Pierre Meystre - Session I

Bromberg:

We start with how you got into physics, because a lot of people when they come to do research want to know what got physicists into physics in the first place.

Meystre:

So this is always a good question. I always was very good at math in school, but I was kind of good at everything except for languages. I got into physics by accident, because I remember when I finished high school in Switzerland, gymnasium, there were four things that interested me, and they don’t seem to make sense when taken together. One of them was physics, one of them was mathematics, one of them was history, and one of them was journalism. I recall that until the day I walked into the university to register, I did not quite know what I was going to do. So by elimination, I decided if I became a journalist, I would probably wind up reporting the accidents in the local news, and it was not going to be very interesting. This is what they call “Les chiens écrasés” as they say in French, “the killed dogs.” So it looked maybe not so interesting. Then history: I really love it, but it looked like the future was maybe not so interesting because I would probably wind up being a high school teacher, which was not what I wanted to be. Then the choice was between math and physics, and because the way the studies were structured in Switzerland it was easy to go from physics to math, and it would be harder to go from math to physics. Sounded like physics was a good idea. The other thing that got me into physics, which is actually a very interesting thing, is that it was claimed to be the hardest course of studies, and that sounded like a good idea, to go for the hardest stuff! So that’s how I started, as an accidental physicist.

Bromberg:

This is Lausanne [Ecole Polytechnique Federale de], and you went right into the undergraduate physics course?

Meystre:

Right. The way courses were structured in Switzerland at the time we didn’t have this concept of undergraduate and graduate as in the States. It’s a little bit more like the German system where after four and a half years, if you go directly through the system, you get the diploma, which is more or less the equivalent of a master’s degree here. So you don’t have the bachelor’s and master’s. So yes, I went right into this and got a diploma in physics. [Note added: my understanding is that Germany is currently revamping its system and has introduced Bachelor and Master degrees. I’m not sure that it’s such a great idea, as I always found that Germany produced superb scientists with the old system. I certainly hope that this continues to be the case in the future.]

Bromberg:

When did you get into the Laboratoire d’ Optique Physique? Was that after this coursework?

Meystre:

What happens is that as part of the program of studies, the last year of studies we had to do a larger lab project that lasted over the year, while still taking classes. Actually, for three months at the end of the studies we were just in the lab working on this project. That’s where I did my diploma in physics, basically, at this lab. It was kind of interesting because I’ve always been interested in optics. I think optics is a fun field, and they had just started this new lab in Lausanne. It sounded like a good idea to go into these things, try to figure out how a laser worked and all these things, which sounded very exotic at the time. [Chuckles]

Bromberg:

What did you do for that yearlong study?

Meystre:

For basically the last two semesters of your studies, you learn your way around the lab a little bit. Then they gave me a project that had to do with measuring the photoelectric effect, but I don’t remember exactly under which conditions. But I do remember that this project didn’t lead me to a Nobel Prize, [laughs] so it was pretty forgettable. I’m not sure that the experiment ever worked. I don’t think it ever really worked.

Bromberg:

Because I think of you as such a consummate theorist. What I mean is I find your papers sufficiently difficult! [Laughs]

Meystre:

I wanted to become an experimentalist; I never thought of becoming a theorist. It’s another accident in my life. I’ve had many accidents in my life.

Bromberg:

So how did that happen, then?

Meystre:

The way that happened is that after I got my diploma, I stayed in the same Laboratoire d’ Optique Physique, and the idea was to do an experiment for a PhD. That was the plan. It became very clear very soon that I was not very talented for experimental physics. First of all, I didn’t have the patience, and I didn’t have this real talent to make things work. You know, a great experimental physicist has these magic fingers, and for me it was always hard. Then I don’t remember because it was a long time ago, but something happened and I think I broke a laser or I broke something, and so it had to be repaired, and I could not do anything in the lab. During that time, it sounded like a good idea to do some theory that would help with the experiment, and I basically never went back. So that’s how I became a theorist. I learned very fast that I really liked theory, for many reasons, actually.

Bromberg:

How did you choice this particular branch of theory that you were working in at Lausanne, which was the atom-field mode interaction? What was going on at the school?

Meystre:

Basically what I was supposed to do for my thesis, which was to be in experimental physics at first, was to study the problem or resonace fluorescence, which became reasonably hot at that time. People were arguing at that time about the spectrum of resonance fluorescence, whether it was a three-peak spectrum, and what the relative height of the peaks in the spectrum were. At the time it was very much an open question, and my supervisor thought it would be good to set up some experiments there. So it led to trying to understand the interactions between atoms and light.

Bromberg:

Who was your thesis advisor then?

Meystre:

When I was doing experimental work, it was Piet Cornaz. Edmond Geneux was his right hand man. My experiment never worked. We had the wrong kind of laser, an Argon-ion laser. We would have needed a tunable laser, ideally a dye laser. Meanwhile, Antonio Quattropani was another young professor, a theorist, who had just come from Genoble. He was an ETH Zurich Ph.D. and had done a post-doc at Stanford. I became his first graduate student. He was a condensed matter physicist, and I wanted to do quantum optics, so he told me about Roy J. Glauber. Quattropani and I had done some calculations and Quattropani said, if Glauber thinks they are OK, we can continue. So I wrote to Glauber. He was still doing a lot of at high energy physics at the time and as it happened he was visiting CERN,, which is just 40 miles from Lausanne. And he suggested I come to see him at CERN. He was very polite and said the calculations were kind of interesting…. As I said, he was very polite… Glauber and I became good friends.

Bromberg:

What kind of world was looking into these atom field interactions at that point? For example, at one point you cite conversations with Cohen-Tannoudji. Were you going to Paris?

