Wolfgang Schleich

Zierler:

Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is June 10th, 2021. I am delighted to be here with Dr. Wolfgang P. Schleich. Wolfgang, it’s great to see you. Thank you for joining me. 

Schleich:

Well, thank you for inviting me to come and talk with you about that. 

Zierler:

We both have Marlan Scully to thank for this. 

Schleich:

Well, indeed, I’m always grateful to him. He was my PhD supervisor, my mentor. He’s one of my big idols. 

Zierler:

Wolfgang, to start, please tell me your current title and institutional affiliation. 

Schleich:

I have a chair of theoretical physics at Ulm University. I’ve had this chair for exactly 30 years now. But I’m also the founding director of a new Institute for Quantum Technologies with the German Aerospace Center. The abbreviation is DLR, Deutsche Luft und Raumfahrt. 

Zierler:

What is the connection between the German Aerospace Center and the Institute for Quantum Technologies? Why would those two be connected?

Schleich:

Oh, that’s a long story. This institute was just recently founded. No, I shouldn’t say founded. It was recently opened with a big ceremony, but for that I would really have to go back 16 years ago — more than 16 years ago. At that time, two gentlemen came and approached me, and the two gentlemen were Hansjörg Dittus and Ernst Maria Rasel. One of them, Hansjörg, was at that time the director of an institute of the DLR which was also operating the drop-tower in Bremen.

So, I’m not sure if you know what the drop-tower is. That’s a building, it’s a tower about 160 meters tall, and it has a pipe in the center of the tower that you can evacuate. And then, you can drop objects from the top all the way to the bottom and see how they fall. Now, why would that be interesting? Well, there’s this old question of, is a feather falling with the same rate as lead? So, the equivalence principle. And here, you can study such experiments in microgravity. It’s called microgravity because now these objects are falling without any gravity. In this frame that falls with the objects, there is no gravity anymore, at least for 4.5 seconds. Then the objects hit the ground. Now, in order to do real experiments in this environment, people have created a capsule that first hangs at the top of the tower, then you let it go, it accelerates, the capsule falls down, and then there is a 6-meter-tall pot of Styrofoam. This capsule falls straight into this Styrofoam, and it is very impressive. You see the Styrofoam fly into the air, but within a fraction of a second, this capsule is stopped. It’s really fascinating. I couldn’t believe — when I visited this place for the first time, I had the impression that some complicated mechanism will stop the capsule. I was thinking of an electromagnetic brake or something. And then they said, “No, it’s just Styrofoam.” I said, “How can you do this with Styrofoam?” And they had a bucket of Styrofoam next to this vacuum chamber, and the guy said to me, “Just put your hand into it. Try to put your hand into the Styrofoam.” So, I put it in, and it was like going into water, at least for the first 10 cm. But as the hand went further in, you could feel that you needed more and more force to go on, and then eventually there was no way you could go on anymore. So, the friction of this Styrofoam was really impressive. It stops this capsule in a fraction of a second. They’re 50 g when you decelerate the capsule. So, 50 times the Earth’s gravitational constant, 50 times as strong.

So, anyways, what happened was that this gentleman who developed, as a postdoc, and created this tower, Hansjörg Dittus, he was the man who in his first postdoc position was asked to construct and design — sorry, let me step back — design, construct, and build this tower. That man came to me and said, “What do you think? Would it be interesting to create a Bose-Einstein condensate in free fall?” Because we’re talking 16 years ago. You know, that’s like 2004, 2003, around this time. By that time, the Nobel Prize had just been awarded to Ketterle and Cornell and Wieman, the three of them, for creating a Bose-Einstein condensate (BEC), which was built around ‘95, ‘96. So, now, ten years later, BECs were standard. You could go anywhere and there were tons of labs that were doing experiments with BEC. However, of course, atoms feel gravity. You can do interferometers with light and gravity wouldn’t play a major role, unfortunately. We would love to see influences of gravity on light, of course. And we will come back when we talk about my thesis and my PhD probably. But now, atoms — and that was a development that also started in the ‘90s, that people were building atom interferometers rather than light interferometers. So, they were using the wave nature of atoms to build interferometers.

And this gentleman, Ernst Maria Rasel, that I’ve mentioned, he did in his PhD with Anton Zeilinger, the first atom interferometer using light beams as beam splitters. So, when you have an interferometer, you needed a beam splitter to split up the beam. You need mirrors to recombine the beams, then you need another beam splitter to mix the two paths. So, you need beam splitters and mirrors, and Ernst in his PhD with Zeilinger did the first beam splitter and the first mirrors with electromagnetic waves, with lasers. So, he had a big history in interferometry and cold atom physics. He had also done a postdoc position with Claude Cohen-Tannoudji at the École Normale Supérieure in Paris. So, these are the two gentlemen. One guy, Hansjörg Dittus who has the tower, Ernst Maria Rasel who has all this experience in atom interferometers and creating Bose-Einstein condensates. These two gentlemen came to me and said, “Why don’t we combine this? Do you think that would be interesting?” And I said, “Yeah, that would be interesting, for sure, because there are many things that we could test in gravity using atom interferometry.” There’s a crucial problem. The crucial problem is — or the advantages if you want — if you have an interferometer, and you send an atom into it, then the phase shift that you measure grows quadratically with the time that the atom spends in the interferometer. So, the slower the atom as it goes through, the larger the signal. So, of course, what you want to do is to have an atom that hardly ever moves. Then it will take a long time for the atom to go through, and it will increase the signal.

Now, if you would do this experiment on the Earth, you remember when you have a water hose in the summer, we go out in the garden, we water the plants, you have a beam of water coming out of the water hose, and of course, that’s curved because of gravity. It bends down. At the beginning, for a long time, the beam is straight, but then it feels gravity more and it goes down. And of course, the longer it can go straight depends on the velocity. So, the faster the water stream comes, the longer it will go almost straight. Then it goes down. So, you can imagine, if you want to build an atom interferometer with low velocities, the atoms will feel gravity and they will not go through your interferometer, they will just pop down. They will just fall down. So, in order to build an interferometer with very long times, or with very low velocities, you need microgravity so that there wouldn’t be any gravity around. And that was the motivation of these experiments. So, now, we had this drop-tower. We had Ernst Maria Rasel, a very gifted experimentalist, and you have my group which would provide the theoretical background for these experiments. So, now the only thing we needed was money. Amazingly enough, the proposal to start a collaboration on this was sent — it wasn’t amazing. Let me start this sentence differently. Well, we decided we would send a proposal to the German Science Foundation, the DFG, Deutsche Forschungsgemeinschaft. Or I think it’s called in English, the German Research Council. And amazingly enough, the proposal was rejected. And the answer was, such experiments cannot be done in Germany. They can only be done at the Jet Propulsion Laboratory in Pasadena. So, the proposal was rejected. 

Zierler:

Scientifically, or politically? What were they saying?

Schleich:

Well, I guess — I can’t remember because it’s too long ago, and I was too pissed. But I remember distinctly the sentence that it can only be done at JPL.

Zierler:

In other words, they had instrumentation at JPL that was necessary for this. 

Schleich:

No, no, no. I later found out who was the referee. He was a theorist. He had absolutely no idea. But one thing one can say is that one has to understand at the time experiments of this type required a big laser table. Experiments to create a Bose-Einstein condensate at that time required very sophisticated beam equipment. So, you needed complicated laser arrangements: lots of mirrors, vacuum pumps. It was just amazing, when you look at these experiments by Ketterle at that time — there’s a famous picture where after he got the Nobel Prize, they showed this experiment. Now, you can imagine, if you want to put such a table into a capsule, which is, by the way, about 50 cm wide and about 2 meters 50 tall. Somebody comes and says, “Look, we want to do this experiment, and we’re going to put all of this equipment into a capsule of this size.” I can understand why some people would say you can’t do it. But that’s exactly what I call “vision.” We had this vision that we can do this. We have to miniaturize everything. You have to miniaturize vacuum pumps; you have to miniaturize batteries; you have to miniaturize the whole experiment. All the lasers have to be redeveloped. You need lasers that are small now because you want to do these experiments. And now, you start hearing why this is relevant for space, because if you want to do something in space, you cannot send a laser table to space. It has to be small.

So, now, the question was, how can we do that? And then, my friend Hansjörg Dittus said, “Let’s see if we can get some money from the German Aerospace Center.” They have an agency that gives grants, so we started talking to them and we got a little grant, not very much, to really start working on this. Over the years, we got bigger grants, the collaboration got bigger, we hired experts on lasers. It was a miniature version of the Apollo, or Sputnik, or something like this. So, we got laser physicists. We got engineers. From different branches of science, we got people together. Computer scientists who would write programs for this. So, the collaboration in the end indeed got the apparatus that was usually on a table, we got it into this capsule, 2 meter 50, 80 cm wide — don’t ask me about the exact measures of this device. One crucial thing, I have to say in this context, was the work of two gentlemen, Jakob Reichel and Theodor Hänsch. Theodor Hänsch got the Nobel Prize later. I think it was — now, wait a minute. When did he get the Nobel Prize? I can’t remember, ‘15, or ‘05. I can’t remember now. Anyways, it doesn’t matter. Anyways, Theodor Hänsch got, of course, the Nobel Prize for the frequency comb, but one thing that really was decisive for us was an atom chip. So, he had a chip with wires on it, and then you send a current through the wire. That creates a magnetic field, and then you can trap atoms with a magnetic dipole in this magnetic field. And of course, this was a completely different trap than the old traps where you had static magnetic fields or complicated arrangements. And with this atom chip, you could suddenly miniaturize traps. So, this thing of Jakob Reichel and Ted Hänsch was absolutely essential to build such a device. And as I said, it was, I think, 2006 or 2007 that we saw the first Bose-Einstein condensate in microgravity. It was then published in Science in 2010. It took us a long time to write the paper and do everything. So, in 2010, this collaboration of many people — as I said, I emphasize again, I’m just a theorist. But as I said, I was from the very beginning involved in this, and pushed it, and said, “Yeah, we should do this.” As I said, we did not get discouraged by the German Science Foundation turning us down. Anyways, we got it done.

And also, I have to say that we got supported by the DLR agency. There was a gentleman by the name of Rainer Kuhl, a funding agent, who had the vision to say, “Yes, I support you guys.” And not only this, one day he came — and you know, we wanted to continue and study all kinds of things in the drop-tower. There was one disadvantage of this whole thing. You had one drop, and it took 4.5 seconds, and you have to appreciate, in this particular experiment, what happened was that you had to create the Bose-Einstein condensate while the whole thing was falling. And then, while it was falling, you had to do the measurement. So, you had about a second or two after the preparation to make the experiment, and then make the measurement, and then it hits the floor. 

Zierler:

Did you or your colleagues have to invent new instrumentation to make this measurement?

Schleich:

Absolutely. Everything had to be miniaturized because you had to get it from the laser table down into this particular arrangement. So, now, we had seen these things, and we were happy. We were writing papers and doing things, and then this gentleman, Rainer Kuhl, our funding agent came — and I still remember. We were in Berlin having lunch in a restaurant close to the Gendarmenmarkt, and the gentleman in the middle of the lunch says, “Okay, now we’re going to put this on a rocket.” And my friend Rasel almost fainted. He said, “Look, we have enough troubles getting this thing going.” You see, the problem was that when you do this experiment, you have maybe one or two seconds to do the experiment. So, you have microgravity that you can use for maybe two seconds. And then, the experiment is done. Now, you have to pull up the apparatus out of the Styrofoam, bring it all the way to the top, and repeat the experiment. But you cannot repeat the experiment more than three times because they have to take the vacuum out again, they have to redo the vacuum. So, you cannot do more than three or four experiments a day. And some other people also want to do experiments. So, this is why this gentleman, Rainer Kuhl, said, “Look, we need to have more time to do the experiment. So, why don’t we take a rocket and shoot it into space? Then, we can do experiments for six minutes.” Six minutes. That’s more than four seconds, or one second or two seconds. And of course, Ernst Maria Rasel said, “Oh my God, I can’t do this. We have temperature changes and all this.” And I said, “Sure, we’re going to do it. If you give us the money, we’ll do it.”

So, this gentleman got the money, and now we started working on the rocket. And then, in 2017, the rocket flew. The project is called the MAIUS project. Again, it is a big collaboration of engineers, of physicists, computer scientists. Because now, in six minutes, if something goes wrong, you cannot reprogram everything. So, everything had to be done automatic. The rocket starts, the program starts running, and it does certain experiments. So, in the end, this device on this rocket did about 85 different experiments. And it flew up to 260 km, and once you do 100 km, if you once pass a line called the Kármán, after Theodore von Kármán — he was a big guy in hydrodynamics and aerodynamics. Anyways, once you pass 100 km, you’re in space. That’s the definition of space. So, we published a paper in Nature, in 2019 probably, where we reported the first Bose-Einstein condensate in space. Okay? In space. And now, we just published the first atom interferometry in space. That was also done on that particular flight. So, we’ve only sent one rocket.

And parallel to this, in America — see, the Americans had a little bit of an advantage compared to us in Germany because, I think, at least in my opinion, a long time before we worried about questions like this, I remember distinctly that there was a workshop in America about putting a Bose-Einstein condensate onto the Space Station. But then, I can’t remember which president, stopped this, and the money was cut. I can’t remember which president it was. But during the time of Steven Chu, when he was Secretary of Energy, he allocated money to restart that program again. And since I have these connections in America, I immediately told the people in America, hey — and by that time we had done already all the experiments in the drop-tower, and we were working on the rocket experiment. So, I told my friends in America, “Look, you guys have to involve the Germans. They have all this experience by now on microgravity.” And the Americans said, “But we can’t give you money.” I said, “The money is not the problem. We can get money in Germany, but we want to be part of this project and eventually do experiments with your Bose-Einstein condensate on the Space Station.” Indeed, this project called CAL, Cold Atom Laboratory — by the way, built by JPL in Pasadena — but many years after we had done the experiments. So, they built this CAL device, and since — can’t remember, 2019 maybe, 2020 — CAL is up and operating, and we are part of this project. So, Ernst Rasel in Hanover and myself, we have groups proposing experiments and working with our colleagues at JPL on CAL on the Space Station.

