INTERVIEWER: You've written that
you began exploring the technical world by playing
with Legos. I don't want to talk about Legos
right away but is there a technical world versus any
other kind of world as you understood it when you
were growing up? What is a technical world? KETTERLE: Well, the world
of gadgets, tinkering, constructing things. INTERVIEWER: And this was the
world that you embraced as young child? KETTERLE: Yeah. It may be difficult to
rigorously define it, but I think if you see young children,
whether they play with stuffed animals, whether
they read books, or whether they want to work with tools,
children are expressing something about their
preferences. INTERVIEWER: And your preference
was clearly grab the tools, build some stuff,
and then build it again. KETTERLE: True. INTERVIEWER: What first made you
think that science was a place to express this
kind of impulse in the technical world? KETTERLE: I found out what I
liked at school and then -- wait a minute, start again-- I think when you are a child
you don't really know what science is, you don't know
what mathematics is. But one aspect is playful
explanation and then another aspect is you take those
subjects in high school and I always this like physics and
mathematics the best. INTERVIEWER: There's plenty of
people, particularly at the time that you were growing up,
who viewed physics and mathematics as a domain of
thought and abstraction and not necessarily a place where
you would build tools to inquire into the nature
of matter. Why didn't you take the path
of maybe more theoretical physics and how did you embrace
the building of tools, so many of which have been
revolutionary in the studying of states of matter? KETTERLE: I actually disagree. I think it was clear for me
from the beginning that science has an experimental
side. What actually drew me into
science was both my love for mathematics, my love for
abstract thinking, but also my love to tinker, to
play with things. I had a motor bike and I was
repairing it and doing some modifications. I had a chemistry set. So I was exploring science on
the hands-on side and I enjoyed it. INTERVIEWER: And so really from
the earliest age, there was a unity of abstract science
and experimental work that you've carried with you
throughout your career? KETTERLE: That's correct. Actually I was quite interested
also in a career in mathematics and I just felt that
physics would connect me more with the real world and the
real world also means the possibility to do
hands-on work. On the other hand, this
interplay between experiment and theory during my career
has taken me a few turns back and forth. For instance, at the end of my
undergraduate studies I did a purely theoretically
Master's thesis. So at that point I actually
wanted to become a theorist. INTERVIEWER: Why did you choose
that at the end of your undergraduate study? KETTERLE: Because I was
fascinated by that. First of all, most of our
education is theory-based. In high school, at least
at my time, we hardly had any lab classes. So it was all about
learning and understanding the concepts. At the University, we had the
equivalent of junior lab. So I went through those
lab courses. But the far dominant portion of
the education is conceptual and theoretical; and
it fascinated me. INTERVIEWER: What was the first
academic setting that you came into where you looked
around the lab and saw machines that were really worth
tinkering with, that had the capacity to explore
questions that you thought were really fascinating? KETTERLE: Probably when
I started my PhD. But I mean it goes
back and forth. I think I've always been a
person who has been talented for both and who has
intuition for both. I remember when I took lab
classes and there was a torsion pendulum I was just
fascinated by it. I could play with it. I was experimentally realizing
the differential equation. So I really felt-- I wanted to feel physics. I wanted to see physics. But nevertheless, a couple
of years later I picked a theoretical topic as
my Master's thesis. I think I was always fascinated
by the power of describing nature. When I learned about statistical
physics, I found it fascinating that you can make
a few assumptions about what atoms do, kind of some
probabilistic arguments, arguments with entropy. You can deduce the equation of
state of the whole system. So from very few assumptions or
very few things you know, you can deduce a lot and
that fascinated me. So therefore I decided to work
on a theoretical subject for my Master's thesis. INTERVIEWER: Is it an accident
that it seems that one of the patterns in your academic career
is that you typically would choose problems and
projects that were beyond the capabilities of the lab
equipment at your disposal, of the machines that were available
at your disposal? KETTERLE: I fully agree. It is the style of my
group to push the frontiers of the field. I tell all the students who work
with me or who want to work with me, if you want to
work with me, you have to be a hardcore experimentalist. Because our job is to push the
boundary of what is possible. So my lab-- my group-- is
extremely focused on exploring new things which currently
seem impossible or out of reach and bring them
into reach. INTERVIEWER: When did you first
have the experience of encountering a question that
required you to make a brand new tool to investigate
that question? KETTERLE: The question sounds
too much black and white. Because new tools are
often not brand new. You have ideas, you see
something and modify it. It sometimes depends on how
familiar you are with the details whether the
modification looks revolutionary or brand new. I always felt what I did was
the logical next step. But viewed from the outside,
it looked to people like a bold move. INTERVIEWER: Well I'm not trying
to over-characterize the boldness or the
inventiveness. But I think there's a classic
story in science there, where you have a question that you're
dying to get to the bottom of and you really need
to invent or modify the existing tool set to enable
you get there. So it requires a number of
jobs to get to the-- KETTERLE: I mean I see an
interplay There's an interplay between technological advances
and experimental advances. Suddenly you realize
I have a tool. Then you find new science. But sometimes you are interested
in solving a burning scientific question and
then you think which tool can help us? So sometimes I have allowed
myself or my group to develop tools in search for the right
question, just to be sort of ready to attack the question. Or there was a question, and we
developed a tool specific for that purpose. So it sometimes depends. Sometimes your research may be
more technological, driven by finding better tools to go
deeper into science. Sometimes you are motivated
