Theodor Hänsch during the interview in Lindau.

Transcript from an interview with Theodor W. Hänsch, 2005 Nobel Laureate in Physics, at the 58th Meeting of Nobel Laureates in Lindau, Germany, July 2008. The interviewer is Adam Smith, Editor-in-Chief of Nobelprize.org.

Theodor Hänsch, welcome. We’re here in Lindau, on Lake Constance, for the Lindau Nobel Meeting, which gives young researchers from around the world a chance to mix with Nobel Laureates, listen to them and speak to them. What do you hope that the young people here will take away from this meeting? What do you think they can learn from this?

Theodor Hänsch: It’s a thrill, of course, to be at the meeting where so many Nobel Laureates whom one only knows as legends and from the literature, where they are there to touch, to ask questions, to get autographs. So it’s a special spirit, but I think it’s enjoyable also to the Laureates, because otherwise they wouldn’t come so many times. But it’s also an opportunity to try to transfer some of the enthusiasm and excitement that we feel for our science, to the young generation, because, after all, science depends on young people entering.

I mean presumably they’re a pre-selected bunch of excited people anyway, but they …?

Theodor Hänsch: Yes, now I was very impressed. We had the students’ discussion yesterday afternoon for 2 hours, and people had actually read up and they asked very good questions, they were knowledgeable, so it was obvious that these are selected students from around the world.

Yes, I was going to ask you what sort of questions they ask, but they’re actually asking about the physics as well as the sort of background?

Theodor Hänsch: Most of it actually was about physics, about my talk and about some other areas of our research that they had apparently read about, and very few general questions.

Right, that’s very interesting. And do you know, I mean every, not every country in the world, but a very large selection of countries is represented here, do you notice big differences in the sorts of questions you’re getting asked by people from different regions, or …?

Theodor Hänsch: But still of course it’s also nice to see people from so many different countries. Some ladies dressed up even with a veil, only with a slit open for the eyes.

Yes, it’s a wonderful look at the world’s diversity, yes. If we turn to your own scientific beginnings, you began as a sort of experimental chemist as a child.

Theodor Hänsch: Not chemist, OK, well if you want to call this a chemist, a hobby chemist.

Exactly, exactly. I mean you’ve previously said how you actually for a while you lived in Robert Bunsen’s house in Heidelberg, the inventor of the Bunsen burner.

Theodor Hänsch: Right. Right.

Auspicious beginning. But you played with chemistry as a child, and then at some point the budding chemist became a budding physicist. How did that transition happen?

Theodor Hänsch: Partly it had to do with an event that left me almost without hearing, and certainly without eyebrows, some mixture, I think, of red phosphorous and potassium perchlorate that blew up in my face.

Oh goodness.

Theodor Hänsch: And I decided that maybe this pastime was too dangerous.

Perhaps your parents decided this too. Right, so you made a pragmatic decision to go for a safer subject. And you went to read physics at university in Heidelberg, and was it … I mean you had liked being an experimentalist with your hands, and you continued to be an experimentalist, was there ever a point at which you wanted to make the decision of whether you should be a theoretical physicist or an experimental physicist?

Theodor Hänsch: Well starting out as a student at the University of Heidelberg, the initial hard work was mathematics, and I started to admire the power of the mathematical formulas, and so clearly the thought crossed my mind, should I maybe do that? But I enjoyed so much experimenting, working with my own hands, being in touch with nature, whose laws we want to uncover, that I decided, no that’s maybe not the right choice for me.

That’s a very nice way of referring to experimental science, being in touch with nature. And you felt that even then? You felt that experiments were leading … were uncovering truths for you?

Theodor Hänsch: Well I had done experiments as a hobby for many years, even before I decided that I wanted to be a physicist, and it’s been one of the sources of enjoyment, so I didn’t want to give that up. But also I felt that the formulas of mathematics, powerful as it may be, it can obscure the genuine nature of physical laws, and our intuitive understanding of course plays a powerful role. If I want not only to solve problems according to the book, but if I want to invent new tools, new directions, I need to develop some intuitive feeling, and as a theorist I think it’s harder to do than as an experimentalist.