Okay, so — new paragraph, as they say. You asked me, how did I get involved with the DLR? Now you understand that we started out in microgravity, we had to miniaturize all these devices, and eventually we’re in space. And of course, atom interferometry is a crucial ingredient, for example, to create navigation systems. Now you can see why we eventually got into this business. So, now, here’s the last part of this story. During all this work on the drop-tower and the rocket, this gentleman, Hansjörg Dittus, moved up in the hierarchy of the German Space Agency, and he became a member of the board of presidents as one of the CEOs. And he said, “We need an institute on the quantum technologies for space applications.” So, in some sense, we had shown prototypes by doing the experiments in the drop-tower and the rocket. So, he and I developed, and together with the people in Hanover, with Ernst Maria Rasel, we wrote proposals to the German government saying that such an institute should be founded. And then, in November of 2018, the German parliament passed a bill to create three new institutes of the German Aerospace Center dedicated to quantum technology. Three. One is located in Hanover, one is located in Oberpfaffenhofen which is close to Munich, and one is located in Ulm. So, this was in November of 2018 that the German parliament made that decision. Then, I was asked — since I was involved in pushing this whole thing, I was asked to become the founding director in maybe January of 2019. Then there was a review process where we had outlined a concept of this institute. What is it that the institute should do? What would be the departments of this institute? That was reviewed in Berlin by a committee of international scientists. And then, the DLR, the board of presidents or CEOs decided in June of 2019 that the institute should be founded.

Zierler:

And Wolfgang, in the middle of these existential questions, what were the considerations between the research focus of the Institute of Quantum Technologies, in so far as basic science and also applied technologies were concerned?

Schleich:

Yeah, so now, maybe I should explain to you what the departments are that were outlined as a scope of this institute. So, the government said, “One thing we need is an institute that is working on fundamental issues, and an institute that transfers the knowledge, builds a bridge between university research and industry.” Now, let me explain. You guys have a national bureau of standards, like NIST in Gaithersburg, there is one. What is it called? National Institute of Standards and Technology. 

Zierler:

NIST, that’s right. In Gaithersburg. 

Schleich:

Yeah, it’s in Gaithersburg. And we have something similar in Germany, the Physikalisch-Technische Bundesanstalt. So, they are in charge of time measurements and so forth. They’re the timekeepers, and they can measure time to fractions of a second that are almost unimaginable. But to do this, you need, again, huge laser tables. Now, today, we are very interested in replacing the clocks that are forming the backbone of the European GPS system. So, we have lots of satellites like the Americans. We have lots of satellites in orbit with clocks, and they’re all microwave clocks. And we have a lot of problems with that. At least, the European equivalent of the GPS system, it’s called Galileo. Only due to redundancies in the system can we operate this successfully.

Now, the question is, to have a completely new generation of clocks in orbit based on optical clocks. And as I said, the PTB, the Physikalisch-Technische Bundesanstalt in Braunschweig, they can produce their most accurate clocks. But you need a huge table for this. So, you’re facing the same question that I had before. You have to transfer the knowledge that is in these institutes and miniaturize it so you can get it up to a satellite. Of course, there’s a lot of industry in Germany that is supposed to do this. I just named a company, Airbus and TESAT, and amazingly enough, they’re all in my state of Baden-Württemberg. So, then it was pretty natural to say, okay, then we have to help to transfer this knowledge from the university into industry and help the industry. And the institute in Oberpfaffenhofen and here in Ulm are tasked with exactly this problem, to make the transfer and make the bridge between university and research and the industry. So, now, when you ask me, what are the concepts, what are the topics in our institute? One is called quantum metrology. That’s exactly what I’m saying we’re building, and this is a gentleman by the name of Claus Braxmaier. He is creating clocks in a project called COMPASSO. This project is supposed to bring an optical clock, an iodine laser, to the Space Station, and in particular, the Bartolomeo platform, and operate it successfully there as a kind of pathfinder for the new generation of optical clocks as a GPS system, or the Galileo system. We have a group which is working on the second generation of CAL. I told you that people are doing these experiments on the Space Station thanks to Steven Chu, and we are part of it. But then, at one of these meetings in Pasadena — and I love Pasadena by the way. I love Pasadena. I always hear this song, The Little Old Lady from Pasadena. You know that song?

Zierler:

Yeah, sure. 

Schleich:

By that way, that has also an interesting story to it. Pasadena was obviously the place where a lot of rich old ladies lived whose husbands had died, and they were always driving in these brand-new cars because their husband had died, and they only drove to the supermarket in that car, and that’s it. And that’s where that song came from. Little Old Lady from Pasadena.

Anyways, coming back; at one of the meetings in Pasadena, I said to my friends at JPL, “What are you going to do once CAL is flying and delivering data and so forth? What are you going to do with the next generation?” They said, “Oh, we don’t have the money.” I said, “Why don’t we, in Germany, build the next generation together with you guys. We get together, we decide what we want to do in the next generation, and then we build it in Germany because we have the experience, and then you bring it up to the Space Station?” Because that costs a fortune, too. So, that was agreed upon, and this is a very successful project right now between NASA and DLR. And it’s not only our institute in Ulm, it’s again the Hanover institute that are really building it. I’m the PI of this project, and in my institute, we have a group called Quantum Engineering. It’s the idea of bringing together the engineers and let them work in quantum mechanics. Usually, engineers are just in ordinary classical mechanics. Car mechanics or designing cars. But now, we have to manipulate quantum mechanics, and that’s why we created the word quantum engineering. So, there’s a whole department in charge of building the next generation to CAL, and it’s called BECCAL. Bose-Einstein Condensate and Cold Atom Laboratory. So, the abbreviation is BECCAL, and that is supposed to fly in ‘26. It’s got to be brought up to the Space Station. So, these are two departments. Then, we have a department that’s going to do quantum cryptography. Again, the idea is to bring quantum cryptography with a satellite, like the Chinese have done. We also have a group that is doing quantum nano physics. I told you that the problem is when you have an atom interferometer, one of the big questions is, how do you make a beam splitter where the atoms have a big angle as they go out? Because the larger the angle, the bigger the area is that you enclose. It’s also the area that matters in the sensitivity of these devices. Our friend Christian Brand, he had the idea to take graphene and send a hydrogen beam, with a high velocity, through the graphene structure, and that will get a coherent splitting of the beam with a large angle. And this is the beginning of maybe a new generation of interferometers. We also have a group working on theoretical questions, because all these experiments require a solid foundation in quantum mechanics. I’m running this group right now. 

Zierler:

Wolfgang, on the university side, are you still teaching? Do you have graduate students? What are your responsibilities there?

Schleich:

I’m having two jobs at the same time right now. I’m the founding director of that institute, but at the same time, I still have a chair of theoretical physics with a group of about 20 people at the university. In the Institute of Quantum Technology, right now we have a total of about 45 people working there. So, I still teach. I teach four hours a week. That’s always Monday from 8:00 to 10:00, and Wednesday from 8:00 to 10:00. It’s called Theoretical Thermodynamics and Statistics. So, that’s the regular curriculum, four hours a week. 

Zierler:

Is there a mandatory retirement to go emeritus that might save you from this responsibility?

Schleich:

Well, I love teaching. I’ve been teaching for 30 years now, and I would not want to miss it. So, yes, there is a mandatory retirement. In my case, it is two years from now. And then there are some possibilities that you can extend your contract by two years, and then another two years. 

Zierler:

I’m sure you’ll try, if what I’m hearing is correct. 

Schleich:

Well, I’ll see, because also people in America have been interested in me joining them, and since we have a house in Texas, of course, it is tempting to go there. 

Zierler:

You can see Marlan more frequently. 

Schleich:

That is correct. I’ve spent some time at Texas A&M. I was extremely lucky. I got a fellowship at Texas A&M. It’s called the Hagler Institute for Advanced Study. This gentleman, Hagler, got his degree at Texas A&M — I can’t remember, maybe 50 years ago or something of that order — in business. As soon as he had graduated, the university asked him for a donation. And at that time, I can’t remember the exact amount, but he gave them a check either for $5 or $2. Something like this. This was after he graduated. Got a degree in business. And he wrote on it, “Once I have more money, I will give you more money.” So, I can’t remember exactly which amount of years — maybe five or ten years ago — he donated $20 million to Texas A&M University to support this idea of an Institute for Advanced Study where well-known people come from the outside and spend some time at Texas A&M, interact with the faculty and people there. So, every year, this institute brings in 10 people from different disciplines — from chemistry, from law, from physics, from music. You name it. And they bring them for one or two years to Texas A&M to collaborate with the people there. The gentleman who came up with all of this was John Junkins, a professor in aerospace engineering. 

Zierler:

Mhmm.

Schleich:

You seem to know him. 

Zierler:

I do — I know the name. 

Schleich:

He is an absolute genius when it comes to establishing institutes and organization. I really admire the man. So, he came up with this idea, went around and asked for money, and he had the great fortune to talk to the Chancellor of Texas A&M, a guy by the name of John Sharp. Also, absolutely marvelous person. So, he got the support, and then Hagler came and gave that money. So, as I said, I was fortunate enough to be a Hagler Fellow, together with Roy Glauber at the same time. So, we spent a lot of time together, wrote papers, and had a great time there. So, now, I told you a little bit about this institute in Ulm, and the goal and the vision. And this institute was now officially opened in a ceremony just about two weeks ago. Yeah, exactly two weeks ago. It was a Thursday, two weeks ago. I’m not sure if yet you can see the opening on YouTube, but it was only an online version. Over 150 people had registered to watch it online, but it will eventually go on YouTube. That’s what they said. So, if you’re interested, I’ll send you the link eventually. They also did a little movie, a three-minute movie introducing the institute. You have to interrupt me if I’m talking too long. 

Zierler:

Not at all. I’ll interrupt when I’m ready, but you’re doing great. 

Schleich:

Okay. So, the next thing is — since I was involved in the planning of everything, they asked me to become the founding director of this institute. And the understanding is that I will not be the director in the end. I’m in charge of hiring the first generation, some of the department heads, get this whole thing off the ground, and at the same time, I will hire the final director. So, this is not something that I will do for the rest of my life now. We had yesterday a faculty meeting, and here is something special that I want to explain. Every department head in the institute is also a professor at the university. So, it’s not just that we have an institute that is all paid by the German Aerospace Center. No. The university also profits because all of these department heads will do teaching at the university. And of course, they will draw students to the institute and educate people there. So, it’s a win-win situation because the university doesn’t have to pay the salaries of these professors. That’s paid by the DLR. But they will do teaching, they will be full-fledged members of the university. So, we have now hired two people, a third offer is out, and we had a faculty meeting yesterday in which we approved a list of two top-notch scientists to take over my job as the director. And as I said, last Thursday, we had the opening ceremony, and it was very impressive. The Prime Minister of the state of Baden-Württemberg gave a speech. The Undersecretary of Economics in Berlin gave a speech. Now, you might say, how is he related to this? The German Aerospace Center is not part of the Ministry of Science. It is in the Ministry of Economics, and that is a big advantage. 

Zierler:

The money flows more freely? 

Schleich:

No, there is no competition. You might know, in Germany, we have the Max Planck Society, we have the Fraunhofer Institute, we have the Helmholtz Institute —

Zierler:

There’s DESY. 

Schleich:

They’re like DESY, yeah. And then we also have the universities. And all of them get the money from this Ministry of Science. But the DLR is the only large science organization in Germany which gets its money from the Ministry of Economics. This is a very important issue. 

Zierler:

I wonder how long that’s going to be the case, given the success you’ve experienced 

Schleich:

Well, we sure hope — there’s an election now, and we sure hope it’s not going to be changed. You know, it was not always with the Ministry of Economics. It was first, of course, with the Ministry of Science. But then, there was a time when the Prime Minister of Bavaria was supposed to become Secretary of Economics, and this gentleman was named Edmund Stoiber, he was supposed to become Secretary of Economics. And he said, “Oh, space industry is very important, and I want to have everything that is related to space in my ministry. Otherwise, I’m not going to come.” Okay, so they moved all of this stuff out of the Ministry of Science into the Ministry of Economics, and then he decided he will not come and take over. He rather wanted to be Prime Minister of Bavaria. Of course, the ministry never gave it back. They kept it. So, that helped us because, as I said, we’re the only big organization now in the Ministry of Economics.

So, now we’re coming to the next point, because we said we have this new institute, and when I was asked to take over, the question was of course, we need a building. We cannot have an institute just on the grass, because you have to do experimental work. The main focus of the institute is experimental work, construction of devices. So, you need space, labs. Of course, you want to be — because all these department heads are also professors at the university, you need space next to the university. Ulm University is on a hill outside of Ulm. I don’t know if you’ve seen a map of that. We’re on a beautiful hill with a beautiful view of the cathedral and the Alps. We call it Föhn — when there’s a warm wind from Italy over the Alps, you’re going to see beautiful skyline of all the mountains from our place at the university. Beautiful view. So, they gave me a piece of land and said, “Okay, here you can build your institute.” But at the same time, I heard that next door, just across from the university, some buildings will become available, because 30 years ago, Daimler — you know the company Daimler — the company Daimler decided to move from Stuttgart to Ulm because of the university and built up a research facility in Ulm.

For this purpose, they hired a New York architect by the name of Richard Meier to build them a campus. You can look on the internet. It’s a really beautiful building. And now, 30 years later, the administration of Daimler, the top CEOs said, “Let’s move back to Stuttgart.” So, this campus is empty now. So, that’s when I said, “Hey, we don’t need to build a building. Let’s just get that building there and get this campus.” So, I talked to the office of the Prime Minister, and explained all this, because they made a mistake. When we got the institute, the undersecretary in the office of the Prime Minister wrote me a letter congratulating me that we got the institute, and if I would ever have a problem, I can count on the Prime Minister to help me. So, one day, I remembered this letter, and I said, “Hey, these guys said they’re going to help us. So, let’s call them up and say we want that building.” Interestingly enough, the undersecretary said, “That’s a great idea. We have already been wondering what we would do with this property. We’ll give you this property.” So, of course, you can imagine this is not something you do tomorrow, you get this property. No, the property belonged to Daimler, so now the state of Baden-Württemberg had to buy it from Daimler, and now we, the German Aerospace Center has to buy it from the state of Baden-Württemberg. So, you can imagine there’s a lot of bureaucracy involved, but there’s a general agreement between the state of Baden-Württemberg and the DLR that the DLR is going to buy this land, including the buildings, and our institute will eventually be housed in these buildings. Now, I’m not sure if I can show you, but maybe Kathy will come in a few minutes — she has to teach right now — and probably with her help I could show the movie. It’s called an image film, and it introduces in three minutes the institute, and there you will see this campus. 