by a scientific question. INTERVIEWER: You seem to have
been drawn to the idea that in spectral analysis of atoms and
molecules the individual signatures can tell you a lot
about the states of matter. You stayed there for
quite a while. KETTERLE: That's true. INTERVIEWER: What did you love
about that particular domain? KETTERLE: Well I found it, and
I still find it, fascinating that when you look at the
spectrum of an atom or molecule, that you can learn
all the details about the atomic and molecular structure
just by looking at the frequencies in the
electromagnetic spectrum at which the atoms or molecules
resonate. I was particularly excited about
it when, during my PhD, I discovered a new molecule and
there was hardly anything known about it. And I had a spectrograph. I looked at the light which was
emitted by the molecules. There were hundreds if
not a thousand of different spectral lines. A lot of them were
not regular. It didn't make any
sense initially. It took me, I think, about
two years or so to solve the puzzle. I had to learn a
lot of theory. I had to do a lot of computer
simulations. I had stacks of papers of
simulated spectra just trying to find a simulation on the
computer which became similar to the experimental spectrum. INTERVIEWER: And the molecule? KETTERLE: The molecule
was helium hydride. It was a molecule consisting of
the two lightest elements in the universe. It was a very special molecule
and after two years I had found the code. I'd really deciphered
the code of nature. For the first time I knew what
is the size of the molecule, how still is the point between
the helium and the hydrogen. All the basic questions one
would ask about a molecule, I could answer. I found it extremely
fascinating to be able to do that. It was even more fascinating for
me since I grew up at an institute which didn't have the
tradition of doing that. So almost everything I had to
learn by myself or learn by asking other people, or by
reading the literature. This was a very important
step in my development as a scientist. INTERVIEWER: That was it
the Planck Institute? KETTERLE: Yes, the Max
Planck Institute. INTERVIEWER: What brought
you to MIT? KETTERLE: Laser cooling
and Dave Pritchard. That is actually an
interesting story. If you'll allow me to fill in
some gaps, we've talked about spectroscopy which was my PhD
work and the work I did during my first postdoctoral
appointment, still at the Max Planck Institute. I've also expressed to you the
fascination and also the satisfaction I had in analyzing a brand new molecule. On the other hand, I felt after
three years as a postdoc I was sort of at a dead end
street because this kind of molecular spectroscopy was done
early on and they're were not many molecules left to
do that kind of analysis. The frontiers of the field
were different. So I felt the molecular
spectroscopy that I did was neither applied nor
fundamental. I wanted to make an important
contribution to science, technology and knowledge, I
should either do something which is clearly applied or
which is clearly fundamental. So I was at crossroads. I decided to work on an applied
project because throughout my studies as an
undergraduate student, also during my PhD, I always wanted
to do something more applied. Applied means something which
is more directly related to the needs of society or the
burning questions which we have to solve as a nation. INTERVIEWER: What are those
burning questions and what do you sense is a scientist's
responsibility in responding to the needs of society? KETTERLE: Well, scientists
can respond in many ways. One of the needs of society is
to protect the environment, to avoid pollution. I felt as a atomic physicist
who understood spectroscopy that there was an opportunity
for me to contribute, namely to work on using spectroscopy
to monitor combustion. So I went into combustion
diagnostics. So I used my knowledge of
spectroscopy and applied it not any more to new and
potentially esoteric molecules, I applied it to
flames, flames in a Bunsen burner, flames inside
a diesel engine. So my goal was, and this is
what I picked as my second postdoc-- and at that point
I intended to make it my career-- to use laser and
spectroscopy as a tool for cleaner environment. To make combustion more
efficient, to make combustion cleaner. INTERVIEWER: Monitor the
combustion and by doing so come up with ways of making it
more efficient and getting rid of pollutants and
contaminants? KETTERLE: Exactly. So the program was that-- the
program was a combination of theoretical and experimental
work. I was the experimentalist who
helped to develop new tools of diagnostics. The goal was to measure
molecular concentrations with high spatial and temporal
resolution. So in the end, experimentally
you can see at what phase of the combustion cycle pollutants
form, to figure out how fuel mixes with air,
and to permission local temperature fields to see if the
flame front is irregular or if it's regular. At the same time, we
had theorists -- theoretical chemists-- who used
models of combustion. The hope was-- which eventually
became true over the last decade or so-- that
by comparing results of theoretical predictions and
comparing that we detailed to accurate combustion diagnostics
that you could verify the computer codes and
help to improve them. So that ultimately we would
wind up with a much better understanding of what limits the
efficiency and what causes pollution during combustion. INTERVIEWER: What did you have
to do to modify traditional lasers to make this diagnostic
tool a reality? KETTERLE: For a lot of the work
you can use commercial lasers but you either have to
adapt the lasers to the special purpose or to do
some modifications. During that year in Heidelberg,
I worked with ultraviolet lasers. One of the -- it's probably not
an earth shattering idea, but I had a nice idea to modify
a laser from a single wavelength into a dual
wavelength laser. With that we had a two channel
capability, which meant we could monitor two chemical
species simultaneously. So that was a nice
development. It was fun for myself and for
the people around it. We even took out a patent. I think in the end, it didn't
really have a big practical impact, but that was sort of one
example where you want to improve your tools, you want to
really master your tools in order to use them for your
scientific purpose. INTERVIEWER: Those skills really
would help you later on in your low temperature work. But before we get to that,
how did you come to MIT? KETTERLE: No I mentioned laser
cooling and Dave Pritchard? INTERVIEWER: Yeah. KETTERLE: Anyway, so I think
we are now at that stage. So I've just told you what I
did before I came to MIT, namely applied research. Applied research was also for me
to eventually realize some of the thoughts or some of