That’s interesting. So it’s by practising and playing that you develop ideas of how things might work, and you can just try them out? And theory is your back-up, in that case, I suppose, or maybe …?

Theodor Hänsch: So I might have an intuition that this is a promising direction, and once we start working of course then I need to do mathematical modelling and all that. But mostly it’s some more or less intuitive thought that starts it, the gut feeling that this might be interesting.

That’s very creative, nice, yes. The course of your research path seems to’ve been set by your encounter with the laser, which …

… I felt more like the crazy professors from science fiction books …

Theodor Hänsch: Right. Actually I was trying to be a nuclear physicist. Nuclear physics was in vogue in the ’60s, that was the highest profession one could aspire to when I started out in the nuclear physics research group of Professor Hans Kopfermann in Heidelberg, working at a betatron. And I even went to a meeting of the German Physical Society where nuclear physicists and particle physicist met. But there I saw that this kind of work, if you do it professionally, means working in big teams, and I felt more like the crazy professors from science fiction books, or something, who do everything in their own kitchen. So I didn’t want to … give up this dream that I could work as a crazy inventor by myself, and laser seemed to be a field that doesn’t require so many people, and you can do many things with your own two hands.

I mean the laser is common or garden now, and we all see laser pointers, and everything, but then lasers were fairly new and do you remember your first encounter with the laser?

Theodor Hänsch: So lasers, I think the first visible laser was invented in … the first visible continuous wave laser in 1961, and in 1964 I saw my first laser in Heidelberg, at the University of Heidelberg, at the Institute of Applied Physics, there was Christoph Schmelzer who had plans to build a heavy ion accelerator using individual radio frequency resonators, and he wondered how to phase synchronise these cavities, and he had the idea that lasers, these new fangled devices, maybe would provide a way to send a synchronising signal along the accelerator. And so he had hired Peter Toschek from Bonn, a student of Wolfgang Paul, as an assistant, and when I saw my first laser he already had, I think, two students. The group was very small, but they actually had built a working helium neon laser, and to me this red beam was the strange speckled pattern that I had seen for the first time, it just seemed like a new world that I wanted to explore. So I told the people in nuclear physics that they shouldn’t count on me, I will start over at the Institute of Applied Physics.

And a correct decision it proved to be. Now you went through the German education system, and then when you were, what, 29, you moved to work with Art Schawlow, who’d been one of the inventors of it?

Theodor Hänsch: I finished my thesis research in Heidelberg, I got my degree in January ’69, on Friday 13th I remember, and I stayed on for another year and a half or so as an assistant of the Institute, and then I accepted a post-doctoral fellowship at NATO to join Art Schawlow in California for one year.

And I mean I imagine that it was quite a transition to go from Germany to Stanford, and the environment there.

Theodor Hänsch: Indeed. But it was common for young German physics PhDs to spend a year as a postdoc in the US. German physics was still suffering from the after effects of the war, and if you wanted to do real science you had to go to the US. And in the beginning I was impressed to see names of companies and then to meet famous people whom I had only known from the literature, to shake hands with them and to be at such an exciting place as Stanford, where Nobel Laureates are the faculty. And I don’t think the laboratories were necessarily better than what we had in Germany, but the excitement of this spirit, one could feel all around, made it nice.

… if you have an exciting idea it doesn’t matter if you are young or old …

And perhaps the biggest difference was that in Germany the bureaucrats played a big role, partly because of the misbehaviour of the professors who had misspent money, but one also wanted to change the university system to not have professors who can direct the institute like a king, so suddenly everything was very bureaucratic. And whereas in California it seemed like anything goes, and if you have an exciting idea it doesn’t matter if you are young or old, people will listen and if they also get excited something happens right away.

It must’ve been enormously empowering. And obviously it was, cause you stayed, the environment suited you. Do you think that it’s still the same? Is the environment in places like Stanford as it was then? Is it as easy?