Meystre:

This is another very wonderful story. The world has such incredible people who have helped me over the years. The way I got to know Cohen- Tannoudji is because as you know I was studying in Switzerland, and at the time the Ecole Polytechnique in Lausanne was not a very big school. There were four French-speaking universities in Switzerland. There were two of them in Lausanne: the Polytechnique and the University of Lausanne, and there was the University of Geneva and the University of Neuchatel, and because they were all kind of sub-critical, all of them, they had this course, the Cours de Physique de Troisième Cycle Romand, or something like that, where we would collectively invite some famous speakers to come for summer schools and to give lectures. So for one of these summer schools when I was a graduate student, we invited Cohen-Tannoudji, who was a rising star in physics. He had invented the dressed atom concept; he had done all kinds of wonderful work already. I think he had just been elected or was going to become a member of the College of France. So he came and gave some lectures.

Bromberg:

Where were the lectures?

Meystre:

In a small village called Saint-Cergues between Lausanne and Geneva. It’s in the mountains in the Jura [?]. And because I was very interested in what he was doing, I asked a lot of questions, and he was curious and asked who is this guy who is asking all these questions? So my supervisor introduced me. And when the time came to defend my thesis, we had to have outside examiners, my supervisor asked Cohen- Tannoudji if he would be an outside examiner, and he agreed. And because he is such a gentleman, when I sent him a draft of my thesis, he invited me to go spend a few days with him in Paris to go through the whole thing, and he basically explained my thesis to me. [Laughs] That’s basically what he did. He was such a wonderful man, and he was so generous with his time and his ideas. So that’s how I got to know him, he was on my thesis committee. That’s really how it worked.

Bromberg:

With this Jaynes-Cummings Hamiltonian, of course as you remarked in this 2007 version of Elements of Quantum Optics, you say it’s a real irony that everybody used his Hamiltonian because Jaynes was trying to defend a semi-classical theory. I’m wondering whether when you were working on this same Hamiltonian, whether that question, this whole business of whether you needed quantum electrodynamics and whether renormalization made any sense, whether that was any kind of issue in Paris or in Switzerland.

Meystre:

Well, I know it was an issue for Prof. Cornaz, who had been my supervisor when I was doing experiments, because he never felt comfortable with quantum mechanics. Of course he accepted it and he used it, but he wanted to figure out whether we need to quantize the light field. And there were a lot of debates. There was Ed Jaynes, who of course was a great physicist, and he had a big bet with Lamb on whether he could compute the Lamb shift classically. Or was it with Peter Franken, I don’t remember.

Bromberg:

I don’t remember. I think Lamb might have been the arbiter, and Franken…but I would have to look that up.

Meystre:

But anyway, so it was really very much a big question, but it was in many ways over my head, too. I was, again, very naïve at the time. But I had a great time. I really had a great time trying to learn all that together with my supervisor. There are not too many supervisors who are willing to do that, to start learning a topic with you, so it was great.

Bromberg:

You came here next as a post-doc. I was sort of struck by the fact that Haroche did that, came over to the United States as a post-doc. And I think even Walther.

Meystre:

Yes, everybody.

Bromberg:

Was everybody from Europe coming to study lasers in the United States?

Meystre:

Basically, the rule was if you wanted a career in Europe, you had to spend time in the States. It was not a written rule or anything, but… And this has to do with the fact that in, when was it? I got my PhD in ’74. In Europe, physics had not really recovered from the war yet. It was well on its way to recovery, but German physics, which had kind of ruled the world of physics before Hitler, was still trying to recover. Well, at the end of the war the country did not exist anymore. The cities had been destroyed, and a whole generation slaughtered on the battlefields. France was also in shambles. All of Europe was in shambles. Switzerland not that much, but it is a small country, it doesn’t count very much. So after World War II, there was no physics left in Europe. And it takes a long time to rebuild. It really takes a very long time to rebuild physics, because you have to bring the teachers back, and you need at least one or two generations. All of physics had basically migrated to the United States, so it was kind of normal that’s where you would go and learn physics before you would go back to Europe. The famous Les Houches summer school was actually an attempt to bring knowledge to France without having to have all the students go to the States. That’s pretty much how it was started. So yeah, it was normal that you went to the States for a year or two or three. That was completely normal. It’s no longer so.

Bromberg:

Is there any particular reason you came here?

Meystre:

What happened is because I was so lucky — I’ve been so lucky all my life — I got my PhD extremely fast. So the question is what next? So I got a Swiss fellowship, which allowed me to spend a year wherever I wanted. It was a very nice fellowship. And I asked various people what I should do, and Roy Glauber told me to go and work for Marlan Scully. I would have liked to work with Roy Glauber, but at the time he had no funding to do quantum optics; a lot of his funding was back into high-energy physics. So he told me go and work for Scully. And that's kind of another big influence of Roy Glauber in my life, right, he sent me to Marlan. That was an interesting time, because it was my first time outside of Europe, first of all. And moving from Switzerland to Tucson, Arizona is kind of a nice cultural shock. And working with Scully was so different from working for my previous supervisor. It was a different world, which I did not know existed.

Bromberg:

How was it different?

Meystre:

Well, my previous supervisor, Quattropani, had a group of two students — I was his first student — and then he had another student, and we would spend an immense amount of time together. Hour after hour after hour going through calculations, going through ideas, discussing. He seemed to have an infinite amount of time for his students. And here you came to Scully, who was running an empire, and basically if you could get five minutes of his time, that’s all. So I learned you don’t work directly with Scully, really. You work on his ideas, of course, but the way it functioned in practice is that he actually had a whole group of brilliant younger professors basically working on the same ideas, and you worked with these younger professors.

Bromberg:

Who were some of the younger professors?

Meystre:

Murray Sargent.

Bromberg:

He was sort of under Scully?

Meystre:

Well, you are never under Scully, he is never forcing anybody to do anything, but de facto he was because Scully was bringing all the money. Murray Sargent was not as interested in bringing a lot of money, so they worked very closely. They wrote the Laser Physics book together: Sargent, Scully, and Lamb. Scully and Sargent were both students of Lamb, actually.

