have never been really good in electronics, but I understood enough of that to have been helpful in settling out where or what the problem is and then helping to resolve that problem. So that put me in a relatively good position within the spin-off. The fact that I was speaking English—maybe not perfectly, but good enough or better than many others—also helped to put me on that spot. So I went up in this company, and I stayed there four years. These four years had been extremely happy years in my career—in my life, I should say.
However, after these four years, some management problems, we were a bit too early. It was a bit too early for the Internet rush because it was still in the ’80s. Ten years later we would all have become billionaires. But then it was kind of too early, and the owners of the company didn’t want to open the capital. They wanted to keep control, and by not opening it, they didn’t get the money that would have allowed them to survive and wait until really the Internet boom happened. Of course we couldn’t know that the Internet boom was coming, but that’s…Maybe I should have, because that was the purpose of the entire company. So the company went bankrupt. Also, that was all kind of fishy. I don't want to tell all the details here.
So there was another company that immediately started in the same location. Simply the door was moved from one place to the other, but behind the door, it was actually even the same rooms. I didn’t like that at all, so I decided to quit that company. I went to the director of the Applied Physics Group at the University of Geneva, Professor Guisan.
His name, the professor?
Guisan, like our general during the Second World War, but had nothing to do with Guisan. He was a professor, a very original person, who was initially in high energy physics, but then changed completely to solar energy. I don't exactly remember, but I knew him because this being a spin-off of the University, we still had contact with the University. So somehow I knew him. So I went to him and told him, “Look, sir. I don't want to continue with Alphatronix or the successor of Alphatronix. But I know a lot now about optical fibers, and I know a lot of people at Swisscom.” Okay, it wasn’t called Swisscom, but our national telecom operator. I think at that time they were called SwissTelecom or something like that. So I know these guys, and these guys need me to understand how to introduce optical fibers in Switzerland and how to write a specification, how to buy optical fibers, how to test them, how to measure them, and all that. So seeing as they need me, they are willing to pay me, but they cannot hire me because in Switzerland at that time, we had something which was called a “Personal stop”. It was in German because the Germans are making the decisions in this country anyway. So the idea was we should not have more state employees, so they were not allowed to hire any state employees. That was good for me because in this way I avoided to become a state employee. Well, actually they paid the University so that the University could pay me so that I could work for them. That was actually a very good position because I was in Geneva. I had just one lab for myself including lab and office were altogether. So I was really paying myself via this telecom operator.
So this was 1988 or 1989?
I started in ’88 or ’89 maybe, and then I continued like that. Actually, the first year I was even doing that, over half time I worked for some software company. I did a one-year, half-time job in a software company CPI.
So if I understand well, your work with optical fiber began at the GAP when you began to work with—
No, no, no. When I was in Alphatronix. It started in Alphatronix.
Okay. It started over there.
Yeah. The mission of Alphatronix was to develop instrumentation for the characterization of optical fibers, but really measurement instrumentation.
Okay. So you brought these skills you acquired at Alphatronix to the GAP.
Yes. Then I left indeed Alphatronix after they went bankrupt and went to the GAP, the Group of Applied Physics. In the beginning, I was doing the same kind of work, which was mostly for the national telecom operator. It was really the time when we were introducing single-mode optical fibers in Switzerland and developed that at that time many instrumentation. So that’s what I started doing, and so at that time, I was also working in the lab doing things which are simpler than what we do today here. But still, I was really working the lab, initially alone, and then hired an electronic engineer and so on and started building a group.
There are maybe two things which are worth mentioning at that time. First, I understood polarization effects in optical fibers. Polarization effects, you can really think about them as a two-level quantum system, and so my quantum background was certainly very useful there to understand it. I worked quite a lot on that and invented a way to measure what is called polarization mode dispersion. It’s a way of measuring how light pulses spread because of polarization effects. That was nonexistent at that time. In parallel of that, because I had still some academic freedom, I could buy a single photon detector. When I was buying the single photon detector, I had no idea what it would be good for. I realized that you can buy a single photon detector, and I found that just fascinating, so I decided to buy one, a silicon avalanche photodiode. It’s a relatively simple thing. So I bought one, and then of course I was trying to invent a reason for using it and measuring decay times of doped fibers and things like that that you could do in a much simpler way, actually. But still I did some things and published some papers which probably haven’t been cited much. But it was a way for me to get acquainted with photon counters, the kinds of things I saw when I was visiting Mandel’s lab in Rochester. So that’s how I started at GAP, at the Group for Applied Physics.