Theodor Hänsch: I think it’s got more difficult, for instance to get research money, if you do fundamental research. In our days you could still get a lot of money easily, say, from the US Navy, and they didn’t require you to do any military work, one could just follow one’s ideas, and we didn’t have to write big reports, we just gave them a list of publications and they’d see what we’ve done last year. If you give us more money, we will do more of the same. OK, here, you have the money. Now I think many of my colleagues are complaining because they spend so much of their time writing proposals and reports. A typical NSF grant might be $100,000 a year, which is just barely enough to have a single graduate student. So I think it’s not quite the same in … but nonetheless I mean in a place like Stanford, still it’s a place where many excellent minds are working and it’s still a very stimulating environment.

Indeed. But just staying with the theme of the sort of the milieu in which one works, you stayed there quite a long time and then in 1986 you moved back to found the …

Theodor Hänsch: Yes, I wanted to spend one year, and I got an extension of my fellowship for another year, and then Stanford offered me first an assistant professor position and after another week an associate professor position.

Another week? That’s pretty good.

Theodor Hänsch: Because I had said I will go back to Germany. But with an associate professor offer I felt like I couldn’t decline, so I stayed and then in ’75 I was promoted to full professor, and so I spent 11 years as a full professor on the faculty of Stanford. And I had, in the meantime, quite a few offers to go back to Germany, and finally I accepted an offer to go back to Munich to the Ludwig Maximilian University and to the newly founded Max-Planck-Institut of Quantum Optics in Garching.

And presumably that was in part then dictated by the fact that the Max-Planck-Institut would allow you to have that same kind of freedom of research that you’d known at Stanford?

Theodor Hänsch: Right. I think that, this combination really is what enticed me. Even the colleagues in the US envious for the amount of freedom we enjoy at the Max-Planck Society. It’s a society that is dedicated to basic research, so we don’t have to promise applications, we can follow crazy ideas, we can start risky projects, and we know that we have pretty much ensured funding for foreseeable years, even if after two years we find, oh, that was the wrong decision, we should re-group. So to have that luxury is something that’s fairly unique of the Max-Planck Society.

So on the theme of crazy ideas and application, I’m interested in the relationship between the sort of fundamental research and the tool you use in your research. Because the laser is a tool, and an ever improving tool, and indeed the femtosecond laser frequency …

… I really enjoy playing with gadgets.

Theodor Hänsch: I think in a way much of my work has revolved around tools, around inventions, gadgets, partly because I really enjoy playing with gadgets. Art Schawlow, my mentor and indeed a colleague at Stanford also liked gadgets. He felt he wasn’t good with his hands, so he couldn’t really build things himself, but he certainly enjoyed clever ideas and clever gadgets, and so we had many hours of exciting talks, about what’s new. The microcomputer revolution was just starting in the mid ’70s, so …

And Stanford was presumably the right place to be?

Theodor Hänsch: Right. There was the Homebrew Computer Club meeting every Wednesday evening at SLAC, the Stanford Linear Accelerator Centre, and people like Bill Gates or Steve Jobs were there. Of course he had no idea what they would be able to move. It was essentially a hobby organisation.

I love the name Homebrew, that’s nice, yes. It think it really captures it. But on the tool and the research, so you like playing and the tools enable you to answer the next question you want to answer?

Theodor Hänsch: Also I mean in the early days, unless you were extremely diligent, it was hard to find out what already had been done in a field, you would have to spend weeks in the library, there was no Google scholar. And I always felt that I make my life much easier if I start a new field where other people haven’t worked before, then I don’t have to go to the library and look up what has been done. And so by having tools that simply were not available before, one could take this easy way.

That’s fantastic. So all you need to know is what hasn’t been invented, and you just go and invent it and off you go.

Theodor Hänsch: Well I mean one of the first things I came up with was tuneable dye laser that was extremely monochromatic. So it was really the first light source that was both widely tuneable and highly monochromatic. It was a nitrogen pump pulse dye laser. But with this we could do fantastic non linear spectroscopy and for sure nobody could’ve done it, because they didn’t have this kind of laser.