Zierler:

Oh, okay. You can send me the link.

Schleich:

Yeah, okay. I’ll send you this thing. 

Zierler:

Wolfgang, a broad question about where things are. For the most important research questions you’re after, what is the state of interplay between theory and experiment right now?

Schleich:

Well, let me put it this way. In these experiments that we did, theory was absolutely essential. And of course, we wouldn’t have done that particular theory if it wouldn’t have been for the experiments. So, originally, I was considered a pure theorist when I was young. But over the years, I have worked with numerous experimentalists. A lot of people did experiments that I had proposed, and nowadays I work with my group with lots of experimentalists. I can give you an example with Israel. I have a friend in Israel — I mean, I have many friends in Israel. That’s not the point. But a friend of mine is at Tel Aviv University. His name is Ady Arieand he has a student by the name of Gary Rozenman, who is a graduate student in the group by Lev Shemer in the mechanical engineering department. These guys have a water tank. That water tank is 16 meters long, and at one of the ends of the water tank, you can move a finger up and down and create a water wave. This water wave will travel down this canal, and there are gauges, and you can measure the wave as it propagates. And then, after 16 meters, it ends in some beach — they call it the beach.

Now, you might say, why do you worry as a quantum physicist about such water waves? Because the equation of motion for these water waves is the Schrödinger equation. How can that be? Well, the Schrödinger equation here is a very special one. Time and space in these water wave equations are interchanged. So, wherever in the Schrödinger equation you have time, you now have space. Wherever you have space in the Schrödinger, you have time. So, it’s a very unusual analogy, but of course, if you want to simulate the Schrödinger equation, or unusual features of the Schrödinger equation, you can simulate that with this funny water wave analogy. Now, you said, how important is this theory-experimental connection? Well, there was a gentleman by the name of Earle Kennard. He was a professor in Cornell — I think it was Cornell — in the ‘20s. He was a full professor in the ‘20s. And he had heard of quantum mechanics being invented in Göttingen. So, he figured, gee, I have to learn this. So, the guy took a sabbatical and came to Göttingen, and then he spent also a couple of months in Copenhagen and learned quantum mechanics. And he wrote several papers on a very elementary problem, the problem of a particle rolling down a linear potential. A linear potential is gravity. That’s the lowest approximation, right? The constant force has a linear potential.

So now, you can see — again, we are ending up on gravity, right? So, you see a wave packet moving in a linear potential, falling in gravity. And what he found was that there is an overall phase that goes with the third power of time. Remember I mentioned to you that the phase shift in an interferometer goes with the square of the time the atom spends in the interferometer? And now you suddenly have an effect that goes with the time to the power three. So, I said to myself, gee, this is interesting. Maybe we can build an interferometer out of this. And I got drawn into this discussion by a gentleman by the name of Sam Werner. I don’t know if you’ve ever talked to him. If you haven’t, you should. Do you know him? Sam Werner. 

Zierler:

I don’t. 

Schleich:

Oh, boy. So, Sam Werner, he should have gotten with his friends the Nobel Prize in physics in my opinion. There’s something called the COW experiment, and it starts for Colella, Overhauser, and Werner. So, these were the three authors of the paper, and what they did is they built a neutron interferometer. So, not a light interferometer; not an atom interferometer. It was a neutron interferometer. And for the first time, they showed the influence of the gravitational field on the phase shift of the neutron wave. I should say, the influence of the gravitational field on the phase of the neutron. So, they saw the gravitationally induced phase shift. And in my opinion, this is an absolutely beautiful experiment. Overhauser died a couple of years ago, and Colella died a couple of years ago, and Sam Werner I think is the remaining one in this area. Some years back, he came to me and said, “Look, I’ve always wondered about that phase.” And we traced it back to Kennard and we called it the Kennard phase. So, for the first time, we could measure this phase they predicted in 1927 with these water waves. So, we created the water wave moving in the tank.

Of course, in order to create a linear potential, you had to apply a current going backwards, and then you could measure for the first time this phase. So, here you see what I’m saying. It’s important to do these experiments. So, Sam Werner came to me as an experimentalist, and said, “Look, this phase has always bothered me,” and I came up and said, “Look, we can measure this phase with these water waves.” Not only this, but we also predicted — we built an interferometer, theoretically at least. We came up with an idea how to make an interferometer out of this phase. And a guy in Ben Gurion University, also in Israel, Ron Folman, built this interferometer, and we saw for the first time a phase shift that scales like the time in the interferometer cubed, rather than T squared. So, it is always in my opinion that we need to talk with each other. Of course, experimentalists have different ideas, and then they bring these ideas. You have different ideas as a theorist, and interact with them, and you build something. Again, this stuff that we did with the drop-tower, Hansjörg Dittus is an engineer. He built the drop-tower. And Ernst Maria Rasel is an experimentalist. I wrote a lot of papers with Ernst Rasel, stimulated by mutual discussions we have had. And his group now, these experimentalists come, and they say, “Have you had this and this problems?” And my guys sit down with them, and they develop a theory behind these experiments. So, I hope that kind of explains what it is. 

Zierler:

It sounds good, as opposed to other fields where unfortunately theoreticians and experimentalists are not talking to each other nearly as much as they should. 

Schleich:

No, but in quantum optics, there is a very close connection between theory and experiment. No doubt about it. 

Zierler:

Wolfgang, let’s take it all the way back to the beginning. Let’s start first with your parents. Tell me a little bit about them. 

Schleich:

Oh, my parents. They’re a very good story. So, maybe I’ll tell you where I was born. 

Zierler:

Please.

Schleich:

I was born in a town by the name of Mühldorf, and it’s on a river, and the river is called the Inn. And the Inn emerges from Austria, and it goes through Innsbruck. You probably have heard of the town of Innsbruck, and if you would translate it, it’s called the bridge over the Inn River. That’s what Innsbruck means. So, if you would follow the river from Innsbruck further to the north, because the Inn eventually merges with the Danube River, it passes through this little town of Mühldorf. Mühldorf has about 15,000 inhabitants, and about a tenth of that population worked at the railroad because there was a major railroad station there. The railroad station dates back to the Orient Express. You probably have heard of the story by Agatha Christie, the Orient Express. So, around 1870-1880, there was the Orient Express going down to Vienna, and then all the way to Istanbul. And it was going through Mühldorf. See, today, if you start in Munich and you want to go to Vienna, you will not go through Mühldorf. You will go through Rosenheim and Salzburg. And if you look at the map, you’re going to see that this is a detour because in some sense you start here in Munich, then you go down south, and then you go up again in order to go to Vienna. But if you would go through Mühldorf, you would go straight like that.

So, Mühldorf is about 80 km east of Munich. It’s not far away, 80 km east. By the way, if you would go further east from Mühldorf about 40 km, you would come to a very, very small place, and that’s where the Pope Benedict was born. And if you go another 40 km further east, you come to a town by the name of Ried. That’s where Anton Zeilinger was born. So, Anton and I come from the very same neighborhood of the Innviertel. It’s called the Innviertel, which means the Inn quarter. So, I was born there, my father worked at the railroad, and eventually, when I was about 10, he got a job in Munich. So, we left Mühldorf, and that was a big tragedy for me because I had all my friends in Mühldorf, and it was a nice neighborhood, and we had a big garden where all the kids of the neighborhood came together and played. And now, suddenly, we got into this big city, and I didn’t have the friends anymore. Of course, then high school came, and I didn’t have that much time. But of course, it also had the advantage that the whole family essentially lived in Munich. So, it was a big family we had at time — uncles and aunts and relatives. Big family.

Anyway, so I moved to Munich and went to high school there, and then the question came up as I approached the last year of high school — here in Germany, we have nine years in high school. So, we go to 13 classes before we go to the university. So, you have four classes in elementary school, and then you had 9 classes before you could join the university. I would say in the middle of high school, I became very interested in music, and I started learning how to play the violin and the viola. I started to learn how to conduct an orchestra. I started writing music, even. And my dream was to really become a conductor and composer, and I didn’t take school that seriously. I didn’t have that good grades, by the way. The average grade when I graduated high school was B+ because I spent a lot of time on music. My dream was to become a musician. My only problem was I didn’t know how to play the piano. I never learned how to play the piano, and in order to study music, you need to play the piano. So, that was out of the question. And also, my parents didn’t think that becoming a musician was the dream that they had for their son. They made it pretty clear that, yes, you can do anything you want, but you need a solid job. And the only thing that I was a little bit good in was mathematics.

So, the decision was made all by myself, and maybe the soft pressure from my parents, that I should become a high school teacher. High school teachers, they show up at 8:00 in the morning. They go home at 1:00 at that time, 12:00 sometimes, and then they could do whatever they wanted to do in the afternoon. So, my parents said, “Look, become a high school teacher and you can do music on the side.” So, I enrolled in the University of Munich in mathematics, and the natural thing at that time was physics. You enroll in mathematics and physics, or you could do mathematics and biology, but the standard thing was mathematics and physics. So, I enrolled at the university, and I kept playing music. I don’t know — have you been to Munich?

Zierler:

I have not. I mean, I’ve flown through, but just the airport. 

Schleich:

I see. Anyway, Munich is like 1 million or 1.5 million at the time. The university has, I don’t know, 40,000 or 50,000 students. We were in the first classes in experimental physics. We had like 400 students. It was a huge lecture hall. We could barely see what the professor was writing there on the board, and you also go the distinct feeling that they didn’t care about you at all. I still remember a professor in classical mechanics, first semester, explaining the Lorentz transformations. And I just couldn’t understand what the hell the guy was talking about. And he turned around, and looked at us, and said, “If you have not understood what I have said here today, you should really start worrying if you’re at the right place.” That was the kind of atmosphere. And as I said, I really did not know what the guy was talking about, so I was wondering if I was at the right place. But on the other hand, I kept playing music in the evening, and I was playing in various orchestras and writing music and all kinds of things. And I said, “Look, I just want to become a high school teacher, so who cares?” And there was a turning point in the third semester. I don’t know if I’m boring you with all this stuff.

Zierler:

Not at all. 

Schleich:

Okay. So, there was a turning point in the third semester, and it came because of a very unusual reason. You know, becoming a high school teacher, you had to take written exams because in the end, you would be employed as a Civil Servant. So, the government was controlling who will become a Civil Servant, and who will not. So, they wanted written examinations, whereas if you would get a degree in physics as a scientist, a Diplom in physics, you would not take written final examinations, and because they didn’t have enough high school teachers in physics, you could then go to the Ministry of Education and say, “Look, I have a Diplom in physics. Can you accept this as a degree as a high school teacher?” And then, you didn’t have to take written exams because the ministry, as I said, they were in trouble. They didn’t have enough teachers at the time. I learned this, and I said to myself, well, that’s a great idea. I don’t have to take these written exams, so I’ll become first a physicist, and then I can still become a teacher.

So, that was a big idea, but for this, you had to take theoretical physics. Theoretical physics is of course part also of the curriculum of a high school teacher, but at a much later stage. So, in that sense, in order to get this degree, you had to take theoretical mechanics in the third semester. So, I looked up who was going to teach next semester theoretical physics. It turned out it was a gentleman by the name of Georg Süssmann. It was third semester, we had no idea who he was, and so I enrolled. So, here we were, 300 students listening to this gentleman. And suddenly, all of this physics stuff that these experimentalists were teaching us over two semesters made sense. This guy could explain in terms of equations how everything was connected, except it was a lot more than a student in this class could take, because this gentleman connected different fields with each there in a class for third semester. So, it was not easy to follow. For us, it was a chaos. Later on, I realized this man, these classes were absolutely marvelous. But of course, for a student in the third semester, you couldn’t comprehend all these connections that the man made.

But as I said, that was a turning point, and I admired this man, Georg Süssmann, because he would come in — he was a very unusual man — he would come into the lecture hall with two huge suitcases, and he would put them on this big table in front in the lecture hall, open them up, and take out five or six huge folders with notes. And he would put them all on the table nicely on various pages, and then he would give this lecture, one and a half hours, without ever looking at these notes. He never looked at the notes. He did it all by his head. He would write on the — he had very nice handwriting, and he wrote everything on the blackboard. But as I said, it was not easy for us to follow these lectures. He had decided this thing that people shouldn’t take notes while he was lecturing. They should rather listen to what he was saying, but in order to have a manuscript at the end, we had a volunteer to write these notes. So, every week, two students would volunteer to write the notes for the class, and then we would have a chance to talk with him about our notes. And as I said, we were 300 students in the class, and as a regular student, you had no chance to meet Professor Süssmann and get a feeling for this man except in the class. And you know, this is a very different — you guys in America have a very different way of talking with students. For us in Germany, this was Herr Prof. Dr. Süssmann. 

Zierler:

More formal. 

Schleich:

Very formal. And you would never address him as Herr Süssmann. You would say Herr Professor Süssmann. And as I said, we would have even contact — the man would come at 10:15. Exactly 10:15 the man would come in, he would give his lecture, and he would leave again. Yes, you could ask questions in between and he would discuss with you, but there was no way that the guy would come for a beer with you or something. So, that’s why I volunteered, together with probably the most beautiful girl in this class, and we decided that we would write notes, because then you would have a chance to meet him. This was around Christmas time that we did this, and sure enough — it was really interesting. The guy looked at the notes, and he said, “Yeah, but you didn’t write down about what I said in this topic.” And I said, “Yeah, we didn’t understand this.” He says, “Uh huh. And I noticed you didn’t write down about this topic that I mentioned also.”

And for me it was amazing because the guy talked so much, I was amazed he would remember what he said. But he knew everything, and he noticed that we didn’t take notes about this. So, he said, “Oh, we need to rewrite this. I’ll show you what we need to do.” So, we had several meetings, and the man liked to meet in a cafe. The Institute of Theoretical Physics where we took lectures, they were very close to the center of town, and there were tons of cafes in the neighborhood. So, we would always meet in these cafes and show him the notes, and then he would look at them and say, “Oh, you have to rewrite this.” He was in some sense the epitome of a professor who is a little bit confused when it comes to worldly matters. I mean, the waitress would come and say, “Herr Professor, the bill is 15 marks.” And he would say, “I’ll give you 14 marks and the rest is for you.” So, he would include a tip, but he took it negative. So, there were lots of things like this. But boy, he knew everything about everything in physics. The man was a genius.