Okay. Let me come back to ask about two people that you made reference in your papers before moving to your work on GAP. One is John Bell. You spoke about how you read this review by Clauser and Shimony, but my question is how did you meet him in person? Here? There are talks and social gatherings. What kind of social relationship did you have with him? And the second one, in one of your papers, I think that one of your first papers, there is an acknowledgement to Jean-Pierre Vigier. So if that was a close connection, because for sure he would be interested in nonlinear Schrödinger equation, and if this relationship with the Fondation Louis de Broglie was related to Vigier or not. So about these two people.
Okay, very good. So we have to come back in time. So during my Ph.D., the Association Vaudoise de Physique, [unintelligible] Physique. So it’s an association of physicists that actually is located in Lausanne (so not in Geneva). But this is really 60 kilometers away, so not far away. Piron was, by the way, living in Lausanne, commuting every day. So this association every year organized one week of a winter school where during the morning and the evenings you would have lectures, and during the afternoons, you’d go for skiing. Yeah, that must still have been during my Ph.D. So we were organizing that, and Piron was in the organizing committee. So for instance, in this case, he had been nice to me because he gave me a chance to talk there. Yeah, there was also Georges Lochak, who was the director of this foundation, the Fondation Louis de Broglie. I don't remember many other people. Of course Piron himself. That’s normal.
There was then a question, a debate. Should one invite John Bell or not? I mean if you want to do a winter school on foundations of physics near Geneva, having John Bell in Geneva, it’s the obvious fact that you should invite him. Of course. There’s no question about that. However, because Piron was in this organization committee and Piron hated Bell because Bell was more successful than himself, at the end, the Swiss compromise—I was not in the organizing committee, but that was pretty clearly the way it went—was to invite John Bell, but without giving him a spot to talk, which was kind of absurd. I mean today everybody would have been proud of having a record of John Bell giving a lecture. Anyway.
So John Bell came to this, and Alain Aspect, by the way, also came. So Alain Aspect was a good friend. Probably from that time, from that meeting that my friendship with Alain started. We skied together a lot during the afternoons. I mean he was already a kind of star with the Bell’s experiments. Of course I was not at the same level, but…Okay, he’s a bit older than me, but not much. So yeah, we got to be good friends. It was okay. John Bell was asking…John Bell in all these meetings—I had seen him at several meetings. He was always sitting at kind of the last row in the very back. He must have had good eyes and good ears. Usually he said nothing, but sometimes there was a kind of eruption. Then he would make some comments, extremely sharp with his Irish accent, “ch, ch,” essentially killing the guy who was just saying too many stupid things. In foundations, especially at that time, you had enormously…Okay, you had many people coming with very stupid ideas and comments and bad philosophy and bad physics quite often. So when it was too much, John Bell would just say a few words, and usually the guy on the scene would just dissolve. But so John Bell was there and not interfering too much. He was quite quiet. I think he would take that as kind of holidays. I think that happened in Montana or I don't remember. Somewhere in the mountain [unintelligible].
So obviously John Bell was a hero for, well, everybody but Piron. So with the same Jean-Claude Zambrini, the same friend, we decided to ask John Bell to give us an evening lecture, a kind of private lecture. At first he said no, that he doesn't have his transparencies. It was still a time of transparencies. We didn’t have computers, no PowerPoint and all that. But then we learned that his wife was going to come to join, and so we said, “Okay, your wife can bring the transparencies. No excuse.” So it was not too difficult to convince him, and so we decided that one evening after the dinner while the professors would enjoy the good local wine, we would organize that for the students.