Bromberg:

And Lamb was here, actually.

Meystre:

Lamb had just arrived. Scully and Franken had brought Lamb to Arizona.

Bromberg:

And Franken was here too?

Meystre:

That’s right. He was the Director of the Optical Sciences at the time. And then another young professor who was absolutely brilliant was Fred Hopf, but unfortunately he died very young, he died when he was 40 or something like that. So I worked a lot with these guys, and with an army of students. It is a completely different way to do science — I had never seen that before.

Bromberg:

That’s so funny, because you think of quantum optics as little science, but now in certain organizations or in a social sense it’s a very big science.

Meystre:

Well, it’s still little science compared to high-energy physics or this kind of things. But Scully has always had very large groups; much larger than typical in quantum optics. He was basically running a group which had Willis Lamb, Murray Sargent, Fred Hopf, and then he had on a given day probably three or four post-docs and maybe about ten students, which is a huge group for quantum optics. It was a different way to do science. Another thing that was different is he was very, very on top of what the hot problems were, so it was no longer the nice academic insulation of Switzerland where you could spend three years working on something at your leisure. He was very sensitive to what was interesting, for instance, for the funding agents, what were the leading problems of the day, which was very different for me, because instead of spending my whole life on just one little problem — You know, for my whole thesis I spent all my time working on just the Jaynes-Cummings model, — then suddenly there was the problem of the week, just about. Very different style. I’m not saying that it was a worse style; just different. So it was a nice cultural shock.

Bromberg:

When you were working, for example, on the free-electron laser or optical bistability, were those things that Scully essentially set as problems?

Meystre:

Optical bi-stability came later. But when I was with Scully, the first problem I worked on was something related to x-ray lasers; he was very interested in x-ray lasers, and we did some work that he liked. Then we worked on the free-electron laser a great deal.

Bromberg:

Was that basically the sponsors would like a more efficient laser?

Meystre:

You know, I’m not quite sure why he was interested in this problem. Usually with Scully it’s a combination of intellectual curiosity and being able to be funded — it’s always both. I worked on something else with him, but I don’t remember now. It’s a long time ago. But I remember trying to tell him what I had done in Switzerland, and he had absolutely no interest in that stuff, which is when I realized that this work was so far, at the time, out of the radar screen of the leading groups.

Bromberg:

It’s curious, because it became so close into the radar screen.

Meystre:

That’s right, yeah, yeah. That’s very interesting. He was not interested. So I remember the first time that I saw Marlan Scully, I had just flown in from Switzerland. I went to his office, and the door was closed, he had not yet arrived, and there was a line of people waiting for him in front of his office. And he suddenly came in with his cowboy hat and all, and he saw me, and I was the new face in town, so he got right to me and he welcomed me. He spent probably 20 minutes talking to me. And then he gave me a paper. He underlined one sentence in the paper, and he told me, “I don’t really understand that. Why don’t you go look into it?” That was it. He also told me, “Oh, and you should go talk to Fred Hopf, but right now Fred Hopf is salmon fishing in Oregon. He will be back in two weeks.” Something like that. So that was my first introduction to Marlan. And it had to do with x-ray lasers, so I started to work on x-ray lasers with him. That worked out pretty well, actually. I remember once after working on the x-ray laser, he said, “Well, we had a good time working on the x-ray laser.” And then he said, “Let’s now work on free-electron lasers.” I know they were building a free-electron laser in Stanford, and there might have been some issues that the funding agents wanted to understand, I don’t really know what the problem was, but it turned out to be a really neat problem. We had a great deal of fun working on free-electron lasers.

Bromberg:

You told me there wasn’t any experimental work going on here; it was in Stanford that the experimental work was being done. [Right.] And that you had two approaches: they had the single electron approach, and you so had some sort of ensemble approach here.

Meystre:

Right. We also did the classical theory because Willis Lamb was always giving us a lot of grief, because Marlan loves quantum mechanics, and he started us all on quantum mechanics approaches to the problems. But Willis Lamb kept telling us, “This is a classical problem. It has to be solved classically. And if you don’t do it, I’ll do it.” [Laughs] So that was a challenge enough. It was really a great time, yes.

Bromberg:

Who funded you after you got through with your one-year fellowship?

Meystre:

Scully did. I was making a whopping $11,000 a year. [Laughs] But it seemed to be a lot of money, you know. I never had problems — it seemed to be enough.

Bromberg:

But then you are also working with Scully. You did this paper with him on the quantum eraser.

Meystre:

Oh, that’s later.

Bromberg:

Oh I see, so that was after you had gotten back to Europe.

Meystre:

To Germany, yes.

Bromberg:

The Bell work, did you do that later too?

Meystre:

Yes, that was later.

Bromberg:

So then where does the optical bistability fit in? Is that something you started here in Arizona?

Meystre:

No. So what happened is I then moved to Germany. Do you want to go there now?

Bromberg:

Should we say more about here before we go there? For example, did the computational facilities here have anything to do with anything? Did they have any especially good computational facilities here that were part of the picture at the Optical Sciences Center?

Meystre:

No, I think what really made the Optical Sciences Center special is this incredible collection of very smart people. We had computers (which by today’s standards would be laughable of course), but I never spent a great deal of time doing computer work because Fred Hopf was kind of a computer genius, and so was Murray Sargent. There was no need to do it because they loved to do it and they did it extremely well.

Bromberg:

So all those little diagrams? I think I’m thinking of later diagrams where you’re already working on micromasers.

Meystre:

Oh, this I did myself.

Bromberg:

Okay, so we’ll leave that. So that was what was attractive about Arizona.

Meystre:

It was a great place, yes. It was a fun place, actually. And Scully is just an amazing guy. He has ideas popping out of his head faster than you can shoot them down.

Bromberg:

Then let’s go to Germany.