Anyways, so you see, I have some admiration for him. I can tell you many things about him, but it’s probably not the moment. Anyways, so this was the turning point, and the real turning point was, for me, when he said, “Herr Schleich, why don’t you expand this relativistic square root?” And I immediately said, “Okay, let’s do a Taylor expansion.” I wrote down the Taylor expansion, and I started calculating the derivatives of the square root, and he looked at me and said, “What in the hell are you doing?” And I said, “Well, I’m expanding the square roots like you wanted.” And he says, “Look, this is something you need to know by heart. You cannot start writing down the square root by Taylor expansion. These are formulas you need to know by heart. What have you learned during the last year?” And then, I said to myself, the guy is right. I spent the whole last year doing music and I heard a little bit about mathematics, but I didn’t really learn that much. And then I said to myself, if you really want to do something in this field, you have to spend time on it. I said that to myself.

And so, then I took the stuff seriously, and I started listening. I took mathematics classes from a gentleman by the name of Ernst Wienholtz — absolutely marvelous applied mathematician at the University of Munich. I learned a lot of mathematics from him, and amazingly enough, I only took one class from Georg Süssmann. That was that particular class. Later on, I had classes from other professors, but by that time, I was determined I would go into physics and become a person like Georg Süssmann. Now I had a goal. Suddenly I had a goal, and music was not that big important anymore to me. I wanted to become a professor of theoretical physics. So, that was in the third semester.

Also, at that time, I had met a young man who was becoming a conductor. He studied at the university, at the music school, and I suddenly realized that his hearing was so much better than mine, that the man had what is called the perfect pitch. We would go to concerts together, and I would say, “Oh my God, that singer was really great today.” And he would look at me and say, “You’re just looking at her tits. She was off-key the whole time.” And you know, I just didn’t hear that. So, he had much better hearing than I did. So, I realized this is not my field. This is not something I could really make a difference because if I can’t hear as well as he, I could become a mediocre conductor maybe, but not really do something. So, then I started concentrating on the university, and I took seriously classes. As I said, I learned a lot of mathematics from Wienholtz. I learned a lot of physics. And then I took a class — there was one professor I also admired. His name was Helmut Salecker. He had been a postdoc of Eugene Wigner at Princeton and has a very famous paper about a clock and measurement of time in quantum mechanics. That’s Wigner and Salecker in Phys. Rev. in the 1950s. Very influential paper.

Anyways, this man had also a chair of theoretical physics in Munich, and he was teaching quantum mechanics and quantum electrodynamics. I liked him a lot. In some sense, I liked him more than Süssmann, because his classes were absolutely perfect. He was well prepared, very logical, he hardly ever wrote anything on the blackboard, and a lot of students were laughing about him because he never wiped out the blackboard. There was hardly anything people would take note. They wouldn’t write anything — they wouldn’t write much down. And they made a dramatic mistake. I understood right away that I have to write down every word that the man would say, and immediately after the class, go to the library and write the manuscript, because then everything is fresh. As I said, the man would write down an equation, and then he would explain on that one equation how you continue. And then, he would write down three or five steps further down the road the next equation. So, if you would not write down how he explained how he got the next equation, you had not a ghost of a chance to understand it.

So, another beautiful thing about his lectures was that the next lecture, he would tell the same story again. He was convinced it would be better for people to hear the story twice. And he was always teaching from 2:00 to 4:00 in the afternoon, because he said that’s the time when he usually takes a nap. And now he can use that time usefully. So, he would teach from 2:00 to 4:00, and he was teaching quantum mechanics and quantum electrodynamics, relativistic field theory. And the guy, as I said, crystal clear lectures, but you had to write down every word the man was saying. Then, you had to work it out in detail, and then you understood exactly. And then, the next time, when you had written your manuscript, you knew he was giving this lecture one more time. So, you could really compare and see if you captured everything that he said. Because he literally, including the jokes, literally repeated everything again. So, I took these lectures, and now I was determined, my field is relativistic quantum field theory, quantum electrodynamics.

So, as I was listening to these lectures, I went to him. I said, “Professor Salecker, I would like to do my Diplom thesis with you.” And Professor Salecker looked at me, and he said, “Herr Schleich, I only take the best of the best.” And I said, “Yep, that’s right. That’s just what I want to do.” And he said, “Well, let’s see. Are you taking my class right now?” “Yes.” “Are you still doing the homework?” “Yes.” “Well, let’s see how you’re doing at the end of the semester.” So, at the end of the semester, I went to see him again, and he said, “Well, we still haven’t had renormalization theory. Let’s see how you’re doing in the renormalization theory.” So, I’m still doing homework for him and all this. One day, I’m meeting him in the street in the evening, and I said, “Well, when do you think I can start with you on a Diplom?” He says, “Well, the way I do it, I give you an office, and then you have to study the book by Jauch and Rohrlich, The Theory of Photons and Electrons. And if you understand that book, then we can start talking about you doing a thesis with me.” He was one of the old professors. When I say old professors, he probably was my age that I am now. But you have to understand, I was like 22. I thought the guy is God, okay?

But at the same time, I had to take my bachelor examination — maybe a year before I had to take my bachelors examination in experimental physics. And I went to a young professor — he was about 40 at the time — and that young lady that I worked with on these notes, she had to take an exam with him. She said to me that he was very nice in the exam. So, I took my experimental physics examination with this gentleman. His name was Herbert Walther, and he had just come to Munich maybe a year or two earlier. He was very different from these old professors that I was telling you earlier. You could tell he was a dynamic person. He gave the lectures differently, and he was trying to teach in a way that people would understand what he’s doing. And he was very friendly. So, I don’t know, by chance I met him, and he remembered that I had my exam with him. And I said, “Oh, Herr Prof. Walther, do you have any positions as a Diplom student?” And he says, “Oh, yeah, sure. I have this big institute, but I only take experimentalists.” And I said, “Yeah, but I want to do theory. I want to do theoretical work.” He says, “Well, let’s see. Why don’t you come and do a seminar with me, and then we’ll see how you’re doing?”

So, one day, now we’re coming towards the end — so, I did this seminar with him, and I explained the free electron laser. I had to give a talk on the free electron laser, and I had a postdoc who helped me prepare the talk. This was Pierre Meystre. Today he’s a big shot in Tucson, Arizona. He was also editor of Phys. Rev — first Phys. Rev. Letters, then Phys. Rev., and he quit a couple of years ago. Anyways, he was a postdoc at that time with Herbert Walther, and he was my mentor, so to speak, for that seminar. So, eventually Walther said, “Look, I’ll make an exception. I’ll let you do a thesis in theoretical physics with Pierre Meystre. So, one day I meet old Professor Salecker, and he said, “Herr Schleich, you can now come and start working on Jauch and Rohrlich.” And I said, “Well, it’s too late. I’m already working for Professor Walther now.” Boy, was he pissed! But that’s how business works. So, I said, that’s it, I’m going to go out — the only problem was at that time that Walther had his labs out in Garching. So, if I would have worked with Salecker, I would have been at the center of town in the neighborhood of the old university. However, the Technical University in Munich had built outside of Garching a little nuclear reactor in the ‘50s, and they centered a whole physics department around this reactor. That also had to do with the fact that a gentleman by the name of — now I’m getting tired — a Nobel Prize in — Rudolf Mössbauer. He was assistant professor in America and got the Nobel Prize for the Mössbauer effect, the recoilless absorption in an array of nuclei in a solid, in a crystal. So, if an atom is in a crystal, and it absorbs, there’s no recoil because the recoil is given to the whole crystal. And he showed this in an experiment when he was a graduate student, Rudolf Mössbauer, and for this he got the Nobel Prize. Of course, the people in Munich wanted to attract him back from California, so they gave him, in some sense, a whole department. And he built up a very beautiful, effective physics department at the Technical University in Munich, but outside of Munich. And outside meant, I would say, about 40 km, 30 miles outside of Munich. So, you had to take bus from Munich to the outside of Munich. So, it took me a whole hour from my parents’ house out to the institute.

So, this gentleman, Herbert Walther, had a chair of experimental physics at the university, and at the same time, he was hired as the founding director of the new Max Planck Institute for Quantum Optics. And this man worked day and night. Herbert Walther worked day and night to build up the institute, to make it famous, and he had attracted a young theorist from America by the name of Marlan Scully. 

Zierler:

Ah, there we go.

Schleich:

So, it was after the seminar. The seminar was the winter of ‘79 to ‘80. In the winter semester of ‘79 to ‘80, I took the seminar by Herbert Walther, and then he said, “Okay, work with Pierre Meystre on optical bistability.” And then, it must have been in May of ‘80 that Scully came to visit. He gave a talk, and I’ll never forget, after the talk I asked a question. I said, “Well, Professor Scully, I have the following question.” And he said, “Well, first thing, don’t call me Professor Scully, otherwise I will call you Professor Schleich. Call me Marlan.” So, that was something for me — 

Zierler:

Some American informality. 

Schleich:

Yes, that’s right. It was very unusual for me, where I was so used to addressing all of these people as Professor. 

Zierler:

Was that good for your development to loosen that formality, to start think of yourself as a scientist on par?

Schleich:

Absolutely. Yes, yes. You know, until the end of Professor Walther’s life, I would never address him as Herbert. He was always Professor Walther for me. 

Zierler:

Yeah. What was Marlan working on at that point? What prompted him to come over? 

Schleich:

Oh, he had a Humboldt Fellowship. I think Herbert Walther had gotten him the Humboldt, and he came to Germany to work with him. And he was interested in ring laser gyroscopes at the time. So, he was interested in quantum fluctuations of ring laser gyroscopes. So, as I said, I asked that question in his — he had an informal seminar. He was just showing what type of calculations he was doing at that time. It was informal, maybe six or seven people, and because I was the new kid on the block, I was there. Pierre Meystre took me along, and I asked this question. And then, after this talk, he had this nonlinear equation, I went to the library, and I remembered certain books that I had looked at when I was in Munich in the university. I was still a student in the university looking and working really on a thesis. I had just started working on optical bistability with Pierre Meystre, but I was interested in this question of this differential equation, and I found some tricks how to manipulate this differential equation.

So, I went back, and I knocked on the door of Marlan, and he said, “Come in. What’s going on?” I said, “Look, I looked up this equation, and you can do the following tricks on this equation.” So, I showed him on the blackboard, and he was fascinated. He had never seen these tricks before. Well, we had a different education in Germany. We learned a lot of analysis, and we had a very solid education in mathematics as a physicist. So, he had never seen this before. He was impressed. He said, “You know what? Stop working with Meystre. You’re working with me. You’re going to be my student from now on.” So, I started working him. That was the beginning in 1980. So, that’s like 40 years ago. It was a wonderful time working with him, and first we worked on these ring laser gyroscopes, and then I did my PhD with him on — it was called eventually Optical Tests of General Relativity.

I had also the chance — at that time, he was in Albuquerque, New Mexico. He was a professor in Albuquerque. He and Herbert Walther made it possible for me to go — so, after my Diplom — I got my Diplom in November ‘81, and then I got a contract as a graduate student in the Max Planck Institute for Quantum Optics with Professor Walther. And then, in ‘82, there were two big events for me. Claude Cohen-Tannoudji came to give a colloquium in the institute. As I said, I was a master’s student at that time. No, no — what the hell am I talking about? A graduate student at the time, because I got my degree in November ‘81. And Marlan told me, “Hey, let’s talk to Cohen-Tannoudji about you.” I said, “What do you mean, talk about me?” He says, “In the summer, in a couple of months, I have to go to Les Houches and give lectures on relativity and optics. It would be just great if you could come with me and help write the notes on these lectures. These are really wonderful summer schools in Les Houches, and you should go there, but in order to do this, we need to get the permission of Claude Cohen-Tannoudji because he’s one of the main organizers of that summer school. Now, you have to understand one thing. I don’t know if you’ve ever heard of the Les Houches summer schools. 

Zierler:

Sure, sure. 

Schleich:

They were founded by Mrs. Cécile DeWitt-Morette. She was the wife of Bryce DeWitt. She just passed away a couple of years ago — a year or two ago. Anyways, so she had invented these Les Houches summer schools, and there was one in ‘64 or ‘63 on — it was called quantum electronics. People like Nico Bloembergen, and Willis Lamb, and Roy Glauber, they gave lectures on the early days of laser physics. And these articles of that summer school were printed in a book, and they were one of the standard references for many years on this field. Glauber’s lectures, Lamb’s lectures on semi-classical laser theory. They were the standard reference on this. And Scully was a student of Lamb on doing the quantum theory of the laser. So, he had studied these Les Houches manuscripts. But he had not been invited for a lecture in Les Houches until ‘82. So, for him, that meant a lot. He had been to Varenna — there was a Varenna summer school on quantum optics in ‘67 or ‘68, where he was lecturing as a young assistant professor on the quantum theory of the laser, but he had not been invited to Les Houches. So, in ‘82 he was invited, so he took me along. So, we talked to Cohen-Tannoudji, and he explained why I should come along, and Cohen-Tannoudji said, “Okay, bring him along.” So, in ‘82, Marlan and I would go to Les Houches, and before we went there, Marlan said to me, “How do you think I should give these lectures?” This was on relativity and optics. And I was a graduate student, and I said to him, “This is how I would give the lectures. I would derive this wave equation, and then do everything with the wave equation, and we can have these analogies to media and optics.” And later on, he told me that he thought that was really arrogant. Here is this graduate student explaining to this famous professor how to give the lecture. But he thought there was some sense in what I was saying, and eventually, that’s how we did the lectures. But it was a lot of work. I had to prepare all these things, and he would give the lectures, and then I would write the lectures. But as I said, at that summer school, I wouldn’t say I became famous, but at least I met a lot of famous people. Haroche was giving a lecture; Dan Kleppner was giving a lecture; then Claude Cohen-Tannoudji gave lectures; Sylvain Liberman gave lectures. As students in this lecture hall was Jean Dalibard, who is now running Les Houches. So, it was an amazing event.