So we went to some basement. It was a room without a chair, and so the students were sitting on the floor. The ceiling was not very high anyway. We just organized one projector. John Bell was staying there, and he delivered his lecture. I remember the way he started that because that has struck me completely, has had a big influence in my life. He started by saying, “I am a quantum engineer, but on Sundays, I have principles.” So essentially he was saying that he’s a guy who of course manages to do computations with quantum physics or quantum theory and he contributes to practical quantum physics in the sense of what they do at CERN and big accelerators—
Yes, and [unintelligible].
Actually, he was also doing a lot of quantum field theory, so he was quite more than an engineer. But anyway, he presented himself as a quantum engineer, so a person using quantum mechanics. However, on Sundays he has principles. He sits back on Sundays, and probably more often than that, and thinks about the foundations and says, “Okay, the foundations have to be clean. You have to clean them, and you cannot use all these sloppy words like measurements without defining them,” and other things like that. So that was his talk. I don't remember much more from his main message, but that message haunted me since then.
For instance, after my talk there—so I was talking about [unintelligible] nonlinear equations that could, a bit in the spirit of Bohm and Vigier, drive particles around so that at the end of a measurement you have a result. But it was still deterministic. That was before Rochester. That was still deterministic evolutions. He was nice because after the talk, he came to me and encouraged me and he was the only one who encouraged me. No, Georges Lochak also was very positive. I have to say so. Georges Lochak and Bell were very positive, but they were kind of the only ones. But okay. You know you had the quality. I had the quality—maybe not the quantity, but the quality. So that was very positive.
Actually, if I continue on that, while I was working in Alphatronix, I wrote a paper where I was generalizing my stochastic equations, the one that went to PRL, to make it fully general, very beautiful, and in which also I was proving that every deterministic nonlinear Schrödinger equation would lead to signaling. In the same paper, I was also combining this very new theory of GRW that just came out with the stochastic differential equations which I knew how to handle. That went to PRL but was rejected by PRL. Actually, it was complicated. I sent it to PRL while I was still in Alphatronix. Then I don't remember. I had two referees, a positive and a negative one, but the negative was not too difficult, I think, to reply, so I replied. At that time, it was not going by email. It was really going by post, so you really had to send a post and then to wait weeks and weeks. After me sending this reply, the company went bankrupt, and then of course I had other things to do. Also, the address I gave was in the company. So actually, the reply from the editor I never received.
Okay. So it went astray.
Yeah. It went to…okay. Then at the end I had to ask again to get a copy, but that was probably maybe a month afterwards. The final decision of the editor was that—So the second referee was also positive. But then the editor, instead of accepting the paper with two positive referees, sent it to a third adjudicator, although there was really no need of an adjudicator because the two referees agreed. The adjudicator said, “This is all correct and nice, but not of the appropriate kind for PRL.” That was indeed the case where if you want to do something fundamental, PRL and any good journal will be extremely, exceedingly difficult. So everybody was positive, but it was not of the right kind, whatever that means. It just was, “You shouldn’t be doing foundations of quantum physics.” That was the message. So I got very upset, but anyway, I was just starting at GAP, at the Group of Applied Physics. So I went to one of the local professors whom I followed his lectures while I was a student, and I knew that he was, at that time, the editor of the Swiss journal Helvetica Physica Acta. So his name is Werner Amrein. So I went to him and showed him everything, all the reports and everything, and he said, “Well, accepted.”
I mean that was in a matter of ten minutes, the time for him to read the referee report, and he said, “I accept that.” So it appeared in Helvetica Physica Acta and is I think today one of the best cited, if not the best cited article in all the history of Helvetica Physica Acta. So it was not a bad article. I still think personally that this is my best article. I don't think I have done anything of greater value. But okay. That was my life in foundations in parallel of my life in industry and then in applied physics. In a way, at that time Piron even tried to prohibit them to allow me to publish in foundations just to mention how stupid he was, or how socially stupid he was.
[Bell also liked my work very much. By the way, as he did know anything about stochastic differential equations he visited me in my lab several times and I had the chance of teaching him about that. I way extremely impressed: I was teaching John Bell! Can you imagine that?]
It makes sense, his attitude, if you consider just a jealous pressure.