Meystre:

I had to go back because my visa expired. I had a three-year visa, and then you have to go. That was kind of interesting, because there was no job in Switzerland. In Switzerland you have to wait until somebody dies before there is a new job, it’s such a small country. I didn’t really know the people in Europe because, first of all, I had just been three years in the States and I had never gone back home in three years. And also, when I was a student in Switzerland I was kind of learning the topic and I just met very few of these people — I met Cohen-Tannoudji. I met a couple of people, but I didn’t know the scene, as they say. So I had to find a job, and I wrote to a number of people pretty much at random. Again, luck came in. As I said, I’ve always been extremely lucky. I wrote to this character Herbert Walther, who I had never met before. The reason I wrote him is that I had read some of his papers and I liked them, and I said well… It was kind of a shotgun approach to getting a job.

Bromberg:

He was mostly doing laser spectroscopy at this point?

Meystre:

Right. He was working mostly on resonance fluorescence,. So I wrote him, and as luck would have it, he was just in the process of moving to Munich from Cologne, and he had just been offered this job as the director of what would become the Max Planck Institute for Quantum Optics later, so he was hiring anybody with a pulse. It was perfect. He offered me a one-year job, and I figured a one-year job is great because I have my feet back in Europe, and then I can probably take it from there. So that’s how I went to Munich.

Bromberg:

Eventually Scully would become very closely associated with that. He wasn’t that early?

Meystre:

No. I will tell you that story, too. So basically I arrived at the Max Planck Institute at the time it was called the “Projektgruppe für Laserforschung”. It had just been started for five years as a trial. And I was the theorist. So suddenly I went from a situation in Switzerland where I had been completely sheltered by a wonderful supervisor, to the big operation here in Tucson, to a place where it was just me. So I had to figure out what to do with my life, which was interesting. Basically Walther told me, “Do something interesting.”

Bromberg:

That’s it?

Meystre:

That was about the job. He was too busy building the place. He figured he could probably use a theorist, but he was building an institute from scratch.

Bromberg:

When he went there, was he already interested in building it into the Max Planck Institute of Quantum Optics?

Meystre:

That was the plan that if it would be successful, it would become that. That’s the way Max Planck starts a lot of institutes. It first gives them a five-year trial run, if you want. So that was the goal. So I started to do things, and I knew a couple of people in Europe. For instance, Rodolfo Bonifacio, who was professor in Milan, and the reason I knew him is that he had spent a sabbatical in Tucson. He was interested in optical bistability. He had actually started work on that with one of his students, L. A. Lugiato. They started that while I was still in Tucson; I was not working on that at all. But when I went to Germany, it seemed to me it was a problem where I could maybe work by myself because it was not that hard. It’s something where you don’t need an army of bright people to keep you going. So I worked a little bit on that, and I worked on this and that, and I didn’t really have a real good focus for a while. But Walther for some reason liked me. I remember one day I was in his office and we started to talk, and he was saying, “You know, I should really strengthen theory here. We should hire a couple more theorists. I’d like to hire a senior theorist to really build the place. Whom do you think we should hire?” I said that it would be great to have Scully. So he got on the phone this very minute and called Scully, and the deal was done. That’s how they made the deal, pretty much. Then there was some horse trading, a little bit. But then Scully agreed to spend the summers in Munich. So I brought Scully to Munich, in some sense.

Bromberg:

Who brought Eberly?

Meystre:

Eberly is different. Eberly was never in any kind of permanent fashion in Munich. He spent a sabbatical with one of these Humboldt senior fellowships. He was another person who had a big influence on my life, actually. So Scully started to come, and when Scully comes some place, immediately people appreciate the place exists. So suddenly you can bring all kinds of post-docs and students, things like that. He has this way of attracting people. So basically this place took off like crazy.

Bromberg:

That’s interesting, because I think of Walther as having been a great institution builder, but…

Meystre:

He is the same, yes. Everybody wants to work for Walther. If you look at his students and what they’ve become, these are just absolutely amazing people. I’m just in awe of the way they do things.

Bromberg:

Everybody wants to work with Walther because he’s got interesting ideas, like Scully?

Meystre:

I think it’s more than that. He’s got very interesting ideas, but he is also a mentor. A mentor is somebody who will not necessarily impose his ideas on you. They will recognize that you have a good idea, and they then let you go and run with it. They will push you. They will get you invited papers. They will help you get jobs. They really will help you a great deal with your career, these people. They are not at all selfish in this way. They are very generous with their time and with their ideas. Glauber is like that, too. So these people in a sense develop schools, right. The great famous schools of physics have all been around some exceptional people. If you look, Max Born had fantastic students, so he was probably somebody a bit like that. And there are people who never had students, like Feynman — fantastic physicist, but very, very few students. Wheeler had all kinds of extraordinary students.

Bromberg:

And he also looked after them very well, didn’t he.

Meystre:

Yes, that’s right. These people are very important.

Bromberg:

Was Walther already thinking of building a micromaser at this point, or was it people like you and Eberly who were working in this field who somehow…?

Meystre:

You know, unfortunately he’s not here to answer that question anymore, because it would be interesting. So how did it work? He was working a lot on Rydberg atoms, Rydberg spectroscopy with a number of students. Then there was the group of Serge Haroche, who was also doing things like that. I think that Serge Haroche (but again, I would have to check the facts) may have been the first to put Rydberg atoms in microwave cavities. But he put a lot of them, and he was studying things like super radiance and super fluorescence.

Bromberg:

But he’s also talking about making one-atom masers. [Right.] This is something I want to get your view on, because you’ve got these two groups that are working, trading ideas, but probably also competing.