And Scully, after he gave the lectures, had to go to the next conference, but the meeting was six weeks. And he said to me, “Okay, finish those notes, and then you come to America, and we can finish them together.” So, during the six weeks while I was in Les Houches, I worked day and night on writing the manuscript for the book of Les Houches. And professors like Haroche came to me, and said, “Why are you working on this manuscript all the time? There are these beautiful mountains here. Why don’t we go for a hike?” And I said, “I can’t, I have to finish. When I get back in four weeks, I have to have this manuscript.” He said, “Yeah, come on. Let’s go for a hike.” This guy Lieberman, there were these two beautiful young ladies, and he was always sitting with them. Famous guy. He was the director of the Aimé Cotton Institute in Paris. He was sitting with these young ladies, and they always had a good time. And he said, “Wolfgang, come. Let’s go for a walk with these ladies.” Well, I didn’t have time, because I was working on the damn manuscript. But at least everybody knew me because I was the guy working on this manuscript on relativity. And eventually, the paper came out, and the book had even my picture in it. So, if you look today in this book, maybe I shouldn’t say this, but it did not have Marlan’s picture in it. It had my picture. Sometimes, in after dinner speeches, Marlan tells the story that when the book finally arrived and it was printed, he would look and there was the picture of Schleich lecturing, and not Scully. 

Zierler:

You’re the big shot apparently. 

Schleich:

Yeah, because the French didn’t think it was nice that I didn’t have time to go with the girls, and I was always sitting in this little apartment I had working on these notes. But then, in the fall of ‘82, I came to Albuquerque with Marlan. As I said, Professor Walther paid my salary, and I came to Albuquerque. And then I worked with Marlan on this manuscript, and it was the first big paper I wrote all by myself — well, not all by myself, but I was the lead writer, and it was 150 pages when it was printed eventually. So, for me, it was a big thing. Then, after one year, I was really happy in Albuquerque. I didn’t want to go back, but then one day, Professor Walther shows up in Albuquerque, and he says, “Oh, by the way, Schleich, you need to come back.” I said, “But I’ve only been here for one year.” He says, “Yeah, but look, I’m paying your salary. Make a decision. You want to have your salary paid, you come back. You want to stay in Albuquerque, it’s fine, but you’re not going to get paid by me anymore.” So, that was pretty convincing. So, I came back to Munich and finished my degree.

So, by the spring of ‘84, I had my PhD. And then, another big miracle happened. Now, I had the question, what would I do after my PhD? I wasn’t quite sure what I wanted to do. While I was writing in Albuquerque this manuscript on relativity and optics, I had met a gentleman that Marlan knew from Tucson, Arizona, Professor Hanno Rund. He was an applied mathematician, and he was born in Cape Town. He had now gotten a big offer to go to Cape Town and take over a chair in Cape Town on applied mathematics. And he came to me, he had read certain sections in this manuscript, and had pointed out mistakes in it. I said, “Yeah, I know the mistakes, but Marlan insists that we explain it this way.” So, he said, “Well, let me talk to Marlan tomorrow.” And then we had a big discussion, and it got fixed eventually. So, then, the next day, Professor Rund came to me and said, “Hey, you want to come with me as a postdoc to Cape Town, once you’re finished with your degree? Why don’t you come with me to Cape Town?” And at the same time, another gentleman I met during all these times working with Marlan — one of the things I really admired with Marlan was that he had brought in lots of different people from all kinds of different fields in optics. He would bring people and they would give a lecture or seminar, and then we as young people could talk with them. These visitors were not the privilege of the boss, but we had access to them. One of them was a gentleman from Helsinki, Stig Stenholm. He was also a theorist in quantum optics, and he and I talked a lot and discussed. And he said to me, “Why don’t you come as a postdoc to Helsinki?” So, I had these two offers. Quantum optics in Helsinki or applied mathematics in Cape Town. You couldn’t think of two places further apart. And my heart would have been in Cape Town, I think. 

Zierler:

Where is John Wheeler in all of this? What is the sequence?

Schleich:

He comes in right now. And one day, just as I’m thinking about what I’m going to do, I get this telegram. First, I get a phone call from Professor Walther, and he says, “Please come to my office. There’s a telegram for you.” Now, at that time, you would only get telegrams if somebody died. You wouldn’t get telegrams. You wouldn’t get an email because there was no email. There was no fax. You got a letter. But if something was really big, it was a telegram. So, I went to Professor Walther’s office, and there on the desk was this telegram. He said, “Open it up,” because it said very clearly, John Archibald Wheeler. And of course, Professor Walther knew Wheeler because of a summer school we had the year before where Wheeler and Wigner and Unruh, they were all giving lectures at that summer school. So, Walther said, “Open this telegram” So I opened it and it says, “Dear Dr. Schleich, would you be willing to come and be a postdoc in my group?”

Zierler:

How did he know about you? From Marlan?

Schleich:

Yeah, see, what happened was at Christmas ‘83, a couple of weeks before Christmas, Professor Walther came to me and he said, “I need your thesis before Christmas. I don’t care if you work day and night, I need your thesis before Christmas. It doesn’t have to be bound. We’re going to put it in a folder, but I need your thesis.” I said, “What for?” He said, “It’s none of your concern. You give me the thesis before Christmas.” And so, I handed it in — I got sick. I worked day and night on writing this thing. But I handed it in. Later, I found out that Herbert Walther had nominated me for a prize in the Max Planck Society for an excellent thesis, for something that is exceptional. And you know, he was building up a new institute, so he also wanted to get visibility for his people because he also had to put up a fight against the old institutes. I can’t imagine, if it was a new institute, the old bosses, the established people, they want to make sure that they remain, and he wanted — here’s a young man who wants to really push his institute, so he said, “I’m going to nominate Schleich for this prize for the best thesis in the Max Planck Society.” There was not just one prize. There were several — I don’t know, four or five of these grants. Interestingly enough, it was maybe 2,000 marks, but if you would be the winner of this prize, the Max Planck Society would pay you your last salary for two years, and you could go anywhere in the world to work with somebody. So, that was the prize, and that was called the Otto Hahn Prize after Otto Hahn, one of the big guys in the Max Planck Society. So, this was the Otto Hahn Prize for PhD thesis. 

Zierler:

Wolfgang, what do you think you were being recognized for?

Schleich:

Well, let me first — I’ll answer this in a minute, because you asked me how I got into this story with Wheeler. So, Walther had to hand in names as possible referees for the thesis, and he had given Wheeler’s name. So, Wheeler wrote a report about my thesis, and not only did I get the Otto Hahn Prize, a medal, I also got this telegram from him. This is how I ended up with Wheeler. But what’s interesting is also that before I accepted the offer with Wheeler, I asked Professor Walther, should I use that money that I got from the Max Planck Society? And Walther said, “No, don’t use that money. Let Wheeler pay your salary, because since it’s his money, he is worried about what you’re doing. So, he’s not going to leave you alone. He’s going to really want to work with you.”

Zierler:

As we say in America, he’s got skin in the game. 

Schleich:

That’s right. That’s exactly it. And I thought that was wonderful advice that Walther gave me. And I said to myself, it’s also good because then I still have a security of two years’ postdoc, even if I don’t find a job now. So, I went and worked for two years with Wheeler, and then Herbert Walther came and said, “Why don’t you come back to the Max Planck and get a habilitation, so that you can become a professor.” So, it was really also Herbert Walther who took me under his wing, and I went back to Munich in ‘86, and got my habilitation in ‘89. And my mentor then was Georg Süssmann. So, the first teacher in theoretical physics then helped me to get my habilitation, and it was the work that I had done with John Wheeler while I was in Texas. And the referees for my habilitation were Wheeler and Cohen-Tannoudji. 

Zierler:

What was Wheeler like? What was it like to work with him? You know what, first, before we get too far afield, the prize. What was your sense of what you were being recognized for with your thesis research?

Schleich:

Ah, yeah. I see. Okay, so that’s what you meant. I thought you thought about the general work in my life what I did. 

Zierler:

No, no, no, specifically about this prize. It’s so important. This is a very important juncture in your academic career. 

Schleich:

Absolutely. So, there were two things. The thesis consisted of essentially three parts. One was the Les Houches manuscript. The Les Houches manuscript was — there was some Russian paper that had shown — and a gentleman by the name of Pleba?ski, a Polish scientist in Mexico City, he had written a paper which relied on this Russian work, which is also mentioned in Landau-Lifshitz, that you can write Maxwell’s equation in general relativity, so in a covariant form, you can write them as normal Maxwell equations, but in a dielectric medium. So, you take the covariant equations with all these covariant derivatives and all of this, and you can cast them into a form so that they look identical to Maxwell’s equation in the medium. This was known, and then I suggested to Scully, let’s take these equations and derive a wave equation from it. And then this wave equation will have polarization terms, and these polarization terms will create all the effects of relativity on light. And we can understand them like classical optics. There’s polarization that creates some effect on light. The medium effect on light. So, I worked all this out, and it was an interesting way of doing things.

Later on, a gentleman by the name of Ulf Leonhardt, who is now a professor at the Weizmann Institute, he did his habilitation with me. And I showed him all this stuff, and he reversed it. We said, we take relativity, and we get a simulation of classical optics with polarization. And he started asking the question, if I have a certain index of refraction, can I simulate a black hole? And he came up with this cloaking idea so that objects disappear that you look at. As I said, I told him these ideas out of my thesis, and he applied them to do the inverse to let objects disappear, to manipulate the index of refraction so that you cannot see these objects. But in some sense, it all came out of that chapter. And then, I also developed the ring laser gyroscope — it was called the dithered gyroscope. That was a mechanism, how to unlock the gyroscope, and I included the quantum noise in that work. That was a completely different thing. And then there was a technique of measuring gravitational waves using nonlinear optics. And there was an issue that people in Russia had published. So, they were Russians, and they had an idea that you could measure gravitational waves using nonlinear spectroscopy. And that was at that summer school in Bad Windsheim, that Scully one day came to me and said, “This Russian paper is wrong. It’s because of this reason it’s wrong, and here is chapter 5 of your PhD. Why don’t you work it out?” And Cohen-Tannoudji was at this conference, and so I went to him and said, “Scully just gave me these notes. I don’t know.” So, we started talking and Marlan joined and other people joined. And everybody had this belief that this paper was wrong by these Russians, and these were famous Russians. But of course, nobody could pinpoint what’s wrong, and where is it wrong?

So, it was my task as a graduate student to point out the mistake. And eventually, I redid all these calculations, and I developed my own formulas to do these calculations. And we found out that these guys — that everybody misunderstood what these guys had said. You know, these Russian papers at the time, they were only two pages. So, they had the beginning of the story and the end of the story, and everything in between was missing because they only published these short papers, and then they were translated. So, you had only these very short papers. Anyways, I worked it out and finally understood it. So, these were the main things in the thesis. As I said, it had different topics, and every time these were at that time published papers that people had noticed. As I said, the reason I got an offer to go to Helsinki was because of the work on gyroscopes, because Professor Stenholm had a consulting contract with a company to solve exactly that problem. He couldn’t solve it. He couldn’t. But I had found a very specific transformation of the equations, and I could solve it. This business with the Russians also caused a lot of excitement. There were people in France working on it, because Cohen-Tannoudji brought the problem to Paris, and then several groups in Paris started trying to understand this paper. As I said, of course, the Les Houches manuscript on this gravity as a dielectric medium, that caused also a lot of excitement, even later. So, that was the answer to that question.

Zierler:

Now, let’s get to Wheeler. What was it like to work with Wheeler?

Schleich:

Well, let me explain again. Now you see, I’ve had people like Georg Süssmann, who was a real professor, and I did my habilitation with him officially. I had worked with Herbert Walther, who built the Max Planck Institute for Quantum Optics. If you look around in Germany — by the way, you guys have stolen one of Walther’s students in America, and he got the Nobel Prize at MIT, Wolfgang Ketterle. He was a student of Herbert Walther, and Wolfgang and I were sitting together in classes in electrodynamics, next to each other. We did homework assignments together. So, we are old friends since the student days. When you look in Germany, who are professors of experimental quantum optics, or of quantum optics in general, you will be able to trace a lot of them to Herbert Walther. The fact that we have a Nobel Prize winner, Ted Hänsch, in Germany is because of Walther. Walther had brought him in ‘86 to Germany. So, Walther was extremely influential in the formation of German quantum optics. Not only the institute. So, for me, he’s also one of my big heroes. In that sense, when I opened the institute, at the opening ceremony, I thanked a lot of people at the end of the ceremony, and I mentioned Herbert Walther. Because as I said, when I was young, a Privatdozent, I was there at the opening of his institute. And of course, at that time, I never thought that one day I would be opening an institute. And then I worked with Marlan. So, Walther was a very good scientist, but also an excellent organizer. And of course, Marlan, outstanding scientist. I still admire him. Every day, the guy has a new idea what to do. And also, he’s an excellent organizer. He gets things done, too. And then I came to Wheeler, and Wheeler — I had a difficult time to relate to him in the beginning because, you know, with Marlan, it wasn’t a matter of talking about things. It was a matter of, let’s do a model. Okay, here is the problem we have to solve. Let’s write a simple model about this problem, and then we’ll solve the equations and get an answer, and that’s it. With Wheeler, it wasn’t solving equations. With Wheeler, he had a picture in mind, and it was all vague. So, here I am, trained with Marlan to crack problems and turn the crank and do calculations, first make a model, then solve it, interpret it. With Wheeler, it was philosophy and quantum mechanics and history. For me, that was very unusual. 

Zierler:

What was Wheeler thinking about specifically at that point? What was he working on? 

Schleich:

He was very interested in quantum mechanics. He had just finished a book with Zurek on the quantum theory of measurement. That had just been finished before, and he was always interested in the quantum theory of measurement at that time. He still had a little bit of relativity, but not very much. But the main thing was quantum mechanics, and entanglement, and Niels Bohr and his interpretation — the Copenhagen interpretation of quantum mechanics. 

Zierler:

Was Steve Weinberg there when you were in Texas?

Schleich:

Yes, he had the office across from me. 

Zierler:

Did you interact with him at all?

Schleich:

No, no. Weinberg was very different from Wheeler. Wheeler was an extremely friendly and open person, and he would never say anything nasty or bad about anybody, and extremely friendly, and he always had time to talk to people. But Weinberg was a very busy person, and I considered him unapproachable. Unapproachable. Now, when I came to work with Wheeler, Marlan was in Albuquerque, and we still had problems that we wanted to work out. So, what I would do is on Friday afternoon I would take a plane to Albuquerque and work with Marlan. And then I would fly back on Monday and would be back in Austin on Monday morning. The first year that I was there, I was talking a lot with Wheeler, but we never really did anything together. One day, Wheeler said to me, “You know, Dr. Scully and I have a wonderful arrangement. I pay your salary and you work with him.” So, I looked at him and I said, “Yeah, but you know, one day somebody else is going to pay my salary and I will work with you.” And that’s exactly how it happened in the end. I was back in Munich and Herbert Walther was paying my salary, but I was writing up all the papers that I had worked on with Wheeler. And I can tell you funny stories about that, too.

