Theodor Hänsch at 75
A Passion for Precision
Have you always known how important precision is?
Hänsch: That became clear to me during my doctoral work in Heidelberg. In those days, we had no frequency combs. Instead, we had helium-neon lasers that we built ourselves. You couldn’t buy such an instrument anywhere then. And we used these lasers for optical spectroscopy. Spectral lines that were normally strongly broadened could be measured with a linewidth that was determined only by the natural linewidth, the rate of decay and the damping of the transition. I found that simply fascinating.
You built your own lasers?
Hänsch: Yes. That was both fantastic and frustrating, because the color of the light emitted by our gas lasers was tunable only over a narrow range. When I moved to Stanford in 1970 to work with Arthur Schawlow, my goal was to “tame” the dye-based lasers, which had been invented a few years before, so that they would emit a similarly narrow band of radiation. They could then be used for spectroscopic studies across the visible portion of the spectrum. And we succeeded in doing precisely that! It was fantastic. For a while, we were the only people in the world who had such an instrument.
Although you are known above all as a basic researcher, you still have a corner in the lab that is devoted to tinkering.
Hänsch: That’s very important for me. Inspiration rarely comes to me when I’m at my desk. The unconscious needs to be fed by confronting it with immediate, hands-on experience. I must be able to see where things get difficult, and why. I have to study the problem intensively. Then there is a chance that an inkling of the solution will occur to me – maybe when I’m out for a walk. And then it’s a matter of extending and refining that first flash of insight.
If you had to rank your inventions in order of importance, which one would be at the top of the list?
Hänsch: I mentioned the further development of the dye laser earlier. That work was done during my first year in Stanford and led to a modest revolution in spectroscopy. Then there was the idea of laser cooling. One can use laser light to cool atoms to extremely low temperatures, practically to absolute zero. Our work in this area was the very first on the topic, and provided the basis for many subsequent developments, for which Nobel Prizes have also been awarded.
So, in this respect, you were ahead of your time?
Hänsch: You could certainly put it that way, yes. When I first formulated the idea, there were no suitable lasers available for us to perform such experiments.
So you didn’t pursue the idea at first?
Hänsch: Exactly. It may have been stupid not to follow it ten years later when we finally had suitable lasers, like Steve Chu who would later win a Nobel Prize. But that was around the time when I was thinking of moving to Munich, and the negotiations absorbed much of my attention.
I notice that the frequency-comb generator is not on your list.
Hänsch: But that was something that was obvious, so to speak. It is a tool which makes it easy to do some things that were previously difficult or well-nigh impossible. For example, it serves as the timekeeping mechanism for the most accurate optically based atomic clocks.
Even astronomers have become interested in the application of frequency combs.
Hänsch: Yes, that’s true. A laser frequency comb that we built will shortly be delivered to LMU’s Astronomical Observatory on Wendelstein. There it will be used to detect the minuscule shifts in the spectral lines of stars that reveal the presence of planets. Observations made with this method at the La Silla Observatory in Chile have led to the discovery of more than 1000 extrasolar planets. Our Astro-Comb, which was developed by our spin-off company Menlo Systems, was installed at La Silla at the beginning of the year.
The range of application of the frequency comb is quite immense. Did you foresee that?
Hänsch: No. To start with, our aims were to measure frequencies and improve high-precision clocks with it. The technique has now taken on a life of its own, and it’s not yet clear where it will take us.
How important is it to you to know that your ideas and discoveries have resulted in successful products?
Hänsch: I spent 16 years at Stanford University in California, right in the middle of Silicon Valley, and it was fascinating to see how rapidly an idea hatched out in a research lab can give rise to a commercial company. I tried to transplant something of this spirit to Munich. But it was very difficult to do in Germany in the beginning. There was simply no entrepreneurial culture of that sort here then. Now there are many examples for successful spin-offs. In my view, it is also interesting for students to see that basic research is not just a useless luxury for which society foots the bill. Basic research is anything but useless. And there are the new firms, and customers interested in their products, to prove it.