Yeah, [unintelligible]. Yeah, he was saying that he is a representative of the Geneva school and I was not in line. So I was not following the line of his party, of his school. [Unintelligible]
And he was not able to appreciate the other line of your research in Geneva because Bell was here, which is kind of an irony, is that Bell wrote to us that he was motivated by a seminar here in Geneva by Jauch. I believe it was Jauch.
Certainly not Piron, so it must have been Jauch.
Yeah, Jauch—that he decided to analyze von Neumann’s proof. That’s the way that he came back and he spotted the problem in the proof. In a certain sense, one can say that there was a kind of competition between the line of Jauch and the line of John Bell. Yeah. Now let’s move ahead. I’ll just make a break on the recording because it’s better to have two files.
Okay, in case one gets corrupted, but hopefully not. [Break]
Now I want to move to your groundbreaking experiments with cryptography and the long distance entanglement experiments. The first one, I can see from your list of publications, was 1993 when you separated photon for 1 km using optical fibers. So in a certain sense, I think that you were the father of the idea of get entanglement at a longer distance using optical fibers. So from where came this idea? Because I think that this was a key idea of bringing expertise from the addition of optical fibers from one side and the other, entanglement. And the second, cryptography. How did quantum cryptography come to your mind?
Well, I think that was kind of very natural because I was working on optical fibers, polarization effect, and all that. Of course I was fascinated by entanglement already, and I had the single photon detector that I bought because I could buy it. So at some point, I was really seeing it would be nice to combine that and do maybe some Bell tests. Also, one shouldn’t forget John Rarity in England. He was working for the military there. What is the name of this? He was in England, so he was also following this kind of research.
So when I was thinking about doing that, the first thing I did was actually to visit Alain Aspect in Paris and tell him about that and try to understand better whether that’s a good idea or not. Alain has been very encouraging and said, “Yes, you should try to do that. This can certainly go beyond what we were doing without optical fibers. You can go outside the lab.” So he had been positive in that, and so we said, “Okay, let’s do some experiments.” Of course it was nontrivial because you needed also a source, and here I also was lucky that I could get in contact with some German post-doc and a German student. I don't remember now exactly. No, I guess we started before them. No, we still were doing that alone, I think, ourselves. I don't remember the timing exactly, but maybe so that the student was Wolfgang Tittel and the post-doc Jürgen Brendel. I think they were already in the first paper, actually. So we did also some preliminary work in Germany, but without optical fibers, I believe. I’m not sure.
Then another problem was that the detector I had was actually a detector that was only able to see photons up to a wavelength of about 1 micron, while telecom photons (so the photons that travel in telecom fibers) are 1.3 or nowadays even 1.5, so longer wavelengths, less energy. So you couldn’t detect them with these silicon avalanche photodiodes. So it was key that the first experiments we had to do using special fibers. I knew that you had special fibers for shorter wavelengths, but there you could not really have them. It’s not the Swisscom network.
Okay. A longer distance.
So if you want to go to longer distances, you had to change the detector. At the same time, I guess in ’92, I read…Yeah, it was in ’92. I read the paper by Charlie Bennett on quantum cryptography. Well, today it is called the B92, or it is called the two-state quantum key distribution protocol. Somehow I missed the Ekert paper of ’91. I don't know why because it was in PRL, so I should have known that one. I also completely missed the Bennett-Brassard 1984 paper because that was in a proceeding in a computer science conference, so I was not following that at all. But the B92 paper I could read and understand pretty easily. It’s pretty easy to understand all that. Then from that, of course I read the Ekert and I read the BB84, and there was also, more or less at the same time, a nice review paper in Scientific American, so maybe not pure science, but enough to understand all the ideas. So I realized that I had everything to do these kind of experiments. I don't know if they were talking about qubits or polarization of photons, but polarization of photons is what my specialty was in the telecom. So it was obvious that I could do that. I also met Artur Ekert at a conference where there was also Alain Aspect. By the way, that was the quantum optics conferences that took place in these times in Davos in Switzerland. Artur Ekert had also been very supportive saying, “Yes, no one has done these experiments. But if you know how to do it, you should do them.”