Anyway, the first year, I spent a lot of time working with Marlan, and I had difficulty to connect with Wheeler because my training was so different. Yes, I listened very carefully to what he said, and I could tell the guy was a famous guy and he understood, but his way of thinking was so different. So, then one day we were teaching a class on quantum mechanics together. He would give a lecture, and I would give a lecture, and then one day, he suddenly said in this class — so, now I have these pinched states, and these pinched states, I can tell what the energy distribution would look like, and he drew a picture there. See, he wouldn’t write equations. That was the difference with Scully. He would write down an equation and model. No, Wheeler would just write a picture. So, for a young person like myself, I couldn’t relate to this. So now, he started talking about these pinched states, and I suddenly said to myself, what the guy is talking about is called squeezed states. They were at the center of attention at that time, squeezed states. I knew the literature on this stuff, so later on I went to him, and I said, “You know, you mentioned these pinched states, but in quantum optics they’re called squeezed states. I’m convinced that what you said is not true, that the photon statistics in my language of these squeezed states would have the properties that you’re saying, because I know the literature on this topic.” Professor Dan Walls, he was the big pope of theoretical quantum optics in squeezed states. I said, “Walls has written a seminal review article on this field, and he never mentioned any of this stuff.” So, Wheeler looked at me and he said, “Well, why don’t we work it out together?” So, this was in the summer of ‘85. So, I started working.

Now I had a problem, because it was a well-defined problem. I could understand what it is that I have to do, and I started working. And then, suddenly, Wheeler came and said, “Hey, you want to stay another year with me?” He offered for me to stay another year, and I said, “Sure.” So, I came back, spent the summer at Max Planck, and then I came back to Texas. And I worked on this, he and I, and suddenly I connected with him. I listened carefully, I could understand the pictures that he had, and I realized that man is a genius. The guy doesn’t need equations to do anything. He just has these pictures. And you know, the last paper that we wrote was in 2000. So, I think I wrote about 20 papers with him. As I said, I did my habilitation on the work that we started there, and he was one of the referees, and Cohen-Tannoudji was one of the referees. I can tell you many stories about him, about the firecrackers, and the cannon he had on his island. He had this island in Maine, and I would come and visit him on the island. There are many things. That in itself is a long story. But as I said, for me, he had an enormous influence on me and my way of thinking about physics problems. He always said, “Never do a calculation unless you know the answer."

Zierler:

Did you learn anything from him about general relativity that you didn’t get in your formal education? 

Schleich:

No, I don’t think we ever talked about general relativity. It’s interesting because my thesis was in relativity, but as a matter of fact, I never had a formal training in relativity. I read books with Marlan. Landau-Lifshitz and Ohanian. I read a lot of books. And Tolman’s book. There were people there who did Regge calculus. Warner Miller did Regge calculus, and there was Ignazio Ciufolini. He tried to measure the Lense-Thirring effect with two satellites. No, Wheeler’s interest was quantum mechanics. 

Zierler:

What about black holes and quantum information? Did that ever register with Wheeler?

Schleich:

Yes. I mean, he was the one who pushed quantum information. Today when you hear all this quantum technology, he always would say quantum theory is information theory. They have to re-derive quantum mechanics from information theory. That was always his big goal, and this was long before today. In the ‘90s, when all this hype came up with quantum technology, Wheeler was already talking about this. And this also had to do with the fact, of course, that David Deutsch was there in the year from ‘84 or ‘85 while I was there. He developed the Deutsch algorithm while he was there. He talked a lot about quantum computing while he was there with Wheeler. So, I think he was strongly influenced too by Wheeler on quantum mechanics. But black holes, yes, we had a visit from Jacob Bekenstein. He came to visit us frequently. Asher Peres came. Also, Wheeler had a lot of visitors, and we learned a lot from them while they were there. So, as I said, I can tell you lots of stories with Wheeler. As I said, I developed a very different point of view once I understood how he was thinking. 

Zierler:

Did you ever think about staying in the United States, making a life for yourself here?

Schleich:

Oh, yeah. Just in ‘85, I met my wife. I met her, and she was a student at Austin, in the art department. She was studying to become a teacher in art and history. We met, and of course in the summer of ‘86 she was graduating, and if they would have offered me a position in Austin as an assistant professor, I probably would have stayed. Later on, I learned that there was some talk. Some people said, “Let’s offer Schleich a position. He would stay. He would take an assistant professor.” But at that time, the physics department had 70% theorists and 30% experimentalists. So, the argument was, we don’t need another theorist.

Anyways, so that was that. And then after I got my habilitation, Kathy and I were separated for four years. I went back to Germany, and she stayed in the United States and had to take care of the two children and was a high school teacher. So, what I did is I traveled frequently to give lectures in America. And every time I traveled to the U.S., can you imagine, she lived in Tacoma, Washington. That’s the furthest place to the west. But as I said, I had a lot of colleagues in America that I knew, and they invited me for colloquia and so forth, and I would show up. So, I was flying a lot, and Professor Walther was very generous with me, allowing me to do this. I can tell you a funny story. While I was in Texas, Wheeler and I did not publish a single paper. I always say, imagine a postdoc working for two years with somebody and not publishing a single paper. That would have been the death knell today. But when I left Texas, I had done tons of calculations on these problems with Wheeler, but I hadn’t published a single paper. And I came back to the Max Planck, and Professor Walther was generous enough to let me work on Wheeler’s problems and writing these papers for the first two years.

So, it was exactly as I had told Wheeler. “One day, somebody else is going to pay my salary and I will work for you.” And at that time, there was no fax, there was no email, so the way the papers were written, I had to get the papers typed with handwritten equations in it. There was no TeX. People typed the text, and then you wrote by hand the equations in it. And then I air mailed the manuscript to Wheeler, and Wheeler would rewrite the whole paper and put it in an envelope and send it back. Then, I would look at what he did, and then I usually would call him. And these phone calls were enormously expensive to the United States at the time, and only the director had direct access to long-distance phone calls. Not a little guy like myself. But I had one advantage. I was in charge of the colloquium of the institute, and because of this, I had the right to make long-distance phone calls. So, I would call Wheeler for an hour or two hours, and we would work on a paper. And Wheeler would say, “You know, Wolfgang, we need this and this introduction here. One second, let me think about it. Do you have a piece of paper?” And then he would start dictating an introduction from scratch. And you could get it typed, and that was the introduction. And Professor Walther had the habit, he would knock at the door, and the next moment, he wouldn’t wait until you said come in, he would be in the office. So, one day, he came into the office, and he sees that I’m talking to somebody in English. It must be a long-distance phone call. And he says, “To whom are you talking?” I said, “It’s Professor Wheeler.” And Professor Walther looked at me and he says, “I’ll come later."

Zierler:

Tell me how the opportunity at Ulm came about for you. 

Schleich:

Well, so you understand that Kathy was in Tacoma, Washington, teaching, and I was traveling constantly back and forth. Also, Wheeler had left Austin at that time, and he was now again in Princeton. But he had retired into a retirement home in Hightsown. There was a big retirement center there. So, what I would do is I would fly usually to Princeton to work with Wheeler on these papers, and then I would go on to another place, and then eventually I would end up in Tacoma and spend a week or two with Kathy. So, after I got my habilitation in ‘89, there were some job offers in America for professorships. So, I started interviewing. I went for example to Fayetteville, Arkansas, and I didn’t get the job because I asked for too much. That was important to learn that you cannot ask for too much. So, then I interviewed for a job in Baton Rouge, Louisiana. There was a gentleman there from whom I have learned a lot about Wigner functions, Robert F. O’Connell, a well-known guy in relativity. He had done a lot of theory on the Gravity Probe B experiment. And he also had worked with Wigner, on Wigner functions. So, I learned a lot from him about Wigner functions. So, he got me a job offer, I interviewed for the job, and I liked the place a lot. They gave me an offer for Associate Professor with tenure right off the bat. And that was very tempting, but then — I can’t remember why I turned it down in the end. I turned it down — maybe Professor Walther said, “I’ll get you a professorship at the Max Planck.” That’s probably what happened. Imagine this, I turned it down with just a promise. Nothing in my hands. I turned due to a promise. I trusted Professor Walther that he would do it.

Then, as the summer went by, a young man showed up who had just moved to the University of Konstanz. Do you know where Konstanz is? It’s Lake Konstanz, south of here, on the border to Switzerland. They have a university about the size of Ulm. And this gentleman, Jürgen Mlynek was his name, he was a young guy, and you could tell he was another Walther. He’s building up and he gives young people a chance. He gives them responsibilities. He’s not running around trying to be behind everybody. Not a micromanager. Quite the opposite. Definitely not a micromanager. And he came to me, and he said, “You know, I have a professorship in theory. Why don’t you apply?” And I looked at him, and I said, “Look, Konstanz. I’m here in Max Planck. I’m here in the center of quantum optics. Why would I go to Konstanz?” And he says, “Look, Schleich, you’ve worked with a lot of famous people. Scully, Walther, Glauber —” by that time I had written papers with all of these guys, “but nobody knows if you did the work, or they did the work and you just put your name on the paper. You have to really get out of the shadow of these famous people, and you have to establish yourself. In a place like Konstanz, you can establish yourself because there’s nobody there.” I thought that was an interesting remark.

So, I went to Professor Walther, and I said, “Look, Professor Mlynek asked me to apply for a job in Konstanz.” And the director said, “Don’t do this. Konstanz is at the Arsch der Welt,” is said in German. “It’s in the boonies. Don’t even apply.” So, I said, “What the heck. I can apply for this job. The worst thing is they don’t even invite me, so what the hell?” So, I sent in my resume. Sure enough, I get invited. So, I go there and give the talk, and a lot of people suddenly said to me, “You know, that was good that you came and gave the talk. We got a very different impression about you.” So, now I realize for the first time in my career, there are people out there that are not friendly. They’re saying bad things about you because they’re jealous, or because of some reason. 

Zierler:

So, you demonstrated that you weren’t just an appendage to the research of these luminaries, that you had your own contributions.

Schleich:

That’s right, because I could answer everything they asked me. I could explain, you could do this, you could do that. People suddenly realized this guy is not just the little guy of Wheeler or Walther or Scully or whatever. Obviously, that was what some people had said about me. So, I got the offer. This is not a simple thing in Germany because it’s not just the president of the university, it has to go through the ministry, and the ministry checks the paperwork. So, eventually I got an offer at this job. But at the same time, I had applied for a chair professorship in Ulm. This was like what you would call an Associate Professorship. But at the same time, there was this professorship here in Ulm, which was a chair. Chair means you get to have a secretary, you get postdocs, and of course, it’s lifetime. That was up here in Ulm, and one of the professors in Ulm, I had worked with, Professor Hannes Risken, a pioneer in theoretical lasers theory. He had come from Hermann Haken in Stuttgart, had been a professor in Minneapolis, and I had worked with him and written several papers. He told me there’s a chair professorship in Ulm, and why don’t I apply for it? I was 33 at that time. So, I applied, and eventually I got it. I got the professorship in Ulm, so I could turn down the one in Konstanz. Usually, you cannot turn down the first professorship you get in Germany, but when you have two offers and one is better than the other because it’s a higher position, of course you can turn it down. It was the same ministry as Stuttgart. Konstanz and Ulm both belong to Stuttgart. And I came here in January of ‘91, and I built everything up here. 

Zierler:

What was your goal at that point? Finally, you have your own professorship. What did you want to do so that you’re not always under the shadow of somebody great?

Schleich:

Well, the first thing I did is I got Kathy to come here. Finally, we were together. We started doing the apartment, and then eventually we got this house that we’re in. But what was the first thing? You know, I still remember the first times I came to Ulm. I took the train from Munich. I don’t know if you have a feeling how far Ulm is. It’s just an hour and 15 minutes with the fast train. There’s a fast train that only stops in Augsburg, and then in Ulm. So, it takes from Munich center to here, an hour and 15 minutes. So, at the beginning, I didn’t have an apartment at the summer of ‘91. So, I would take a train in the morning, and in the evening I would go back. And every morning I came into the institute in Ulm, I would say to myself, it’s wonderful. I don’t have to ask anybody anymore. In Munich, you wanted to travel, there was an infinite amount of money to go to conferences, to hire people, but every time you wanted something, you had to ask the director of the institute. Not that the director wouldn’t give you the money, but just the fact that you had to ask provided a threshold, at least for me. So, now I realized, I don’t have to ask anybody anymore. I’m free. Except I didn’t have any money. So, I had to go out and get money.

I still remember, in December of ‘94, I had a group of probably 20 students, and the dean came to me and said, “You know, Schleich, you have all these people. Two questions for you. What are they going to do eventually when they graduate, and secondly, how are you going to pay these people in half a year from now?” And I said, “I have no answers to any of these questions. I can tell you what they’re doing right now. I can tell you the science. But I don’t know how I’m going to feed these people half a year from now, and I don’t know what type of jobs they’re going to have in the end.” But these guys were excited about what they were doing. As I said, Professor Risken was 60 — not even 65. He was 58 when I came and was hired. He was a wonderful person, a very nice colleague. He developed a brain tumor, and we only had an overlap of two years, and then he died of a brain tumor. Very, very sad. So, he helped me a lot. He gave me his manuscripts to teach the courses and everything. It’s a small department. At that time, we were like 18 professors, and we had to teach all the courses and everything.

One day he came to me, and he was the dean at the time, and he said, “You know, there’s a prize by the German Science Foundation, which is called the Leibniz Prize. It’s for young people who have shown some promise and done already some work that was recognized. I’m going to nominate you for this prize because you’re probably one of the youngest professors in theoretical physics in Germany. You might have a good chance of winning this prize.” And that prize was one million marks, €500,000. In today’s measure, it’s probably nothing, but in the ‘90s, this was the biggest prize in Germany you could win. So, Professor Risken nominated me together — I think all the physics faculty nominated me except one colleague. He decided, no, Schleich is not that good. So, they decided to nominate me, and then Professor Risken died. I had forgotten about this prize. As I said, this dean came to me and asked how I want to pay all these people a year from now. As I said, I had a little money from here and there, and I combined it, and sometimes the dean had money left over and gave me money. As I said, I really didn’t know.