So what did that allow you to show? What were you able to demonstrate with this?

Theodor Hänsch: Well what we did at Stanford was Doppler-free spectroscopy of atomic gases, of molecular gases, atomic line /- – -/. Hydrogen was one of the first atoms we studied.

So you eliminated the Doppler, and brought a more accurate picture?

Theodor Hänsch: Right, right. So in atomic hydrogen many of the old time physicists, they remembered that in the ’30s there was a big debate whether the Dirac theory would correctly describe the line profile of the red Balmer alpha line of hydrogen. And some people suspected that it doesn’t, and these suspicions led to the discovery of the Lamb shift, but with our laser we could see plainly resolved the Lamb shift for the first time in the optical spectrum. So that was one little thing. But there were many other things we could do. We could excite fluorescence in atomic gases at very low density. For instance, in sodium vapour we could work at temperatures, I think, of -50°C where there is one hundreds of an atom in the volume of sight, but we could see this, it was fluorescing. And a lot of other things followed. And so it really revolutionised in a way optical spectroscopy, and the trick to do it was exceedingly simple, so it was like once one knows it, one can build such a laser in an afternoon, and many laboratories did so. There were hundreds of laboratories that followed in the footsteps and built these so-called Hänsch design dilator.

That’s nice. But it’s just having the intuition to see how to do it. It may be a silly question, but when you’re playing, are you thinking about the tool, or are you thinking about the question you want to answer with the tool, or both? Is it kind of a reciprocal …

… if you hit the right frequency it’s like a soprano singer can burst a wine glass.

Theodor Hänsch: It depends. I mean once we started to be interested in precision spectroscopy of hydrogen, hydrogen is the simplest atom and allows unique confrontations between experiment and theory, then that became a goal, to try to achieve the highest resolution and highest measuring accuracy that one can for this simple atom. And so in doing that, of course you can try to work harder with it, that’s not so successful, it’s better to come up with new ideas and new techniques. And so quite a few things that I invented were motivated with this goal in mind, how can we get higher resolution?

So one thought was one should try to slow down the atoms. Doppler-free spectroscopy is good, but still they move very quickly through the laser beam with transit time broadening. How can one slow atoms? And so what about laser light, what about radiation pressures? Well the idea of laser cooling was motivated by that, and of course other ideas, how do we measure the frequency of the light? We have extremely sharp resonances in hydrogen, hydrogen is extremely selective. If you shine in a strong laser that doesn’t have just the right frequency, it’s ignored. But if you hit the right frequency it’s like a soprano singer can burst a wine glass if she hits the right note. But then we would like to know, what is this frequency? And so there were many increments of ideas, but in the end there was this simple tool, the frequency comb that could now measure not just the frequency of hydrogen, but any optical frequency.

And turning to the frequency comb, for which you and John Hall received the Nobel Prize jointly, it’s a device that allows you to measure high frequency light with extreme accuracy, extreme precision, and there are an enormous number of potential applications for this.

Theodor Hänsch: We hope so.

So I wanted to ask you which ones you’re most excited about, which ones are most likely to be realised soon?

Theodor Hänsch: It depends on whom you ask. In a way, I’m also an entrepreneur, I’m part of a start-up company that is actually selling frequency combs, and there I would have to say those are of interest to the largest number of people. But if you ask me as a scientist, well I’m interested in using combs to ask questions like, do we already understand quantum physics well, or are there some small level shifts that are not included in present day theories, or second stage Lamb shift? Our fundamental constants …

So by reinvestigating spectra with greater precision, you’ll be able to reveal that, yes?

Theodor Hänsch: Our fundamental constants, or can we do laboratory experiments that would uncover slow drifts of fundamental constants?

Fundamental constants have never had any drift associated with them, they are just … as far as everyone knows?