the aspirations I had when I was a student. I wanted to work on something
practical. I've already mentioned that I
had sort of two forces in me. One was a desire to work on
something applied to solve practical problems. The other aspect was my
talent and passion for abstract thinking. This drew me to fundamental
problems and it also drew me to theory. I mentioned to you earlier
that I had worked on a theoretical Master's thesis
and then I switched to an experimental PhD thesis. Then eventually after my PhD and
first postdoc, I went into hardcore applications:
combustion research. So while I worked on combustion
research and did applied research, I learned
a lot about myself. I saw on the one hand the deep
satisfaction to work on problems for which it is
self-evident that somebody has to work on. If you work on clean combustions
using laser light, you can talk to all your
friends, you can talk to your parents, and everybody
understands what you are doing. It's so easy to explain why it
is important to use modern technology and laser
technology to make combustion cleaner. INTERVIEWER: It's a little
harder to talk about Bose-Einstein condensation? KETTERLE: Yes. If I now tell people that there
is this tiny little amount of gas, it's smaller than
a millimeter, but it's terribly cold and it's terribly
exciting to study its properties, I'm not sure that
everybody understands me. So I think you get the point. I felt the satisfaction to
work on applied problems. I also felt I was in
the middle of life. I talked to engineers at the
Volkswagen company, because we had a collaboration with
Volkswagen, and even used our laser equipment there
on a test stand to look at a diesel engine. I was really relating to
real-life problems with real-life people and diversity
and all that. But I was missing something. I was somehow missing the
long-term challenge. I was working on problems
and solving problems. But I felt the desire to work
on problems where you don't even know if there's
a solution. So I felt that while I did this
applied research, I want to work on something else. There was also one other
thing I learned. I liked that applied research
was more directly contributing to the solution of
urgent problems. But while I did the applied
research, I also realized how much applied research draws
from fundamental research. For instance, my training in
fundamental spectroscopy enabled me to do applied
research with a different approach than the people who had
done applied research all their life. So while I did applied research,
I clearly realized that we need a certain division
of labor between people who do fundamental
research and people who eventually take care
of applications. So after realizing that, after
also realizing my talent and my desire to do fundamental
research, I then decided to change my career again. I wanted to work on fundamental problems in physics. This was a big step, and it had
a lot to do with finding out something about myself and
my own motivation, and in some sense I found out what
I really wanted. INTERVIEWER: We associate lasers
in the popular world with heat not cooling. How did you suspect that a
laser would be a cooling device at the extreme low
temperatures that you wanted to explore? KETTERLE: Laser cooling
is pretty cool. Laser cooling is an amazing
phenomenon, and it attracted a lot of attention when it was
developed in the '80s. So when I was about to make a
decision again, I wanted to work on something fundamental,
how did I pick my area? Well, I felt at that point I
was around 32 years old, I couldn't start completely
over again. So I wanted to work in a
fundamental area of physics where I could continue to use
some of my tools, namely lasers, optics and
spectroscopy. I talked to many people. I looked at proceedings
of conferences. What I identified as the most
promising and most exciting area was laser cooling. So laser cooling was
already quite well developed at that point. But I felt there was a big
potential to study the properties of laser cooled
atoms and explore other science with cold atoms. I concluded that it's the most
exciting field in fundamental physics that I can contribute
to with my tools. So what I did is I sent letters
to some of the best groups all over the world, and
asked if they would offer me a postdoctoral position. Eventually Dave Pritchard
here at MIT offered me such a position. That led to the fact that
eventually in early 1990 I arrived at MIT. Now I haven't really addressed
your question about why lasers cool, or when do lasers cool
and when do lasers heat? INTERVIEWER: I noticed that. Yes. You haven't addressed that. KETTERLE: Let me do that now. INTERVIEWER: Please. KETTERLE: Lasers are,
of course, a directed form of energy. If you hold your hand in a laser
beam, you feel the heat. Because the light, which is
energy, is absorbed by your skin, and it is converted
into random motion. We feel that as heat. Well, with some of the lasers
we have in our labs you can even burn your flesh. So this is how lasers heat. But if you apply laser radiation
to atoms, there's one important difference. The atoms cannot
keep the light. When atoms have been excited to
a more energetically higher state, they have to re-emit
the photon. So a photon which is absorbed is
re-emitted with 100 percent probability. Therefore, the energy which the
atoms absorb is emitted again as florescence. INTERVIEWER: But that's a
straight ahead trade. KETTERLE: So that would be
a straight ahead trade. Now on a much more subtle scale,
when we use the right laser light with the right
frequency, we can arrange for situations where the emitted
light is slightly more energetic than the
absorbed light. INTERVIEWER: So that means as
you apply the laser to the atom and the atom continues to
re-emit it loses energy. KETTERLE: Exactly. It's quite fascinating
how that comes about. What happens is we need a
frequency shift of the light. But in its simplest form, this
is provided by the well-known Doppler effect which makes the
frequency of light depend-- if a moving object emits light--
the frequency depends on the direction of the emission. That is sufficient to
make lasers cool. INTERVIEWER: When did you-- I mean, you say that laser
cooling was fairly well known at the time you came to MIT,
what potential did you want to explore using this tool as you understood it to be so exciting? KETTERLE: I was drawn to the
field by its novelty and its general potential, also by the
possibility of analyzing and exploring properties of
cold atomic matter. Cold atoms can form
cold molecules. That was quite exciting and that
was actually the project I was hired for by professor
Dave Pritchard. INTERVIEWER: The mission would
be to explore matter at states of extremely low energy in a
whole variety of settings? KETTERLE: Yeah, that was