And I never forget, it was in December, and I was teaching quantum mechanics, and I had a terrible cold at the time. Terrible cold. And I just barely made it through my class, and I got back into the office, and I told the secretary, “Look, I need a few minutes break to relax.” So, she said, “Okay, you sit there.” And I closed the door, and then the phone was ringing. I said to myself, shit. I told this lady not to give me any phone calls. I can barely speak. And I picked up the phone, and there’s the press secretary of the university on the phone, and says, “So, Professor Schleich, what do you think now that you’ve got the Leibniz Prize?” I said, “What do you mean, the Leibniz Prize?” He says, “Yeah, it’s in the news. You got the Leibniz Prize.” I said, “I don’t believe that.” He said, “Haven’t you heard this?” I said, “No, you’re the first person to tell me.” So, I called up Kathy. I said, “You know, I got this phone call that I got the Leibniz Prize.” Kathy says, “Don’t believe these guys. They’re doing a prank on you.” So, I called up the German Science Foundation, and asked them. And they said, “Oh, yeah, sure, you got the Leibniz Prize. Haven’t you heard this yet? Absolutely, it’s true.” So, I always felt it was a prize from heaven because Professor Risken was dead by the time I got the prize. And now, I could pay all the students. So, it worked out in the end.

So, I was very fortunate over the years. I got all these recognitions and money, and I could pay people. I was always concerned that I would use the money that I got on these prizes to help young people enhance their career with it. And also spread the money, not just keep it for myself. I invited lots of people with the money. Roy Glauber came and spent a lot of time. One of the persons — and I owe this to Ted Hänsch, by the way — Willis Lamb, the PhD supervisor of Marlan, came and spent several summers in Ulm with me. The reason I could do this was because of Hänsch. As a young professor, I was walking at a conference with Hänsch in the forest. We knew each other from the Max Planck. He was one of the directors when I got my habilitation. He said, “Well, how is it going in Ulm?” I said, “One problem is I would like to get Willis Lamb to come to Ulm.” And he says, “Why don’t you get him a Humboldt award?” And that’s what I did, and Lamb came and spent the summers in Ulm over about ten years. And the students were really impressed that there was a Noble Prize winner in Ulm, and they could even talk, and he was extremely nice with these students.

I can tell you many stories with him, too, and about the physics and what we discussed with him. And his first and his third wife was the grand-niece of the man who wrote the national anthem of Germany, Hoffmann von Fallersleben. She was German and related to Hoffmann von Fallersleben. And then his third wife, she was also in Ulm, was Bruria Kaufman. She was the last postdoc of Albert Einstein. I don’t know if you’ve heard of her. So, both Bruria and Willis spent a lot of time — almost every night they came and had dinner with us in our house, or in our apartment. So, I learned a lot about the life of Bruria and Einstein, and there was a lot of talk. And Willis told me about how he found the Lamb shift. That was all interesting. I had students and postdocs, and as I said, when I got the Leibniz Prize, I got a lot of visibility. Eventually, I got into all these committees, but I always try to do science and write papers myself with people.

So, I think that’s pretty much what I can tell you. And as I said, we started in a funny way. We started in the middle of the story and then went back. So, I worked in many different fields, and I was never interested — and I’m still not interested in just one field, but connections between different fields. I can tell you a funny story, when this business of quantum information came up, and quantum computing, a lot of colleagues of mine would go around and say, “Oh, it’s interdisciplinary and it involves mathematics and number theory and blah, blah, blah.” But I noticed that these people never really learned number theory. And one day, I had a visitor, experimentalist, and he asked me a question about the Riemann zeta function, and whatever. And I couldn’t answer the question. So, I said to him, “You know what? I know who can answer the question. The guy has a chair here in Ulm on number theory, and he’s a well-known person in analytic number theory. Why don’t we get him over here and discuss with him about this topic?” So, I called up my friend Helmut Maier, and he came and gave an impromptu lecture on analytic number theory on my blackboard. And I said, “Hey, that’s great. Why don’t you give us a crash course in number theory? Just four lectures. Important topics out of analytic number theory.” And I took notes like I did when I was a student, and then in the summer, I worked out these notes and did homework assignments for myself. And then I said to myself, let’s hire a student who puts all this into TeX and helps me write this book.

So, I learned analytic number theory. And then I started working with this Professor Maier. We wrote several papers, he as a mathematician, I as a physicist, and we wrote this unusual book. Unfortunately, it’s not even finished yet because I always start something new, but I lectured out of this manuscript. I gave many courses. Quantum Mechanics and Analytic Number Theory, it was called. So, it was a mixture of quantum mechanics and analytic number theory. And I used this book that I wrote. And then people started doing experiments. This was a method to factor numbers which I called the Gauss factorization. It relied on so-called Gauss sums. And suddenly, people call me up and say, “You know, I did this experiment that you proposed.” The funny thing is, I never published these papers. I talked about it at conferences and people — there was a guy, Michael Mehring, at the University of Stuttgart. He had listened to a number of my talks on how to factor numbers using Gauss sums. And one day, he showed up in my office, and I said, “I want to show you a plot. Can you understand what this plot is about?” I said, “Yeah, you factored this and this number. So, you did it with Gauss sums?” He said, “Yeah.” So, we wrote a paper in Phys. Rev. Letters together. Several experimentalists suddenly came and said, “Oh, I did what you said.” As I said, also, this quantum mechanics/gravity. It started out in my thesis, light and gravity. Now it is matter and gravity. So, it’s never one field. If I would have stayed with Professor Salecker, I would have stayed in quantum electrodynamics my whole life, I’m sure. But it was Marlan who always said, “Let’s connect this with this.” Wheeler also said that if you want to understand a theory, you have to go the extreme of that theory. You test the borders of the theory. You don’t stay in the center of the theory. You go to the borders. 

Zierler:

Wolfgang, going into the 2000s, what were some of the advances either in computers, or instrumentation, or technology more broadly, from an experimental point of view that might have been compelling to you theoretically?

Schleich:

Well, the point, I think, why quantum optics developed into such a big field that eventually quantum technologies evolved out of them was because of people like Walther and Haroche. They could suddenly manipulate individual quantum systems, like individual atoms. Now, of course, I should mention people like Dave Wineland, and Jerry Gabrielse, and of course a gentleman in Hamburg, Peter Toschek. You probably never heard of him. 

Zierler:

I did not hear of Peter, no. 

Schleich:

Peter Toschek was the PhD supervisor of Ted Hänsch, and he did the first experiment of a single barium atom in a trap, and you could see the light from that single atom. So, it was really, in my opinion, in the ‘80s and the early ‘90s, that people learned how to manipulate individual quantum systems. Make transitions. Not in an ensemble of atoms, like an atomic beam or a gas cell, but rather have, like Herbert Walther, he had a very thin beam of atoms, and it went through a cavity. And only one atom at a time was in the cavity. So, you could really see one atom interacting with one mode of the electromagnetic field. We saw, for the first time, quantum dynamics. The technology that was there was microwave technology, and cavities of high Q-value. So, the decay rate was extremely slow. And so, you remember, Haroche was the big competitor of Herbert Walther in Paris. By the way, there’s an interesting thing, and I always felt — of course I don’t know about it, but my interpretation — when the Nobel Prize was awarded to Haroche and Wineland, there were only two people. Herbert Walther had died before, and I always felt that if Walther had been alive, he would have been the third one. He died very young. He was only 72. He died of cancer. So, it was people like them, in my opinion, and also two gentlemen that should be mentioned definitely in this context. These were experimentalists. It was Leonard Mandel in Rochester. Len Mandel was certainly also instrumental in understanding quantum mechanics of photons. He did beautiful experiments, pointed out beautiful effects of light and single photons. So, he was one, and then of course, Jeff Kimble, first in Austin, and then at Caltech. I don’t know if you’ve talked with Jeff Kimble yet. 

Zierler:

No, but I will. 

Schleich:

Yeah, Jeff Kimble is certainly somebody you should talk to. He did beautiful experiments in Austin. You know, there was a competition in the mid ‘80s, who creates the first squeezed state of light? And it was at Bell Labs. A gentleman by the name of Richart Slusher. He did the first squeezed state, but it was very small squeezing. Already at that time, Jeff Kimble in Austin, a young, by that time probably Associate Professor, had worked day and night and had seen squeezing but he wasn’t convinced. He always said to me, “You know, I’ve seen it, Jan Hall has seen it. But until my wife can see it, I don’t believe it myself.” He was a very critical person to himself too. And then, one day, he came, and he said, “I’ve seen it now.” He had seen — I can’t remember how much. People had seen 10% and it was a big sensation, but he had seen 80% of squeezing. That was a sensation when it was brought up. And then Jeff and [?] did really beautiful experiments in quantum electrodynamics, cavity QED in the optical domain. Walther and Haroche worked in the microwave domain, and he worked in the optical domain. And all of these things were essential ingredients in order to do quantum technologies and quantum information processing as we know it today. A lot of these things developed based on these experiments. We could have done the theory of this stuff before, but — look at Scully’s PhD, the quantum theory of the laser. At the time when the thesis was written, there were hardly any effects that needed the theory. There was the line width that needed the theory. You couldn’t explain the line width of the laser without Scully’s quantum theory. And the photon statistics of the laser, you couldn’t explain that either. So, you needed the quantum theory, but that was about it. But suddenly, with all these experiments that came up — Walther, Haroche, Kimble — suddenly, the Scully theory of the laser could be applied, and opened up this whole field that we have today of quantum technology and quantum information. You know, I’ve always felt Scully deserved the Nobel Prize for the quantum theory of the laser. Of course, Professor Hermann Haken, he’s over 90 by now. He was the big man in Germany who developed the quantum theory of the laser. I always felt that both of them — and they did this work independently and different approaches — both of them, in my opinion, deserve a lot of credit and recognition for this work, because we wouldn’t be today where we are without their theory. 

Zierler:

Wolfgang, what were your motivations in writing Elements of Quantum Information? Was the idea that the field was now mature, and it needed that kind of standard work, or was your sense that the field was heading into maturity, and you wanted to get your ideas in with the textbook at an early stage?

Schleich:

Let me make sure that I understand the question correctly. I wrote a textbook called Quantum Optics in Phase Space. That’s really the textbook. The book that you mentioned is a summary of points of views of different people. So, the Elements of Quantum Information that I did together with Herbert Walther, shortly before he died, that was a meeting where several people in the field gave talks and then summarized their work in these articles. 

Zierler:

But to make this into a book, is the idea that quantum information is now a mature field?

Schleich:

No, not at the time when we did this. Not at the time. It was still flowing, and you could tell that there was something important happening. That’s why we felt we bring together people who were working in this field, and they had to summarize what they were doing in this field and say where it would go. Of course, that was very different from this textbook I wrote. The textbook is also, I felt, unique, because a lot of people wrote books on quantum optics, but nobody had the phase space concept behind it. This is what I learned. If you ask me, what have I learned from Wheeler? If I would like to talk to Wheeler, he would say, “Tell me in one sentence what it is that you want to say.” So, you couldn’t spend ten minutes explaining. He wanted it in one sentence. So, that forced you to really think about it and boil it down to the one sentence. What is it that I learned from Wheeler? Never do a calculation unless you know the answer? No, I learned from Wheeler that quantum mechanics becomes simple when you look in phase space. All this talk about Hilbert space, and all this other noise that goes around, it only becomes simple when you look into phase space. And then suddenly the transition between classical and quantum is just a difference between positive probabilities and negative probabilities.

For example, the famous particle that diffuses, and you solve the Schrödinger equation, and it’s a nightmare to even solve that problem in the Schrödinger equation. It’s very simple what’s happening. You have an ensemble of particles in phase space, and they just spread classically, in the same famous spreading of the wave packet, there is no quantum mechanics whatsoever. It’s a classical effect. The only quantum mechanics is the initial state, and to look at quantum mechanics from phase spaces, what I learned from Wheeler, of course, you could ask lots of details but that’s the bottom line. And to look at quantum optics from that perspective, that’s what’s in that textbook. You asked me at the beginning, what is it that is associated with you or with your work? And I think it’s phase space. So, first you asked the question about my PhD thesis, but I thought at that time you were thinking more in general terms. I would say, everything that goes through my almost 400 papers is phase space. It doesn’t matter if you talk about number theory. I wrote a paper about analytic continuation in complex analysis, and entanglement in quantum mechanics. There’s a connection between analytic continuation and entanglement. You can interpret this also in phase space. So, I feel like — Wheeler once had a theorem when he was young that said everything is fields. There was a time period in his life where he believed everything is fields. I have to explain everything in terms of fields. Well, a couple decades later, he said, everything is particles. So, I have this thing, everything is phase space. 

Zierler:

What was it like to win the Lamb award in 2008?

Schleich:

Oh, that was a big honor for me, I have to say. 

Zierler:

Thinking about Marlan Scully, it must have been amazing. 

Schleich:

Yes, and since Willis spent the summers with me, and we spent so much time, and suddenly I would get a prize that is associated with his name, and I could say, “I won the Willis Lamb award,” that was a big honor. I don’t know if you noticed, but this year, ‘21, I got the Herbert Walther Prize of the Optical Society of America. And I told you how important he was in my career. So, as I said, I’ve been very fortunate, David, that I had these professors who helped me in my career. I always say, I want to give back to the next generation, the present generation, by helping them in their career like they helped me. 

Zierler:

Did you ever get involved in LIGO, or were you just an interested observer?

Schleich:

I was an interested observer. I had a lot of relations to the people. For example, in the Max Planck Institute, we had a group on gravitational wave detection. And the first man, the first experiment was in the neighboring institute by Professor Heinz Billing. He eventually retired — so, Billing first worked on Weber bars. Joe Weber in Maryland did experiments on these resonating aluminum cylinders, but then people moved to this Rai Weiss concept of laser interferometers. So, the people in Munich, there was a group of maybe ten people, they also moved to the interferometer, and it was run by a guy named Billing. It’s interesting, the man died when he was like 104 years old, and gravitational waves were detected maybe two years before he died, and he still was smart and with it, and he knew it that they were found. So, he spent a lot of his time on these gravitational waves, and eventually he lived long enough to see that they were found. Now, one of the guys in gravitational waves is this guy, David Reitze. I knew him when he was a graduate student in Austin. He was the first graduate student of Mike Downer in Austin. And Downer, when he was hired as an Assistant Professor, stayed in the same apartment complex that I lived in. And of course, the gravitational wave group in Munich eventually was taken over by Gerd Leuchs, who is now Max Planck director, or retired, in Erlangen. He was also a student of Herbert Walther. So, now we’re coming to almost 7:00 evening here, and we passed three hours. 