Meystre:

[Chuckles] Oh yes, they were competing quite a bit. Mostly it was a friendly competition, but sometimes a little less so. I think that the history is that they both learned a lot on how to manipulate Rydberg atoms. Again, I hope I’m not wrong on that; I need to check. I think that Serge Haroche then started to put them in microwave cavities, but he had very large numbers of atoms. And I’m sure that Walther, who is a really clever guy, said, “Wouldn’t it be interesting to see how few you can have and still see interesting things? And so reduce the number of atoms.” A key point here is that he realized that he would need a very high quality microwave resonator, with mirrors that would have an exceedingly high relectivity at the microwave frequencies involved, and where the microwave photons would live a very long time, so as still be present when the next atom from very low density atomic beam would enter the cavity. Haroche’s mirrors were not of a sufficient quality to hope to achieve laser action with just a few or possibly just one atom at a time. Walther realized that a solution would be to use superconducting mirrors. He had contacts with a group developing superconducting cavities for accelerators, and the realization that such cavities might do the trick must have been a key element in his realization that it would be possible to build a maser where the field is build one atom, and one photon at a time. I’m sure that Haroche had similar ideas at about the same time, but I suspect that maybe didn’t have access to the superconducting resonators and required cryogenics at the time.. This is kind of how it evolved to the micromaser idea. But I don’t think it was somebody specific who woke up one morning and said, “Let’s build a micromaser.” I think it was an evolution and a combination of advances in the manipulation of Rydberg atoms, improved detectors, and the availability of superconducting resonators. At the same time, theory kind of converged on that, which is very interesting.

Bromberg:

It is, because these one-atom models that you were dealing with… I mean I sort of expected that Eberly would come over and say, “Look, I got these results. Let’s test them.”

Meystre:

No, it didn’t quite work like that. Actually it’s very interesting, because it kind of closed an interesting loop for me. Much of this started because at some point in Rochester, Eberly got back into the Jaynes-Cummings model. I’m not sure why he did it. He did it with a Mexican student, Javier Sanchez-Mondragon. They found some very strange things in the Jaynes-Cummings model. They found that you had these oscillations in the population inversion, and then things were very quiet, and then they came back. He called that revivals, and he got a lot fame and credit for that, as he should. The interesting thing is that I had that stuff in my thesis. But, my supervisor was not sure what it was, and I was not sure what it was. We kind of thought that it was the Poincaré recurrence. You know, when you have a finite size system, it always comes back on itself. So we thought it was an artifact of the system. Eberly, who is a lot smarter than us, realized it was something important. For a while when he was giving talks, he was always giving me grief because he knew that I had seen a little bit of these revivals, but we had never published it. I show a tiny little bit of them in my thesis. So he always gave me grief, saying jokingly that it was not serious enough for Swiss science, or something like that [laughs]. So this brought us back to the work that Walther was doing, because he had this one atom in a beautiful single-mode cavity, and it was basically the Jaynes-Cummings model with some complications. So suddenly this problem I had worked on in my thesis and that I had totally ignored because Scully thought it was not important came back to the surface. By that time, I had learned a lot of laser physics from Scully, so it was not very hard to put it all together to do a good description of the micromaser. Plus, I had two fantastic post-docs working with me by then.

Bromberg:

Were they Javanainen and Filipowicz? [Yes.] And they were post-docs at the Max Planck…?

Meystre:

Yes, because by then I had gained a little bit of seniority — I was no longer the youngest, greenest guy. Walther, who I said liked me, let me hire post-docs to work with me. So it was great. And he had given me a permanent position by then. I was a happy camper!

Bromberg:

But Scully and Lugiato and Walther were doing something a little bit different at that time?

Meystre:

Yes, they did afterwards. At the time I was a little bit annoyed, because I was young and I was more easily getting annoyed than I am now, I guess. So the way I saw it is that we did this beautiful theory with Filipowicz and Javanainen, and I still think it is a very beautiful theory. We were very proud of it, and we got it published and we got some good credit for it. And then Walther and Lugiato and Scully came with this other theory, and I thought at the time, which was completely wrong but I was too stupid to see that, but I thought they wanted to steal the sunshine from me.

Bromberg:

Oh? How could you think that?

Meystre:

Because I thought we did this paper with three unknowns, and here come these three big shots and they are going to be remembered as the people who did it. Of course it was really not at all like that. They just wanted to understand it from another angle. But I misunderstand things some times, and I remember being a bit annoyed. And when they asked me if I wanted my name on the paper, I said no, I don’t want anything to do with this paper. [Laughs]

Bromberg:

Then they wrote a paper saying they were just laser theories?

Meystre:

Right, actually not quite: they did point out the fundamental differences with conventional lasers, just as we did with Javanainen and Filipowicz. But their theory looked much more like a usual Scully-Lamb laser theory. Anyway, after that we were still very good friends. It was just that I was annoyed.

Bromberg:

Now I’ll go back to these questions I was asking you. So you write a paper with Scully on the quantum eraser because people are objecting to it, but this was that just sort of a little extra thing, it wasn’t very central?

Meystre:

Right. It was never central to what I did, but it was very central to what Scully did. He’s always liked these things. Scully is this kind of guy. As I said, he is a very inspiring fellow, so he would come to Max Planck for the summer and he would get everybody all excited about his latest problem. Some of these papers did not take a whole lot of time at all to write.

Bromberg:

The same thing on this business on Bell’s work and these negative probabilities?

Meystre:

This is something that has bothered me, these Bell inequalities. I’ve never made a profession out of that, but I’ve always tried to figure out what went wrong, and so I wrote a little bit on negative probabilities with Marlan. It’s a way out, but what does it mean? Nobody knows. Then I also wrote a paper with somebody named Asim Barut. But I never found anything that made me feel happy about it. So that’s just the way quantum mechanics is, I guess. And I had long discussions with Alain Aspect, who is another good friend, and he is also to some extent uncomfortable with quantum mechanics, but we all are, and that’s just the way it is. So it was kind of a side theme, and I’m not spending too much time thinking about that. I try not to [chuckles].

Bromberg:

The whole community, it seemed to me, got very interested at that point in these foundational issues, and then did the whole community just go back to calculating after that?