the fascination. I had certain questions about
the wave nature of atoms. I was intrigued but as far as
I remember I wasn't really focusing on one specific
thing. I just felt it was really cool--
a cool area to work in. But let me give you my
assessment, which is I think interesting. In 1989, I thought that laser
cooling had gone through extremely exciting times. Sort of between the early
'80s and late '80s, new technologies, surprising cooling
technologies were developed, and it all
came together. that one could cool and
manipulate and trap atoms. My assessment was that the field
had reached a certain peak and I was expecting that
now the field would be at a very high plateau for at least
some years to come. This is where I wanted
to contribute. Of course ideally as a
scientist you want to contribute to a field
while all the pioneering work is done. Well, I didn't see such a field
or such an area within my reach given my background. I felt laser cooling was
the best I could do. But I had the impression that
laser cooling had reached sort of its peak and now
there was a long plateau for several years. In some sense I was right. Because when the Nobel Prize was
given for laser cooling, it was given to the people who
contributed to the development of cooling techniques in
the mid and late '80s. But what I couldn't know at this
point is that when laser cooling was combined with
evaporative cooling that the whole field would jump up to
another level which was completely off scale. So at that point, the best of
laser cooling-- the very best of laser cooling-- was
still to come. INTERVIEWER: The combination
with evaporative cooling, which is another technique--
when did that come into being or occurred to you and
your team as a tool? KETTERLE: Well being at MIT,
you are of course familiar with evaporative cooling. Evaporative cooling was
pioneered by Professors Greytak and Kleppner at MIT. Of course, being at MIT
I knew about it. INTERVIEWER: Now, evaporative
cooling: I think of that and I think of the very lay,
traditional notion of how an air conditioner works, where the
energy from evaporation is removed from a space and by
doing that continually you can cool air or cool the room. But on the scale that
you're talking about it's very, very different. Can you explain? KETTERLE: I was referring to the
work of Dan Kleppner and Tom Greytak who applied
evaporative cooling to cool hydrogen atoms in
an atom trap. So it's more talking about how
can you cool atomic samples and to find-- and this was
really pioneering work-- to apply evaporative cooling to
this whole new system in a whole new range of
temperature. INTERVIEWER: Basically you're
taking the high energy atoms relative to the system and
bumping them out of the picture so that the average
energy goes down, down, down. KETTERLE: It's as
easy as that. INTERVIEWER: Yeah easy
for you to say. But on an atomic scale how do
you identify the hot atoms and kick them out of the system
without adding more energy? KETTERLE: Well that can
be done in two ways. One is, we keep the atoms
in a trapping potential. So the atoms are like in a-- you
can think of it as marbles in a cereal bowl and they
move randomly around. By lowering the rim of the
bowl, you allow the most energetic ones to jump
over to the rim. INTERVIEWER: That's good. KETTERLE: Another method is
to use radio frequency or microwave spectroscopy, but
it's actually a variant of that scheme. INTERVIEWER: By combining this
with laser cooling, the low temperature potential you
could see was what? KETTERLE: So that's now an
interesting question, because now we are talking about
the early '90s. So what happened is with laser
cooling and evaporative cooling, the cooling techniques both were well known. There was some speculation
that those two powerful cooling techniques could be
combined, but the general opinion was that it will
not be possible. Laser cooling only works at low
density where the laser light is not absorbed
by the atoms. Evaporative cooling only works
at high density because the atoms have to collide and stay
in thermal equilibrium. So it seemed-- that's what many
people said-- that there was a big gap between the
highest density you could reach with laser cooling and
the lowest density at which evaporative cooling
would set in. INTERVIEWER: So did you set
about to bridge that gap or--? KETTERLE: When I was a postdoc
with Dave Pritchard, we bridged the gap. It was interesting
how it happened. I think it was just the ideal
combination of somebody who joins as a postdoc from a
different field and somebody who has all the experience
like Dave Pritchard. I know as a postdoc I have to
learn, read all the papers in a very short time because
I have to come up to speed very quickly. I can't go through a year or
two of preparation like graduate students do. As a postdoc you have already
learned how to learn, and you should be able to
learn rapidly. So I was reading all the papers,
and asking questions, and having my own thoughts. I had discussions with Dave
Pritchard as asked, "Why wouldn't evaporative
cooling work? Why not?" He referred
me to some papers. I analyzed the papers and said,
"But shouldn't there be a window?" We started
talking about it. Our conclusion was that there
was a gap to be bridged, but the gap was not very large. Ultimately, we were building
a new cold atom machine. Dave Pritchard had a grant with
work-- with approval for a proposal to study the
formation of molecules from cold atoms. We were building an experiment
along those lines, but kept on discussing how can we
bridge the gap? How can we do one more
improvement to laser cooling which would then hopefully jump start evaporative cooling? INTERVIEWER: How did you
acquire lab space to investigate these ideas? KETTERLE: Dave Pritchard
had labs. I was working in his labs. INTERVIEWER: But is it unusual
for a postdoc to come in and suddenly get lab space? KETTERLE: No, I was assigned
to an experiment which was started by Dave Pritchard. He had an earlier postdoc
who had worked on that, and I took over. What was usual was that the
experiment at that stage was sort of the concepts were
not fully worked out. So it was an experiment which
was just being put together. There was an opportunity to come
up with your own ideas and your own concepts. INTERVIEWER: But in any
institution there's a limited amount of lab resources and it
seems that a lot of your colleagues were as excited by
your investigations as you were and voluntarily gave up lab
opportunities so then you could have. KETTERLE: That actually came
later after I was appointed as an assistant professor. But we are just talking about
the postdoc time when the opportunity arose to bridge the
gap between laser cooling and evaporative cooling. INTERVIEWER: Alright, well let's
talk about the postdoc moment then before we get to the