[At that time I was a real “PMD star”; at large telecom conferences, like OFC with tens of thousands of attendees, there were several parallel sessions on the topic and I was systematically invited. So, I was delivering one lecture after the other. No one in Geneva realized that, but in the huge telecom community I was very well known]
So then with a few students—it was a very small group at that time, maybe three, four, or five people; I don't remember—we said, “Okay. Let’s do these experiments.” We started on short distance with the special fibers and polarizers and all that. But there we had the detectors, and at the same time developing detectors for the telecom wavelength so that we could then go outside the lab. So the first detectors were really very, very simple. Take just an avalanche photodiode but InGaAs or germanium. Actually, initially it was germanium, so sensitive to the 1.3 micron wavelength. You dip it simply in liquid nitrogen in a very brutal way so the diode would not survive very long, but it would survive a few cycles. Then you do a bit of electronics to have a good—how do you say that?—thresholds, and then you could detect the avalanche. It was far from perfect but good enough that we could do our first experiments. Certainly the first—Okay, the one in the lab was the 1 km that you mentioned. But then there was the one under Lake Geneva, “quantum cryptography under Lake Geneva”. That had an enormous impact.
Yes. This got to the press.
It got to the press, and then there was this entanglement distribution between Bellevue and Bernex, two villages north and south of Geneva over more than 10 km. That also attracted the attention of the press. The New York Times had a full page on us. The BBC came to shoot a movie and so on. So that was for me a big change.
I didn’t know about the BBC movie. I just knew about the New York Times.
With the New York Times—well, with both of them I learned a lot. For me, that was completely new. I mean no journalist until that time ever talked to me. But then the guy from the New York Times—
From then that today you became a kind of celebrity, kind of a new Geneva school.
Well, to the journalists. Yes. Journalists torture me now a lot. But at that time, it was the very first. I remember I was so shocked by the New York Times because…So the interview went. That was over the phone. It was all fine, and then I could see the text. It was all fine. But what I didn’t see was the title because the title is added at the last minute by the editor essentially. The title—I don't remember exactly how it was saying. “Faster than Light Communication,” or something like that, something completely wrong. So the title was totally wrong, but I take no responsibility for a title, but only for the text. But anyway, it was a nice experiment. It was all very nice. Also when the BBC came, I was so scared. I didn’t even know how to behave in front of this camera, but okay. It was nice.
I think that’s for the public at large. That kind of chronicled your relationship between somebody who is doing cryptography in Swiss and that idea that everybody in the world had that the most secret, the most safe banks are in Swiss, so something that originated in the popular imagination.
Yeah, I guess that is certainly also true. You have also to remember that at the time at least of these first short distance experiments—and you would have to sort out the chronology exactly—but at least at the beginning I had actually still no permanent position at a university because when I came [unintelligible]—
Yes. You want to come back to this.
When I came back to the University Group of Applied Physics, I was paid for these projects by Swisscom. So I had just no position, and I had to pay my own salary even. I was growing a little group, but only with money coming from outside. I was also starting to have some money from some kind of European projects, very early European projects at that time, Eureka and Cost [???] projects.
How did it come, your position at the University of Geneva? What moment did the University realize that it would be important to have you as a professor?
I would say the University, probably never. The University is such a big thing that when at the very beginning I was asking sometimes the directors why they could not give me one position, maybe a half position of a student, the answer was always no. I never got anything, any support. The only guy—there was one guy in physical physics, Henri Ruegg, a professor there, and at some point when he was head of the theory department, he gave me half a position. So that was actually the first position I ever had. It was half a Ph.D. student position. But I would say that the guys at the University who started realizing that I might be a good asset for them was probably Olivier Guisan, the director, and another professor who was initially also a theory professor, but also at that time an applied physics professor, was Marcel Guenin. Probably they were closer to me and so they could realize what I was doing and realizing that I was actually bringing quite some money into the University, enough to pay my small group, and that I was actually having nice publications. You know, doing this quantum cryptography below Lake Geneva went to Nature and things like that.
You did an impressive list of publication.