Meystre:

Yes, so basically what happened is that we pretty much all convinced ourselves, well, that’s the way it is. But the interesting thing is that while many of us kind of walked away from this, there is a whole group of very brilliant physicists who have gone one step further and said, “Well, can we use that to do things? Can we use these very weird properties of quantum mechanics?” And that’s led to this whole business of quantum information. So in a sense, it was very important to look at these problems, to understand that yes, that’s really the way it is, there are these bizarre effects, this entanglement. And all of the work on quantum information is based on that. I just happened not to work on that. One day I decided I’m not going to touch quantum information theory, and I wonder if it was the right or the wrong decision sometimes. I decided my group was too small to do everything I wanted to do, and I stayed away from it.

Bromberg:

Just because you’re group was too small? [Yes] Not because the National Security Agency and all these military…?

Meystre:

No. I actually even ran a small workshop here in Tucson a number of years ago on quantum information, and I walked out of this workshop saying, “I’m not going to get there.” It was a strategic decision.

Bromberg:

That’s interesting, because you’re going in another direction that is very philosophical; it seems to me, in the paper with Filipowicz and Javanainen. You go into these trapping states, and these macroscopic super positions, and whether in quantum mechanics you have macroscopic superposition. That seems also very fundamental stuff.

Meystre:

Yes it is, and it’s not — Okay, so by the time I made this decision, I was back in the States, or I was moving back — I don't know, I don’t remember exactly. But at this time I had become very, very interested in laser cooling, and I figured I was going to go full speed in laser cooling and stuff like that. And I typically don’t have a very big group, so you have to decide; you can’t do it all. So that was kind of the decision. And you never know if they are right or they are wrong, but it’s a strategic decision, right.

Bromberg:

I think that’s perfectly understandable. And I’m not back in the United States yet, because it seems to me you were still in Munich when you get very interested in the mechanical effects of radiation.

Meystre:

It’s laser cooling. Radiation pressure, and then laser cooling. And I’m back into that now, closing another circle.

Bromberg:

So you’re getting interested in that already in Max Planck. [That’s right.] And you’re doing this special issue with Stenholm on that.

Meystre:

Yes, that’s right, and I’m very proud of that, because we were ahead of the curve.

Bromberg:

Tell me how you got interested in that so early.

Meystre:

What happened is that, again, I had this incredible position at Max Planck, where I could do whatever I damn pleased. I didn’t have to write a proposal, I could hire post-docs; I could have incredible visitors from all over the world. It was the best. One of the things we were working on was optical bistability, and optical bistability has to do with the fact that if you use the right arrangement, you can have two values of the output in the light field for one value of the input, depending on the history of the system. I don’t remember how I got it, but one day I got the crazy idea that you don’t need to have a nonlinear medium to do that; all you needed is to use radiation pressure on a moving mirror. The calculation was about five lines, and I was really proud of this calculation and I thought it was really cute, because it was a minimalist way to do optical bistability. Maybe not very practical, but I thought it was very elegant. I went to see Herbert Walther and I said, “You know, I have this great idea!” And he liked it too, and he said, “This is really cool. We should do the experiment.” So he “sacrificed” one of his students. It turned out the experiment was very hard to do, but the student succeeded eventually and this worked. This really got me into appreciating the beauty of radiation pressure and changing the motion of massive particles with light.

Bromberg:

This was in a laser cavity?

Meystre:

Right. One of the mirrors was moving, was allowed to move.

Bromberg:

So that was really an important little step in there.

Meystre:

Yes, I really liked that. So we did the experiment, and this got me into this business of radiation pressure. And if you go into the business of radiation pressure, pretty soon you read the old articles by Ashkin, and then you get into laser cooling. Stig Stenholm had been working on laser cooling for several years before that, he was working a lot with the Russians: Finland was in this very privileged situation when it came to such interactions because this was still the Cold War, and it was hard for the Russians to go to Europe or to the West, but they could go to neutral Finland somehow. And I knew Stig Stenholm because he had also been hanging around with Scully, and he had been a post-doc with Lamb, and he used to spend time at Max Planck, and Javanainen had been his student and now he was my post-doc. So there was kind of a lot of inbreeding there [chuckles]. So one day we decided, you know this stuff of laser cooling and radiation pressure, this is going to be hot, so let’s put together this special issue. This special issue could not have been timed more perfectly because it came just before laser cooling became important in the lab and people figured out how to go below the recoil limit, and do all these marvelous things that led to the Nobel Prize of Bill Phillips and Cohen-Tannoudji and Steve Chu, and then to Bose condensation. So it was perfect timing. This little special issue was wonderful this way.

Bromberg:

But now the Russians were doing all this work in atom optics.

Meystre:

Right. Letokhov and Kasentsev and their collaborators.

Bromberg:

I guess I’m a little confused about these instruments they were setting up, mirrors and interferometers and stuff like that for atom optics. Do you need to cool things down for that?

Meystre:

No, you don’t really need to cool things down. You need atomic beams to be very monochromatic to do atom optics. But it’s very much like in optics where you can do interferometry with filtered light — you don’t need a laser to do interferometry, you can take a regular discharge lamp and you can filter it to death, and then you can do interferometry. Which is probably how you did and how I did interferometry in physics labs, right? But having a laser helps a great deal to do interferometry, and having ultra-cold atoms for the same reason helps a great deal to do atom interferometry. Perfect analogy.

Bromberg:

I wanted to ask a little bit more on the theoretical work on the micromaser, because I don't know how important trapping states turned out to be and what happened to them, whether they were exhibited experimentally as well as theoretically?