assistant professorship-- when did you know you
had bridged the gap? KETTERLE: Well, that's a moment
I will never forget. I was two years into my postdoc
and we were laser cooling atoms and we used this
new idea which Dave Pritchard and myself had come up with. What we saw were completely
pitch black clouds of red atoms which absorbed all the
light that we were sending at them, because they
were so dense. So we had reached higher density
in large clouds than anybody had ever
reached before. Very soon it became clear that
this could bridge the gap. INTERVIEWER: What element
were you using? KETTERLE: It was the
element sodium. Sodium used to be the workhorse
of atomic physics in the '70s and '80s. Dave Pritchard has worked with
sodium for most of his scientific life. That was our natural choice
for laser cooling. What then happened was also
a formative moment. We started to discuss
what can we do next? I-- and it wasn't first
very clear. Dave Pritchard had worked out
all these ideas about how to form cold molecules
from cold atoms. He felt this higher density
would just be a boost to it and he felt we should continue
along those lines. Myself, as a postdoc, I said
"Well, why don't we go for the holy grail?" Let's combine laser
cooling and evaporative cooling-- there is a chance
it will work. For -- I don't know how long it was--
for a few meetings, we discussed it back and forth. The graduate students who worked
with me were ambitious. They wanted to go for kind
of the big challenge. I will never forget that after
a few such discussions, Dave Pritchard just said, "I'll let
you do what you want." So Dave Pritchard had really
the greatness of abandoning his own ideas. Some other people got famous
with photoassociation, you know, putting cold atoms--
associating cold atoms into molecules. But he allowed me as a postdoc
to go for this combination of laser cooling and evaporative
cooling, which some people regarded as uncertain
and speculative. I know in 1995, a few months
before Bose-Einstein condensation happened, I got
reviews on my proposal where people felt that there was
such a long way to Bose-Einstein condensation it
would probably never happen. So it wasn't clear that this
was a golden road. But Dave Pritchard was
a great person. He realized that I was so
enthusiastic about it, he wanted me to try it out. INTERVIEWER: It's fair to say
that that was a moment of extraordinary generosity
in your career? KETTERLE: Yes. Number one; and number two was
that it was also a moment of sudden change. We had this new technique called
the dark spontaneous optical trap, where we had
reached higher densities than people before. But before even writing and
submitting the paper, we ordered a list of critical
pieces of equipment in order to go for evaporative cooling. So there was really this sudden
moment where kind of all your thinking, all the
resources of the lab-- whether it's student manpower or
equipment money-- everything became suddenly focused
on a new goal. INTERVIEWER: So you made the
big bet on your question. All the resources were devoted
towards figuring out if there was an answer. An exciting but scary moment. KETTERLE: I thought
it was exciting. I wasn't scared at this point. INTERVIEWER: Yeah, you
don't get scared. Not much scares you. So what was the target then? Explain perhaps at the end of
that pathway of low energy lay Bose-Einstein condensate. KETTERLE: I tried to be
reasonable and conservative. I knew about Bose-Einstein
condensation. But it was also clear that with
laser cooling, we were missing a factor of a million
in density, or more accurately, in phase space
density, which is a more technical term. So we were a factor of
a million away from Bose-Einstein condensation. I felt it was almost frivolous
to say we are now going for Bose-Einstein condensation. So for myself and for my lab, we
declared the goal: we want to combine laser cooling with
evaporative cooling and we want to get at least a little
bit colder and a little bit denser than anybody before us. We knew that ultimately this
could lead to Bose-Einstein condensation but we felt we
were far away from it. INTERVIEWER: Let's describe
what that condensate actually means. At a certain density and at a
certain low energy, the atoms would do what? KETTERLE: The atoms would
fall into lock-step. Instead of moving randomly, they
would literally all march in lock-step, which means they
would form one big coherent wave, a giant matter wave. INTERVIEWER: This matter wave--
does it merge quantum events and other events in
a way that you can't see anywhere else in the world? What is it about that matter
wave that is fundamental? KETTERLE: Well, coherent matter
wave is for atoms what the laser is for light. If you have light from a light
bulb, the light is emitted randomly in all directions. There are little waves going
everywhere in contrast, the laser light is just
one coherent wave. Similarly, in an ordinary gas,
atoms move randomly and in a Bose-Einstein condensate,
they march in lock-step. So it's not that the atoms are
merged, it's just that all the atoms do to the same thing
exactly as the photons in a laser they all do
the same thing. They're not moving in random
directions, they're just in a single mode of the
electromagnetic field. INTERVIEWER: Does that
constitute a different state of matter? A new state? KETTERLE: It is a different
state of matter which is very, very different from ordinary
states of matter. However, I would actually adopt
the suggestion made by Fritz London in the middle of
the 20th century or so, that we should use the fifth state
of matter for systems which are quantum mechanically that
show a macroscopic coherent quantum state. This class of matter would
include superconductors and all superfluids. The Bose-Einstein condensate
would be another member of this very distinguished class
of quantum matter. Some people have labeled the
Bose-Einstein condensate as a completely new state
of matter. I disagree. INTERVIEWER: You would
say it's part of the class of states? KETTERLE: Yes.The Bose-Einstein
condensate has a lot in common with
superconductors and superfluids, which are a very
special club of systems, and the Bose-Einstein condensate is
the first gas in this club. The fact that it's a gas makes
it distinctly different, but still I would put it into
the same category as these other systems. INTERVIEWER: Can you describe
the experiment and the time frame of the project
that actually delivered that result? Getting the atoms to be of a
sufficient density that they constituted this Bose-Einstein
condensate state? KETTERLE: Yep. So we've talked about the
combination of laser cooling and evaporative cooling. In 1992, we bought some vacuum
equipment, electric equipment, optical equipment and
put it together. INTERVIEWER: That