Theodor Hänsch: When they were named fundamental constants, people assumed that they must be constant, and on the other hand astronomers have speculated that spectral lines in the light of distant quasars, that they might carry some indication that the electromagnetic fine structure constant was a little bit smaller in the early universe than it is now. But of course nobody can go there and check what are the circumstances. But the rates of drift that they have speculated about should be of a magnitude that one can observe them in a laboratory experiment if you compare two different clock now, and again in a year or two years from now. And such comparisons have been made, and so far there is no evidence for any changing constants.

But the evidence is not that it’s not changing, at the moment it’s …

Theodor Hänsch: But I don’t think we are at the limits of possible accuracy yet, and so I think in the future it should be possible to measure 100 or maybe even 1,000 times more precisely than we have been able to do so far. And at least if nothing changes, then we can assume maybe these are really constants that are not arbitrary, and that if another universe should exist, maybe it has to have the same constants.

That’s a pretty big finding, or rather a pretty big speculation. The one thing that people have talked about is the generation of optical atomic clocks, the cesium atomic clocks that operate now operate on microwaves, and optical clocks would be 100 times more accurate.

Theodor Hänsch: Well in principle the microwave clock uses cesium atoms oscillating at 9GHz, 9 billion cycles per second. Optical clocks work 100,000 times faster, and if one goes through all the relevant steps of cesium clocks, eventually you might hope to get 100,000 times better clock. But we are already at a stage where there are clocks that are 100 times better than the best cesium clocks, in particular at NIST in Boulder in the group of Dave Wineland and Jim Bergquist.

So are they now widely accepted as the standard clocks, or is it still experimental?

Theodor Hänsch: Now there is a race who gets to redefine the second? And of course I believe that NIST would like to do that. On the other hand, I think for the clocks, it’s not just important that they are accurate, they should also be reliable, easy to work, easy to replicate, so that lots of people can have one. And I hope that the committees will hold back, that they won’t redefine the second too quickly.

That’s interesting, yes, cos then everybody is just left depending on one source, I see, yes. What about your commercial applications, what is your company selling these things for?

But the dream is that we can do much better in the future …

Theodor Hänsch: I think most of the labs that are working on optical clocks, by now have bought a frequency comb from our company, there are 45 systems worldwide been sold. And of course we are looking for applications beyond precision clocks, so one project right now involves the European Southern Observatory, ESO. We are trying, also jointly with our research group, to make frequency combs fit to calibrate large astronomical spectographs. For that we have to thin out the comb spectrum.

Right now our comb lines are typically spaced maybe 100 MHz to 1 GHz, we’d like to have them further apart, like 30 GHz. And one way to do that is to filter them out with an external cavity, and we have already done some first steps in this direction. We brought one of our fibre based frequency combs to the vacuum tower telescope on the island of Tenerife earlier this year, and superimposed a comb to a solar spectrum, in the infrared range around 1.5 micron, and so you see all the nice Fraunhofer lines irregularly spaced, and then you see the comb spectrum very regular, like a ruler. And without any big analysis, immediately we could improve on the calibration accuracy that they are used to with this telescope. But the dream is that we can do much better in the future, and that you might be able to, for instance, watch directly the expansion of space with the evolution of the universe by taking some objects with certain red shift of the spectral lines, and see if this red shift itself drifts with time.

Over what period would you have to look to actually witness the expansion?

Theodor Hänsch: It depends how far away the objects are. But of course your hope is that this can be done with observations of months or years, or human scale times. But on the other hand I mean we could measure now, record it, and astronomers a century from now might go back to this data and try again.

Would this be the first direct visualisation of the expansion of the universe?

Theodor Hänsch: Another thing that has already been done, is you could search for planets or distant stars, by looking at variable Doppler shifts of Fraunhofer lines of these stars. Because the star and planet, they really orbit around Earth, come with a centre of gravity, and so this has been successful in the search for Jupiter-like big planets. But if you can improve the accuracy with which one can calibrate these spectral lines, you might have a chance to discover Earth-like planets in distant solar systems.