Meystre:

Yes, they were demonstrated experimentally in Herbert Walther’s group, and maybe also in Haroche’s group. I’m not positive on that. So they have been seen, and I think that they are — okay, well, definitely important [chuckles]. They are certainly not important like the Lamb shift and they are certainly not important like a lot of things in physics, but they are very cute. The reason is that they are a very beautiful demonstration of the quantized nature of light. And okay we know that light is quantized by now, but still... So I can tell you a little bit of the history of these trapping states, because it is kind of amusing. I was back in the States when we figured that out, but I was spending the summer in Munich, which I like to do. It’s hot here in the summer, but its okay, you get used to it. So I was in Munich, and in Munich they had this bright young student named Gerhard Rempe, and he was doing experiments on the micromaser. He had put together a program to do some simulations of his experiment, and he was getting all this weird stuff. The curves didn’t look like in our paper with Javanainen and Filipowicz; they just didn’t look the same. You know, I don’t like it when my theory is being questioned! [Laughs] So I started to talk to him, and I looked at his code, and I realized that what we had done in our calculations always had a finite temperature because we were working at I think 2 degrees or something like that, and he was working at 0 Kelvin — he had no finite temperature effects. But his code was correct. So we spent about a week trying to figure out what was going on. I took my old codes, and I saw that if I decreased the temperature, you started to see this kind of structure that he was seeing in his code. It took me maybe a day to figure out what this structure was, and it worked perfectly. It was beautiful. Rempe is really a smart guy and fun to work with, and we did more runs to make sure that we had the correct story, and we basically wrote a paper within a week. The funny thing is that Herbert Walther was reluctant to put his name on the paper. He said, “No, no, I’ve not done anything. I don’t want my name on the paper.” I said, “But you should have your name on the paper because we think it is a nice paper and it would not have happened without you.” So in the end he did put his name on the paper, and then he was so proud of it afterwards, so it was great.

Bromberg:

I never really understand, when you see a person’s name on a paper, like Walther when he is the head of the institute, does he usually go on every paper because it’s his institute…?

Meystre:

No. Some people do that, but he was not like that. He had plenty of papers; he didn’t need more. He didn’t need any of that. I’ve never had a fight with anybody about whose name should be on a paper in my whole life. But I thought he should be on the paper because he was the intellectual leader behind all that, and without him this would not have happened. He was a very, very dear friend, and I thought it was kind of nice to have papers with him. And Rempe, by the way, is a Max Planck director now.

Bromberg:

He had curves at 0 degrees, but he was doing experiments?

Meystre:

No, he was doing experiments at 2 degrees, so there was no way he could see that. In this sense his program was too simple. But it’s just because he is an experimentalist, and he only wanted to do some quick and dirty stuff to match his experiments. Then to actually see the trapping states was very hard because they had to go to these very low temperatures.

Bromberg:

I see, so it was done pretty recently, I would guess. [Right.] But now the trapping states lead you to macroscopic super positions, doesn’t it? [Right.] And then there is something going on because you have one scheme for macroscopic superposition, and now Haroche has another scheme. Was that an interesting thing? He was doing non-resonant and you were doing resonant.

Meystre:

Yes. I think that in the end (and I hate to say that because I don’t want to say anything bad about Herbert Walther, who was a dear friend), but I think that Haroche recent experiments, which he has just published in the last year or two, are so beautiful that I wish Walther had done them, basically. I think that going non-resonant was a smart move.

Bromberg:

Are they in the Physical Review?

Meystre:

Yes, and in Nature. They are just this last year or so. They are absolutely fantastic experiments. Haroche is probably going to get a Nobel Prize for that stuff at some point, I’m sure. Just amazing. In a sense, it was again a strategic decision — you have to do that all the time in physics — am I going to work resonant or non-resonant? And there are advantages to both. The micromaser work calls for resonant interactions, but if you want to look at things like macroscopic super positions, there are considerable advantage in going non-resonant. Of course Walther is unfortunately no longer around to compete and to argue this point. It would be interesting to see how he would compete now. What Haroche has done this last year or two is just simply gorgeous. I’ve always found this competition to be great because they kind of pushed each other to get better and better. And mostly it was a friendly competition.

Bromberg:

I’m always telling historians of science it would be really great to go to Europe and study those two groups, because the experimental apparatus seems to be to have been rather different. [That’s right.] And the traditions might have been different. I want somebody to do that bit of history.

Meystre:

That would be interesting, I agree.

Bromberg:

So you get this proposal for creating Fock states from the trapping states. Did that ever get done experimentally?

Meystre:

Yes, but it’s never been that clean. And I think that the reason that it’s never been that clean in the Walther group is basically because he became very sick and the whole program slowed down. Then he retired — they made him retire. So he had a very small group after he turned 67 or so.

Bromberg:

So a lot of what you do theoretically gets experimentally investigated at Garching.

Meystre:

Right. Was, actually. Now it’s different because I’ve changed direction.

Bromberg:

Where now?

Meystre:

I will actually spend six weeks in Garching this summer, but last year I spent a month with Alain Aspect, and now I’m back to this moving mirrors business, which is kind of strange. You know, you can never bury your old crimes [laughs]. Actually there is a group in Garching doing beautiful optomechanics experiments, but it’s not associated with Walther; it’s associated with Hänsch. The main force in the group is Tobias Kippenberg, who is in the process of moving his group to Lausanne, by some strange coincidence. And there is a group at Yale, and I have a collaboration with Keith Schwab at Caltech. So things have changed a little bit.

Bromberg:

There are a couple of things I really don’t understand that I wanted to ask you about. Several of your papers are dealing with whether or not you should use a semi-classical approach or a quantum electrodynamics approach, and there are two things I want to understand about that. Is this just a matter of what’s the best way to calculate something?