was fun, right? KETTERLE: --that was
a lot of fun. But what I had never expected
was that only a few small ideas were needed to take the
system to Bose-Einstein condensation. We're talking about six orders
of magnitudes-- a factor of 1,000,000, and it was almost
like we took it in one step. I expected that every time we
get a little bit colder, we find something new, something
unknown, we have to learn something about it and implement
something new. But it really happened that
there was only one modification to the atom trap
we had to implement. But other than that, we could
jump over those six orders of magnitude with one big jump. It was really dramatic. INTERVIEWER: That is dramatic. KETTERLE: So we got the
machine to work. There was one major modification
we had to do just to built sort of a
leak-tight trap. A trap that atoms couldn't
leak out. INTERVIEWER: A magnetic trap? KETTERLE: It was a
magnetic trap. I've used a combination of
optiomagnetic trap, but eventually there was a leak in
the trap, we had to plug it. But after that, Bose-Einstein
condensation just happened. INTERVIEWER: You saw it how? KETTERLE: We looked at the
atoms by now I would say digital photography. You can image a cloud of atoms
by illuminating them with light, and then the atoms cast
a shadow on a camera. The shadow told us how the
atoms were moving. In a normal cloud of atoms, if
they're no longer in a trap, just move away from
each other because they have random velocity. But if the cloud is very cold
it's a Bose-Einstein condensate, the motion is so
slow that the cloud is hardly expanding at all. So it was a little
bit like that. If you trap a normal cloud
it just expands. If you trap a Bose-Einstein
condensate it drops like a rock. So that's what we saw. INTERVIEWER: Do you recall
what you said to your colleagues when you observed
that result? What did Pritchard say? KETTERLE: Everybody
was delighted. I remember the moment we saw it
in the lab, which was full off tension and anxiety. The situation was slightly
more complicated. Over several years-- for about three years --
before Bose-Einstein condensation happened, my group
was in a head-to-head race with my colleagues
at Boulder. The group in Boulder actually
pushed ahead of us in the summer of '95. They had observed Bose-Einstein
condensation in June of 1995. INTERVIEWER: You were using
different elements, right? KETTERLE: We used different
elements, we used different methods, we used a
different trap. But in both cases it was about
the combination of laser cooling and evaporative
cooling. So when we discovered
Bose-Einstein condensation in sodium-- that was in September,
three or four months later-- the first reaction
was a big relief. We realized that Bose-Einstein
condensation would start a new era in atomic physics. We were, of course, afraid that
with our atom and our techniques, we may have bet
on the wrong horse. We had been pioneers
of the field. I had put all the resources
into the pursuit of Bose-Einstein condensation,
and came very close. But then, after the success in
Boulder, we were afraid of missing out on all
the excitement. So when Bose-Einstein
condensation finally happened at MIT, the immediate reaction
was a big sigh of relief rather than the excitement of
a new discovery, because the first observation had been made
a few months earlier. INTERVIEWER: How did you extend
on your relief and get into an area that was
fundamentally new? I guess I'm speaking of the
expanding or contracting clouds that begin to tell you
a little bit about how the atoms behave. KETTERLE: Well, in hindsight, in
1995 after we had observed Bose-Einstein condensation here
at MIT, we made the right decision for the wrong reason. In these moments of anxiety,
where we didn't know if our method would work, I was
feverishly analyzing everything I knew. I was in a very competitive
situation. It was an exciting time. I was well-positioned
with my group. But I had to make the
right decision. I had to steer the group into
the right direction. I thought very hard about what
used to be a bottleneck in the experiment, namely the
magnetic trap. So I came up with a new design
for the magnetic trap. Even before we observed
Bose-Einstein condensation, I divided the effort of the lab
and had some people work on the new magnetic trap. The moment when we observed
Bose-Einstein condensation, the new design was sufficiently
well advanced that we took the risk and
decided that we're not even studying the Bose-Einstein
condensation, which we had observed. We had only seen it
on two nights. We had written this important
paper on our observation of Bose-Einstein condensation,
which was crucial for the Nobel Prize. But only after two nights, we
took the whole machine apart and implemented the new idea. When I said it was the right
decision for the wrong reason, my analysis was at that point
that there were so many groups in atomic physics who were
immediately jumping at Bose-Einstein condensation. I knew that those groups had
more resources, more experience with-- I mean all these established
groups-- and I felt that our set up needed an improvement
just to be competitive. I also thought that we would
put in the improvement in a few months. What happened was that for six
months we couldn't produce Bose-Einstein condensation. The machine was down. We had lost the vacuum. But six months later, we had
a set up which was more controlled, produced 10 times
more atoms, and was robust. We had a dream of
an experiment. More what had happened is, the
competition hadn't caught up. All the other labs were
struggling to learn how to combine laser cooling and
evaporative cooling. They had to learn what my group
and the Boulder group had developed over two years. It took all the groups, even
with abundant resources and experienced people, it took all
the groups two full years before they could repeat
Bose-Einstein condensation. For those two years, my group
and the group in Boulder had the only working machines
to produce and study Bose-Einstein condensates. INTERVIEWER: Your machine
was better? KETTERLE: Our machine
was much better. We had 1,000 times more signal
than the Boulder people. We had a better trap. We could just study the
condensate in one experiment after the other. That was an amazing time. It's a dream for a scientist. You have the stuff which
everybody tries to get, and nobody gets it, but
you have it. You can just study it,
look at it, have ideas what to do next-- it was a gold rush. INTERVIEWER: When you say right
decision for the wrong reasons, it sounds like you're
saying that that is the position that every scientist
wants to be in-- finding yourself with the machine
and the ability ahead of everyone else. But that's a great thing. but
the competitive motivations and the worry about being