Zierler:

I want to ask a few questions for the last part of our talk, if you’re able. 

Schleich:

Yeah, sure. 

Zierler:

As we come closer to the present in our narrative, where you do see the reality, and where do you see the hype in true quantum computing?

Schleich:

That’s a good question. I can tell you how our institute, the institute that I was fortunate enough to open two weeks ago, where we are interested, where we are positioned, I should say. I mentioned in the very beginning, Hansjörg Dittus, the man with the drop-tower, now CEO in the DLR. He and I convinced the Ministry of Economics in Berlin to invest a lot of money into quantum computing. In contrast to the United States, we don’t have Google, or Amazon, or huge internet companies. We don’t have this in Germany. We don’t have this environment. I would say that we have at least an ecosystem at universities in quantum physics that is at least equivalent, if not superior, to the one in America. So, we are very strong in the sciences and in the foundations of quantum information and experimental quantum computing. What’s lacking is the industry. So, we convinced the ministry to invest €740 million to build a German quantum computer, and not by giving them money. As I said, we belong to the Ministry of Economics. Not by giving it to universities, but to the industry. And there are startup companies.

You see, the problem is in Germany, huge companies like Siemens or ZEISS, they have to think carefully about where they invest money. Where is the immediate gain? Somehow that mentality does not exist in America. The big companies — some like Google and so forth, they invest on long-term goals. Somehow that’s not in Germany. I had a conversation with a CEO about a year ago of a huge company. A big, big, mega company in Germany. And I said, “Look, we give you money, and you help us build a quantum computer. We pay for it, but you have to take people in your group, and we pay them.” And the answer was, “We can’t do this.” I said, “Why not?” He said, “If I hire 100 people to help you with this big initiative, and after four years we haven’t built it, and we don’t get more money, I have to fire these people.” I said, “Yeah, and?” He said, “That’s a big scandal. I cannot fire 100 people in my big company. You can only do this with startup companies.” See, in America, you wouldn’t have this thought. If there’s no money, they cannot hire people. No. In Germany, we have to be very careful, because if a big company makes a headline that 100 people were laid off, oh boy, that’s a big problem. So, we thought okay, we’re going to do it with startups. And my institute, because of its background in quantum technologies, will play a major role together with a big company in north Germany, in Hamburg, and various other DLR institutions, to build this quantum computer. I call it something like a Manhattan Project. Of course, the Ministry of Science has also invested money, about the same, maybe even more, but they are working mainly in the universities. But there are many startup companies in Germany, mainly by professors, that are trying to do this now. And my feeling is, and this is why I understand when you say hype, my feeling is we are ready to take over the thing — we can go on and do basic science in quantum computing for the next 20 years, and that’s what the universities want to do because that’s where that funding is coming from. But I’m sure that we can take the knowledge from the universities together with the companies and really do something. You know, your friend Elon Musk — 

Zierler:

Not my friend. 

Schleich:

I love this guy, okay? He built a big factory in north Germany now. He came and said, “Let’s build the biggest factory for batteries.” And people said, “We don’t even know how to build the best battery.” And he says, “Look, we build the factory, we develop the batteries, we all do it at the same time. We cannot do it consecutively. We cannot do it in a sequential thing. We build it all together, and then we develop it.” That’s exactly what I’m saying. Sure, we don’t know what the best platform for quantum computing is. We don’t know that. It could be NV centers, it could be ion traps, it could be anything. So, people say, “Let’s first find out which one is the best.” And I say, “Come on, let’s just do it all at the same time and work it out with the industry."

Zierler:

So, it makes me wonder about the analogy to the Manhattan Project, because obviously there, there was a very specific end product in mind. 

Schleich:

Yes, and the same end product exists here too, a quantum computer. But it wasn’t 100% clear which path you should go to build an atomic bomb. 

Zierler:

No, but what I mean is, there was a very specific understanding of what the atomic bomb would be “good” for, “good” obviously in quotation marks. Do you say that there’s a specific understanding for what a quantum computer will be good for? 

Schleich:

I’ve heard that question in some sense before too, but then I say, look at the laser. When the laser was invented, lots of people said that’s a solution looking for a problem. And yes, indeed. Well, there are certain algorithms that exist in quantum computing, and we know they’re better. But I can tell you, look at the Deutsche Bundesbahn. You guys don’t have this in America. Well, you have it little bit, but you’re working with freight trains. Of course, we have freight trains but also, how do you call it, people trains. 

Zierler:

Passenger trains. 

Schleich:

Passenger trains, yeah. So, we have passenger trains, and these passenger trains make a lot of the traveling in Germany. That’s a little bit on the East Coast and maybe on the West Coast in America, but it’s not across the country anymore. But in Germany, passenger trains are key. It really doesn’t matter if it’s passenger or freight trains. Imagine, a train cannot run in that day, because the engineer is drunk, or the engine broke down. That has immediate consequences on the whole schedule of other trains. And this is not a trivial problem because it’s a network that is interconnected, and you start fooling around at one point, it has immediate consequences. People think that maybe you could calculate with a quantum computer these effects and find out what you need to do. People at BMW or Volkswagen come and say can we find better ways on a conveyer belt, on an assembly line, how to make this more efficient when you’re putting together a car? There’s no way you can easily calculate this in my opinion with a classical computer. So, people are wondering, is there a way of doing this with a quantum computer? People at Boehringer Ingelheim are trying to get new pharmaceuticals, new medicine. Of course, you can put it all together with chemicals and try it, but that takes forever. If you could do this with a quantum computer, it might work. So, Boehringer Ingelheim supposedly has a big department on quantum computing. So, I could list many more. 

Zierler:

The question is provocative because it has a certain bias towards applications, but what you’re saying is the basic science value of quantum computing is self-evident. 

Schleich:

You know, there’s one thing, I’m talking with a gentleman. He’s a physicist and he works in the ministry in Dresden. I talk with him regularly. I talk with unusual people. I talk with a gentleman and his group in the Bundesdruckerei. I wouldn’t know what the English version for it is, but they are printing the passports. They are printing sensitive documents. So, for a long time, they were part of the German government. Now they’re an independent company. But their main issue is security. How do you print money that cannot be forged? How can you give information to somebody and the person who gave you the information does not have the information anymore? Security issues. And how can quantum mechanics help us? Or is it that quantum mechanics does not allow you to do such a thing? My first reaction was, let’s take a two-level atom — trained by Scully, we always take a two-level atom — and we excite the atom. And we give the other person the atom, and the guy now has the information that the atom is in the excited state. And immediately, I said, “No, that’s impossible because you needed light in order to excite the atom.” So, the other photon that excited the atom is missing in the cavity of the person who created the atom. So, even if the atom is over here, and the man on the left-hand side can’t touch the atom anymore, he has the information stored in the photon. So maybe quantum mechanics does not allow you to build such a thing. I find there are so many questions, and as I said, I would have never thought about the question if it wouldn’t have been for the people in the Bundesdruckerei. So, in the government, there are many questions related to security, of course. And now people are waking up to these properties of the quantum. 

Zierler:

Wolfgang, one big question, retrospectively, before we end looking to the future. In what ways have you seen theoretical advances in quantum optics advance our understanding of quantum mechanics itself?

Schleich:

I think that we have learned, in the years that I’ve been in this field, we have learned that quantum mechanics does not only describe ensembles of atoms, but it describes individual events as well. However, of course, in a different averaged system. I don’t know 100 years ago, 50 years ago, people would have thought that’s possible, that you can use quantum mechanics to describe individual systems, and even employ phenomena. This is what I find is the amazing thing. When you think back at 1935, and Einstein, Podolsky, and Rosen published this paper about, is quantum mechanics complete? And they look at this measurement here influencing a measurement here. We learn from this, as Wheeler would say, that reality it doesn’t exist. Properties of quantum systems don’t exist unless we make a measurement. You would think that is irrelevant. That’s an interpretation. That’s a philosophy. You cannot tell if something exists or not exists. You know, as Einstein once put it, I can’t believe the moon is there just because a mouse looks at the moon. So, the important role of the measurement is there.

And then came John Bell, and Bell found a way to put this question of philosophy into equations. This was in ‘63. ‘35, ‘63, and again nothing happened. People admired this paper, but it was only then, David Deutsch with his algorithm, that he showed that you could do something with entanglement that you couldn’t do with a classical system. So, it’s not so much that we learn something new about quantum mechanics, but we are applying quantum mechanics to something that we wouldn’t have applied it to before, to applications. And then people started wondering, can we use entanglement as a resource to do things — it’s what I call quantum dense coding, or to make secure communication? Maybe this is a good story to end. I always liked this story, and I told this story also at the opening of the new institute. Michael Faraday was visited one time by the chancellor of the treasury, William Gladstone. And Faraday showed him how a current going through a wire would turn a magnetic needle. Gladstone said, “Yeah, but what is it good for?” And Faraday said, “One day, your excellency, you might tax this effect.” So, there’s a long way from the foundation to the application. And ‘35, EPR, entanglement. Schrödinger says entanglement. The mathematical formulation of bringing out what’s the difference between classical physics and quantum physics with the Bell inequality, ‘63. And then in the ‘90s, of course prepared by the path of Deutsch and Feynman, finally, what is it that we can do with this? And we finally discover we can employ entanglement and do things that we couldn’t do before. There’s even people talking about quantum thermodynamics, that you can build machines that are run by quantum thermodynamics. Not the classical thermodynamics, but quantum thermodynamics. And people are talking, can we violate the second law of thermodynamics based on quantum mechanics? The answers are no, but maybe a little. Whatever. It’s still an interesting field. 

Zierler:

Last question, Wolfgang, looking to the future. You’re working in a field right now where there are going to be fundamental advances for decades, if not centuries. What do you want to do personally, for as long as you want to be active in the field? What’s most important for you as you think about your own contributions?

Schleich:

Oh, you see, my own contributions, this sounds a little bit arrogant if I say my own contributions. 

Zierler:

I asked you. You don’t have to worry about being arrogant. I asked you to talk about them, so it’s on me. 

Schleich:

Actually, a very close friend of mine, by now an elderly gentleman and a big pioneer in quantum optics, at a dinner party in Washington once said — we were sitting and having dinner with a lot of people, and jokingly I said, “You know, when I was young and arrogant,” and the guy stopped and looked at me and says, “When did that change, Wolfgang?” So, no, my feeling is still, as I said earlier, giving back to the new generation. Big discoveries are made — let me put it this way. I think I had the great fortune to have worked with many famous people, and I’ve learned a lot, but as Newton said, we stand on the shoulders of giants. The funny thing is, by the way, he didn’t mean this in a nice way. He referred to Huygens, who was a dwarf, and of course, he couldn’t stand the guy. So, it was a sarcastic way of saying — but we always quote it nowadays, of course, in a positive way. 

Zierler:

And you would mean it un-ironically, given who you learned from. 

Schleich:

Yeah. So, I say, we stand on shoulders of giants when we talk about these things. And what is it that we want to understand? If you ask me, where do I think that we will find — you know, for me, I’m still fascinated with this Riemann hypothesis. The old question of, why are all the zeros of this function on one line? And people are still working on this. As I said, I have also written some papers on this. And I had this question, can we learn something out of quantum mechanics? People have worried about quantum mechanics and the Riemann zeta function. I’ve had a different approach towards it with a time-dependent question. And that’s when I found out that in the context of this Riemann problem that there must be a close connection between the complex analytic continuation and entanglement. My feeling is that that’s — every time we will make progress and a deeper understanding of our field is when we go outside of this box, when we connect two different fields. I would expect that the work on cold atoms at the interface to gravity — you know, the stuff that I mentioned earlier — that will shine some more light on quantum gravity. We still don’t have a good idea what quantum gravity will be about. And some people like Danny Greenberger — I don’t know if you’ve talked to Danny Greenberger — I wrote a lot of papers with Danny, and Danny is convinced that by understanding the equivalence principle correctly, we will finally understand how to quantize gravity. He’s convinced we’re barking up the wrong tree right now, because we haven’t understood the basic principle. My feeling is always, indeed, that something new will emerge not by doing — that’s what Wheeler would have said — not by doing complicated calculations and complicated mathematical questions. It will be the right question, the basic principle that starts everything. If you find a way to make that collapse, the whole theory will collapse, and you will find how to build it again. 

Zierler:

Is that to say that Wheeler was not holding his breath for string theory?

Schleich:

I remember distinctly — here is a funny story — when I came in ‘84, Wheeler took me to the astrophysics lunch. So, every month, on a particular day, the astrophysicists and physicists would meet to have lunch together in the faculty club. So, Wheeler took me along, and I was deeply impressed when I was sitting at this table eating lunch, and there was Prigogine, there was Weinberg, Sudarshan, Wheeler — all big names. And there was a discussion — it was the beginning of the semester, and Weinberg said, “I just heard a lot about Kaluza-Klein theory, and I read about Kaluza-Klein. I’m working now on Kaluza-Klein. Kaluza-Klein is going to solve all the problems we have in quantum field theory. And a lot of the young professors were nodding their head, and they were deeply impressed. You know, Weinberg spoke. That wasn’t my field, I couldn’t judge anything. I couldn’t say anything about it. Well, a year later, again the semester starts, the first astrophysics lunch, and Weinberg says, “Oh, by the way, during the summer, I learned about superstring theory and superstring theory is going to solve all the problems we have in quantum field theory.” And then I realized if a year before it was Kaluza-Klein, and now it’s superstrings, that’s not worth bothering for me, because every year it’s another fad. So, one day, somebody came and said to Wheeler, “Professor Wheeler, what do you think of superstring theory?” And Wheeler looked at the guy and said, “I’ve learned one thing in life. Never run after a bus, a girl, or new unified field theory, because there’s always another one coming.”

Zierler:

Wolfgang, it has been so fun spending this time with you. I’m so glad we were able to do this, and I’m delighted that Marlan connected us. Thank you so much. 

Schleich:

Well, thank you.