Meystre:

It’s both. Clearly if you can get away with doing a classical theory, it tends to be simpler. Simpler at least if you have to go to a computer, because quantum mechanics blows up in your face on a computer. The Hilbert space becomes so big so fast that you run out of memory. But this is just a technical point in many ways, but at the same time a fundamental limitation. What interested me for many years, and I think interests all physicists, is the interface between the quantum world and the classical world. We don’t really understand that interface very well still. And again, this is not things that I did then but things that I do now. That’s why I went back to these moving mirrors, and it’s an attempt at putting the things I’ve done over the years together. People have now demonstrated that you can cool vibrating mirrors or cantilevers agonizingly close to the quantum regime, and these are microscopic systems — they weight milligrams or even grams. And there is little doubt that we will be able to cool them to the point where they behave truly quantum mechanically. So here you are. You have a system that everybody would say, “That’s a mirror. That’s a big chunk of glass.” And we try to push the limit. I’m not sure we’ve learned very much at that point, except that we can make bigger and bigger systems behave quantum mechanically. And I don't know if there is something profound to learn, frankly. But it’s always been kind of in the back of my mind to try to understand if there is something profound in this transition, or if it’s just because of noise and all these other things that make systems classical.

Bromberg:

And you don’t just solve it by saying decoherence?

Meystre:

That’s kind of the party line, and that’s what we all tell our students. I don't know, there is this wonderful lady, Nergis Mavalvala, who is a professor at MIT and who is cooling LIGO mirrors, mirrors that weigh a ton, to a point where they are almost behaving quantum mechanically. Are there going to be surprises there? I don't know. But I think we owe it to ourselves to check. And even if there are no real surprises, this work should lead to novel detectors of feeble forces and fields of extraordinary sensitivity, so you can’t lose, really. This question is kind of always in the back of my mind. But I don’t lose any sleep over it, let’s put it that way. It has to do with quantum measurement and all these things, because at some point things become classical. Of course the party line is it’s decoherence, coupling to the environment, and things like that. And it works in practice. From an operational point of view, it certainly is true that if you put decoherence into your systems, they start to behave classically, there is no question.

Bromberg:

Something you just mentioned that I want to ask you about, when you go down to single systems like micromasers, and suddenly you are getting calculations that say that whether you measure it in the upper state or lower state is going to change the dynamics of the cavity field you left behind.

Meystre:

Isn’t it cool!

Bromberg:

But what does it mean? Well, where did that take you — let me ask you that.

Meystre:

The real question is where did it not take you. I did this work some time back, and that was again a very interesting thing. Now I remember. I was doing some simulations on my computer. That’s when I still had time to do things myself, which I don’t anymore. I was doing this crazy thing. I was just doing some very simple simulations. I would take one of these micromaser cavities and I would put an excited atom inside the cavity, and then I would simulate a measurement on the computer and say find the atom that came out again in the upper state, let’s say. And then I would look at the state of the field after the measurement, and I got some very strange results. For instance, I would compute the mean number of photons in the field, and let’s say the mean number of photons was 2. Then I would put an atom that did nothing, clearly, because it came in the upper state, got out in the upper state. And after the atom had gone out, the mean photon number would be let’s say 10. I spent weeks looking for a mistake in the dumb program, and I couldn’t find the mistake. Then I realized the mistake was in my brain. I was just thinking about it completely wrong, and it is actually completely trivial. I can give you a classical example that is almost the same. Suppose you talk to a graduate student on a Friday in the morning, and let’s say that graduate student makes $500 a week, and he gets his paycheck on Fridays. Now you don’t know if he has cashed his check or not. So what is your best bet for how much money he has? Well, it’s ½ of 0 + ½ of $500, so it’s $250. That’s the best guess that you have. You ask him, “Give me a dollar,” and he says, “No I can’t, I’m broke.” Then suddenly his mean number of dollars has gone to zero. Or if he gives you a dollar, then his expected number of dollars went to $499. That’s in this sense in a way that the measurement changes what you know about a system, and what you know about a system is all that quantum mechanics is about, really.

Bromberg:

So the conclusion is that all quantum mechanics is information.

Meystre:

Yes, that’s what I think. My understanding of the wave function is that it’s your best bet about the state of the system, and you change your best bet with each measurement. So I understood that, and I understood that my computer program was actually telling me something that I had failed to understand. I wrote a little paper, and I left it at that, which is the story of my life. And then a couple of weeks or months or maybe a year later, there was this beautiful work coming out by Klaus Moelmer and others in the Cohen-Tannoudji group, as well as a couple of other groups using quantum Monte Carlo simulations to do quantum trajectories. And I was that far from having done that.

Bromberg:

That’s inevitable when you do physics that you get situations like that.

Meystre:

I was so annoyed with myself! I should have finished this problem. But I was so happy to have understood that what my brain was telling me was wrong I left it at that, and I should have continued. I should have persevered. But I never did so, oh well.

Bromberg:

Does that have anything to do with it being a single atom? I even brought these few pages with me because I was so puzzled, because it says to describe the dynamics of single quantum systems properly. Does that have something to do with the fact that the field is a single mode?

Meystre:

Yes. You see, if you go back to the history of quantum mechanics and you read the founders of the field, one of the founders is of course Schrödinger, and he said that it was insane to even think of a single system. He had some words about that. You had to always think of an ensemble. And all you do in quantum mechanics is actually think about quantum averages, expectation values for large numbers of systems. So I always wonder what Schrödinger would have said when Peter Toschek first and then many other people managed to trap a single ion or a single atom or a single photon. And you look at this single photon. You don’t have an ensemble average. You don’t repeat the experiment over and over and over again. You just have the single atom and the single mode of the light field and you keep looking at it, and the question is what can quantum mechanics tell you about that. So you have to kind of rethink a little bit. And its okay, it works. So that’s kind of the spirit of this sentence, that you really have to see that once you have your single mode of the field, and you want to send atoms one after the other to look at the state of this field. The state of the field is your best bet about what you know about the field, and each time you put an atom, you change your knowledge, basically. That’s what I’m trying to say in somewhat clumsy words. And again, I wish I had done more on that, but I tend to be a bit intellectually lazy sometimes. Once I’ve understood it for myself, I kind of quit before finishing.