Yeah, it was really important. So I think they were the only people who realized that. At some point, Olivier Guisan decided to take an early retirement, but that was his decision independent of me. Then Marcel Guenin somehow—I don't know exactly—decided or could manage to convince everyone to reorient this activity towards my activity, towards optics. So instead of the position being reopened in solar energy, it went to optics. Then of course I had to apply and there were candidates, and I was chosen.
This was 1990…the year? Do you remember?
’94 or ’95. I don't remember exactly when I became a professor. Something like that. So I was over 40.
After you had become a celebrity.
Yeah, yeah. Then things went pretty well, pretty fast because more or less at the same time I became a professor, there was all this press. Press got attracted by our results, and I started publishing a lot. Yeah, so in the ’90s, my career exploded. I published some years 30 publications, all very well cited and so on. So my status certainly changed.
Okay. Let me come back for more detail on your experiments with optical fibers, about the kind of source of your photon-pairs you were using. You spoke about Tittel. So my question is from the very beginning, you and he were aware that it would be better to use parametric down-conversion, or you had doubts?
No. No, we had no doubt about that. That was very clear from the beginning that we should go for parametric down-conversion. It was also very clear that we should…What we could do originally there is to go to telecom wavelengths, so the wavelengths compatible with the telecom optical fibers. Then of course the problem was a detector that we had to develop. But that was, I think, very clear from the very beginning that we wanted to do that, although the very first experiment was still at the silicon wavelength (so below 1 micron) because there we could immediately use the detectors.
This is an important piece of information, I think, because it’s true that Mandel was the first to use parametric down-conversion to entanglement, but the first results were not better than Alain Aspect 1982 results. Only in 1993, something like that, is that the results. So just only 1993 or even 1995 that results with parametric down-conversion overcame the previous results.
Yeah, indeed. So at the lower wavelengths, you had also Shih in Baltimore that was doing very good work, and some more that I’m probably forgetting quite some people here. But at telecom wavelengths, we were probably, and maybe we still are, kind of the leaders. So we were very active and successful, and that has been extremely…Yeah, it was a good move, let’s say.
I have one additional question about publications related to quantum cryptography because not only you have an impressive list of publications, more than 400 publications, but one of them, this review 2002 about quantum cryptography, it’s kind of a bestseller from the point of view of citation because it’s almost 200 citations every year. So my question is did you expect such a success? What happened with this paper?
So I was invited to write this review. I never wrote a review before. I mean I was very eager to publish and to be very active in the lab and on the theory side, so my first reaction was it’s too much work to write a review, and it’s too exciting a time to spend all the time necessary to write a review. But on the other side, I also realized that that’s a way of putting together much of our knowledge. So I was invited by Peter Zoller. He’s an extremely respectful person, of course, so I decided to invite some of my students and collaborators to help in writing this review. So at the end, in addition to me it was of course Tittel and Grégoire Ribordy, and was Hugo Zbinden the fourth one? I’m not even sure now, but I think these four. So of course then I had to write only a quarter of a review somehow. Of course all the coordination is not trivial [???] also. But so yeah, we decided to do that. But at that time, I didn’t even know that you could count citations. I had no idea that even an h-index existed.
So I was not paying any attention to these kind of things. Actually, we should come back to Vigier because you can really learn something about that. So I had no idea about that, so of course I had no idea. I couldn’t even say at that time whether 100 citations was good or bad. I had no idea at all.
But what you are telling me, it’s very, very interesting because…So how the scientific world had changed. Nowadays it’s impossible to discuss anything without citation and so on.
Exactly. So actually I had no idea until maybe 2007 or 2008, something like that, when one colleague, a professor here in Geneva, Markus Büttiker, someday informed me that I’m now with the second highest h-index in physics in Geneva. He was the first, and he was telling me that I’m the second. I didn’t even know what he was talking about. Then he showed me how you can look up on the Web this kind of information. Over the following year, I was applying to an ERC grant, these new grants that the European Science Foundation, I think it’s called, gives. When you fill these application forms, you have to put your h-index and how many citations you have. So I had to go this webpage because without that, you could not even apply. So that’s how I learned about all that. Actually then on my publication h-index, I grew and very quickly I overpassed also Markus, and yeah, nowadays I’m probably, in Geneva at least, by far the…I still don’t think that this is the most important thing in life, not even in scientific life, but okay. It’s always a satisfaction.