Goodness. With so many applications it must be hard not just to concentrate on selling these things and getting them installed everywhere.

Theodor Hänsch: Well I mean selling them means that other people can be clever and they can do things with them. For instance a French group, it was Nathalie Picqué who right now with her students actually is in Garching, in our Institute, but they are exploring the use of frequency combs for molecular Fourier transform spectroscopy, where you cover a broad spectrum but with sharp, evenly spaced lines, and you can use the tricks of Fourier transform spectroscopy to get complicated roll vibration of molecular spectra very quickly, rather than moving a mirror or a prism in a traditional microscope, in a traditional Fourier spectrometer, you use two frequency combs that are tuned to slightly different spacings of these combs, slightly different repetition frequencies. You send these two beams through a gas, and you look simply at the modulation of the light as it has passed through the gas you get an interferogram, and you Fourier transform that mathematically on a PC and you’ll get a highly resolved spectrum. And so right now Mrs Picqué can tell me that she can record a spectrum of /- – -/ with all the rotational lines resolved in a small fraction of a millisecond. To get the same spectrum at home with her traditional Fourier spectrometer, she would have to accumulate data over an hour. So now Fourier spectroscopy might become applicable to, say, diagnosis of combustion processes, things like that.

So with all this to do arising from that innovation, it’s perhaps surprising that you don’t stop there, but you go on. I mean your talk here, for instance, is entitled Quantum Laboratory On A Chip, and you’re moving into new areas, you’re constantly innovating. Is it just again the need to keep playing and keep experimenting?

… I want to be able to make mistakes quickly.

Theodor Hänsch: Right, I think my approach has been, I want to be able to make mistakes quickly. So I don’t like to have large scale complex experiments. Some of my students, though, do, and some of my postdocs, so we are doing some of the serious research also, but in my own work I like to be able to improvise quickly, and if I see, oh, that’s a bad idea, move onto something else.

What’s quickly? What defines quickly?

Theodor Hänsch: Say a week timescale, to get the first feeling whether an idea is feasible or not.

And yourself, do you still go into the lab regularly and play?

Theodor Hänsch: I actually have my own lab. I have downtime at the University of Munich, the so-called toy store or toy lab, which has a collection of toys, I have to admit. But for me it’s the only lab where I know where everything is in the lab. So if a student works, if I need a screwdriver I spend 15 minutes to open all the drawers. In my own lab I know where the screwdriver is and which lenses I have, and where to find them. So I can try out things quickly, and I take advantage of that.

I imagine it’s important not to let the students into your lab.

Theodor Hänsch: Indeed, right. They are at least supposed to ask me before they take anything out.

And turning to the students, when you pick students to come and work with you, what do you look for in the people that you want to bring on?

Theodor Hänsch: Well of course I look at grades and letters of reference. Typically I talk to them for a while, I show them experiments. Other people in the group show them, and I try to judge from the kinds of questions they ask, and whether they show interest or not, whether they have the potential to be successful. And I think one indication is that indeed they can get excited about something, and that they have in the past done things like hobby holograms, or who knows? Just something that shows that science is indeed something that they enjoy.

That makes sense. If you are looking around the world for people, though, how do you tap the resources that are now coming out of China and India, and things like that? Are you able to reach out to students worldwide and bring them to work with you in Munich?

Theodor Hänsch: We have students from different countries. We have a very good student from Japan. We have French students. Not so many from China, I think there are very good Chinese students, but in the past they would go to the US universities. But maybe that’s changing, in particular with the visa problems in the US.

I suppose that must be working to Europe’s benefit, yes, to a certain extent, yes. And a last thought, the students here at Lindau, would you have any particular advice to them that they should take away about managing their scientific careers?

Theodor Hänsch: I think it’s very important to find something that one is deeply interested in and that one enjoys. If they find something like that, they don’t mind working day and night because they’re obsessed. Then I thin they’re on the right track.

With that thought, thank you very much indeed, it’s lovely to talk to you.

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