upstaged by your colleagues is maybe one of the unfortunate
things in experimental science that you wish wasn't there. KETTERLE: No absolutely not. First of all-- INTERVIEWER: So what was
the wrong reason? KETTERLE: I was in an
advanced position. I had one of the two machines
in the world which could produce a Bose-Einstein
condensate. Most reasonable scientists, if
they have reached a stage where they can do unique
experiments, they wouldn't take the machine apart. But we took it apart and
improved it by a big margin. The reason was that I
underestimated what we had accomplished. I underestimated the
uniqueness of our accomplishment and felt other
groups could soon follow up and would have improved
setups. INTERVIEWER: Your worry
was misplaced? KETTERLE: My worry
was misplaced. On the other hand, I think
competition brings the best out of us. Especially in our field, it's
friendly competition. It's competition where your
competitors acknowledge what you have done, where people are
excited about real good science even if it hasn't
happened in their own lab. This is competition, which
brings the best out of you. Without competition I would not
have thought so hard about what is the best solution. We worked even harder because we
knew there was competition. INTERVIEWER: You've talked
repeatedly about wanting to do things that are rooted in real
world problems and to find results that have the potential
of helping real world people. How does this experiment satisfy
that in any way? KETTERLE: To some extent I have
detached myself from the goal by a time scale which is
maybe 20, 30, 40 years. The work I'm doing right
now is fundamental. It improves our understanding
of matter. It leads, in conjunction with
theoretical colleagues to an improved description
of matter. But right now the matter which
we use as a test object to observe and understand new
phenomenon is sort of so cold and you can say so special that
it cannot be used for anything applied. On the other hand, a better
understanding of materials should lead to novel
materials and novel devices in the future. INTERVIEWER: Do you think there
are applications of these extreme low energy states
for the development of superconductors and materials
which will make energy use more efficient? KETTERLE: That's the hope. The hope is that cold atoms may
provide critical insight for future development
of superconductors. INTERVIEWER: I've heard
described that some of the behavior of matter that you've
observed at low energy is remarkably similar to the
apparent behavior of matter at extremely high energies in
things like plasmas as or inside of a neutron star. Do you buy that or do
you think that's an oversimplification? KETTERLE: That's not an
oversimplification because we work at extremely low density. What happens at low temp-- very,
very low temperature, very low density, can be
equivalent if not identical to what happens at higher temperature and higher density. So the scaling is completely
sound and in neutron stars for instance, you have much higher
temperatures but also much, much higher densities and you
may encounter physics which is similar or exactly the same as
we study in our laboratories. INTERVIEWER: Is it possible to
understand the nature of the interior of a neutron star
billions of light years away by observing the atoms
on your desk? KETTERLE: In principle, yes,
but it is not realistic. But it is possible to contribute
one piece of understanding. For instance, when you want
to figure out if you have extremely cold neutrons and they
strongly interact, what is their energy? We can simulate that
with cold atoms. But there are, of course, many
other properties of neutrons, many special properties in the
environment of the neutron star which we cannot
stimulate. But a question like that, what
would be the base energy for a neutron star at zero
temperature, we may find the answer with cold atoms. INTERVIEWER: That's a result
that you might not expected to come out of this journey that
you began in the begin-- middle of the '90s
or early '90s. KETTERLE: You're absolutely
right. If I remember-- if I think
back about the year I described in '95 and '96 when
we were brainstorming what other new experiments we can
do with Bose-Einstein condensates we had
some good ideas. But even our boldest imagination
was not sufficient to anticipate the developments
which the field has taken in the following years. INTERVIEWER: So there
was no plateau? KETTERLE: No. It was just shooting up and some
people feel the field is still expanding. INTERVIEWER: When you became an
assistant professor, what explains the generosity with
which your colleagues appeared to donate, voluntarily give up
lab space, to allow you to explore the questions that
you were working on? KETTERLE: The support and
mentorship I had as a young scientist and as a young
professor here at MIT was unprecedented for me. There is the issue of lab space,
but what came even earlier was the generosity of
my mentor and postdoctoral adviser, Dave Pritchard. Dave Pritchard has been a
pioneer in laser cooling, and in collaboration with
me as a postdoc we pushed things even further. Dave Pritchard at this point had
three experiments in three different subfields
of atomic physics. It was probably clear at this
moment, but definitely clear in hindsight, that the laser
cooling experiment was his most promising and
best experiment. When there was a possibility
that I would join MIT as an assistant professor, he told me
that, well, he didn't want to create a shadow problem, that
I would do laser cooling in his shadow. He told me if I were to stay
at MIT, he would get out of laser cooling and hand
me over all the experimental resources. This is a highly unusual offer--
almost unheard of-- that a creative, active
scientist is getting out of the field he has help to create,
is handing over his best experiment, his
best lab, to a young assistant professor. At the same time he told me,
"This is your own experiment, we will not publish
papers together. You can ask me for advice, but
I want to make sure that you have your full independence." INTERVIEWER: Can you imagine
you yourself doing the same for some young assistant? KETTERLE: Yeah, I've
just done it. I handed over one of my labs to
Martin Zwierlein, who's a young assistant professor, and
one of our young stars. But coming back to 1993 when
Dave Pritchard made me this offer, it was hard for me to--
it felt hard to accept it. He was this famous person and
with what expectation would I take over this experiment? I wasn't sure that I would
really succeed, and whether I could earn this enormous
trust and credit which was given to me. INTERVIEWER: Well he knew one
thing about you-- pressure works with you. KETTERLE: Yeah and I know
there was one of these remarkable discussions. I sat down with him and told