Yeah, yeah. It’s [unintelligible].
Let’s come back to Vigier. So when this paper was refused by Physical Review Letters because—okay, the referees were positive, but the adjudicator decided it’s not of the appropriate kind—I got very upset. It was a time where my life or my professional life was not that easy because I was quitting this industry, trying to establish myself. So having half-time working for the software company, half-time at the University but where I had to pay my own salary, it wasn’t too easy. Certainly comfort was kind of minimal. Then the stupid adjudicator came with his comments and the editor. I even called the editor, but it was kind of useless. So I decided that PRL, I will never go back to them. I decided that I would publish in Physics Letters. That’s the journal of Vigier.
Okay. He was one of the editors.
So I got in contact with Vigier, and I also said that I’m willing to do a lot of refereeing work for him. So I thought, “Okay, that’s a journal where I can publish,” and indeed, I think I’ve never had any paper rejected by Physics Letters A. Of course today I know that Physical Letters A cannot be compared with Physical Review Letters. Probably I could have some more publication in Physical Review Letters if I decided to continue to submit there, but somehow it’s so annoying, all these referees. Sending my paper to Physics Letters A, and after I send it and I knew it’s going through, maybe I have to correct a little thing, but it’s going through. So I can work on. I don't need to spend my time answering and arguing with referees. When you argue with referees about some work that you think is very good, it also affects you personally. It gets emotional. So it’s so much easier to go to a lower level journal, but where you can get rid of all this emotion and can just work and publish and be effective and so on. So some of my very good papers like a paper on optimal quantum cloning, the derivation of optimal quantum cloning from those signaling principles, for instance, which I think is a very nice result, was published in this Physics Letters A journal, and many more. I could certainly cite many.
So, I think at the end of the day that was not wrong because…It was not wrong. It’s only when I had students, and the students insist to publish in journals because the students today know everything about this h-index citations.
Yeah, it’s a new generation completely adjusted to this time.
So you have to—Of course I can’t speak against my students, but I have to respect them. They have a career to develop, so they insist that we publish in Nature, in PRL, in these journals where you have so much fighting to do with the referees. I would probably never have done it.
Okay. I have now two final questions, more speculative questions. The first one is that in the field of what we call quantum information, you have on one side quantum cryptography and on the other side, for instance, quantum computers. The idea of quantum computers till today seems to me more kind of a promise, while quantum cryptography seems to me something that came to stay. I want to know your personal views about this distinction I am doing between quantum cryptography—so distinctions in which concerns application and quantum computer. This is one thing.
The second thing is independent of this. One is that you analyze a number of different interpretations of quantum mechanics in your papers. Consistently you worked on nonlinear quantum mechanics and so on. So I would like to know if you have a kind of preferred interpretation for understanding the two very—
So you’re really asking two very different questions.
Yes, very different.
Let me answer the first one. Maybe after that we make a break because I need probably to drink. But let me answer the first one about quantum cryptography and quantum computer. I agree with you that quantum cryptography is here to stay. I don't see any reason for physics-based cryptography not to become main stream or at least to take some significant share of the market. However, I think that quantum computer, at least if you consider by quantum computer quantum information processor in general—not a universal quantum computer, but quantum information processor—I think they are also here to stay. Of course there exist not yet large-scale quantum information processor. You have quantum random number generator, which is certainly a quantum information processor, but on a very small scale. The large-scale ones don’t exist today, but I don't see any reason for them not to exist in the relatively near future. The first kind of results are already more or less existing, so I guess really the breakthroughs will come in five to ten years. So it’s not the far future, and I think it’s going to change everything. There will be a lot of quantum information processing capabilities in the decades to come. This is also to stay. There’s no reason for that to pass away. Yeah, history will remember the ’90s and this early part of our century as the time of this quantum…some people say the second quantum revolution, this quantum information processing revolution. But that’s going to stay.
Okay. Let’s talk about your preferred interpretation.
Can we make a break so that I can get something to drink?
Yes. Okay, good.