Dave all about my uncertainty whether I would succeed
or not. I told him that I'm here with
children and a German wife. I can't even say for sure
whether I will stay forever at MIT or in this country." Dave
just said, "I know all that. All I expect from you is to work
hard and continue what you have started." So he took
the pressure off my shoulders, and just said, "Well,
just go with it. INTERVIEWER: Focus on science. Do what you do best. KETTERLE: What happened
is almost history. Two years later my group had
realized Bose-Einstein condensation. If I had started at any other
university, I would not have been able to build up
a lab in two years. So after two years, my group
contributed to one of the most exciting developments
in physics. We were right in the middle. So what I did at that point-- I had actually thought
Bose-Einstein condensation could motivate me for
my whole lifetime. I would reach colder and colder
temperatures, and study this and that. That this is the big goal which
keeps you focused on good science for years
and decades. But after two years
I was there. So I went to Dave Pritchard and
said to him, "I would not have done it without you. I was to some extent
executing ideas we had developed together. I was taking over the
lab from you. I want you to co-author the
paper." It's now one of the most highly cited papers
in physics. Dave said, after thinking about
it for a moment, he said, "No, I want you to
take the full credit." INTERVIEWER: So finish
the story with Dave. KETTERLE: So I still feel that
what Dave Pritchard has done for me, how he has supported
me and mentored me, was highly unusual. There were moments it was just
difficult for me to even understand it. So let me try to use some words
which Dave Pritchard used to explain his magnanimous
behavior. At some point he said, "Well, I
gave up an experiment, but I got a wonderful colleague at
MIT." At another time he said, which I think it's just
wonderful expressing kind of the feelings-- he said, he said
it in a speech-- he said, "I gave Wolfgang the keys to
the family car because he could drive it faster than I
could." So, driving the family car is sort of the two of us
being a couple for so many years, collaborating on various
levels, has really been a wonderful experience. INTERVIEWER: Is that a value
that permeates MIT in your experience? KETTERLE: At MIT, there is the
value of enjoying science and being exciting and using that
as one of the, I would say, one of the most important
motivations I think is true. I think the special
circumstances of supporting somebody are very unusual. I think the atomic physics
groups at MIT has with Professors Kleppner and
Pritchard, has sort of this wonderful tradition and history
and it's a very, very friendly family. INTERVIEWER: Describe
your relationship with Professor Kleppner. KETTERLE: There are
two more things about Professor Pritchard. So that's how it developed and
what was very satisfying for me, after I was established and
became as famous as Dave Pritchard, or to some extent
even more famous, the two of us could collaborate again. Dave Pritchard told me we won't
publishe papers together because he wanted me to
get fully independent. But then as two independent
people, we have now worked together, jointly supervised
students for about a decade. In a way, what I also find
remarkable that initially he, of course, was the leader. He taught me many things. He mentored me. But right now Dave Pritchard
is very focused on educational science. So he doesn't really have the
capacity to run a lab completely by himself. But in collaborating with
me, he can still do wonderful physics. So it's for me almost like a
life cycle of the two of us have always been a team. A team when I was his postdoc,
a team when we were independent, but-- the
independent research groups, but that was a lot of advice and
camaraderie and now we are collaborators again. It's just so life cycles. One way how I was able to
express what Dave had done to me was when he we
came back from the ceremony in Stockholm. Back at MIT, I went to his
office and gave him the original of the Nobel medal. I told him I couldn't share the
honor of the Nobel prize with him, but that he has been
a big part of it and this is how I want to express it. I know it meant a lot to him. INTERVIEWER: What was it
like to get the call? KETTERLE: Not bad, but
it's a crazy moment. It's sort of a moment where--
you're called up at five o'clock in the morning. So your brain is not
fully working. You hear somebody telling you
that you just wake up, get to the phone, and somebody tells
you that he's from the Swedish Academy of Sciences and they
want to tell me that I've been awarded the Nobel Prize
in this year. I reacted with joy, with pride
for MIT, for my collaborators, for myself. It's a wonderful
acknowledgement, and yeah, it's very, very special. It's sort of great. You're struck by it,
you are hit by it. INTERVIEWER: How did the family
react here at the Center for Atomic Physics? KETTERLE: It was joyful. It was a celebration
of pride, sharing. It brings together the
whole community. It also brought together
the whole field. Because for Bose-Einstein
condensation, the Nobel Prize was given only six years after
the discovery; the field was in full swing. So everybody who was working
in the field felt elated because the whole field became
distinguished as one of the important frontiers
of physics. You asked about my relation
with Dan Kleppner-- INTERVIEWER: Yes. KETTERLE: --well,
let me fill in. Dan Kleppner was the PhD adviser
of Dave Pritchard. To some extent, the way how
Dan Kleppner was educating Dave and nurturing his career
and allowing him to become independent and famous, that
this sort of repeated itself between Dave and myself. Now Dan is the senior member of
the atomic physics group. I've always-- I mean, I've appreciated his--
the science he's done. He has enormous scientific
skills. He has shown great taste
through his career. He has worked on many
important frontiers. But he also has this wonderful
balanced personality. When I was an assistant
professor, and I felt I was in competition and needed his
advice, he always gave advice. So his balanced personality
and his friendship and his calmness were very,
very important. As a young assistant professor,
I had this dynamic environment at MIT with all
these wonderful students. But with my two mentors, Dave
and Dan, I also had the experience, the tradition,
and the wisdom. I felt when I-- also I was
professor-- but there were situations where I didn't know
what to do, I needed a sounding board. I always found it. INTERVIEWER: Is that something
you're passing along to your colleague Martin Zwierlein, a
young physicist who's somewhat in the position that you were
nearly 20 years ago? KETTERLE: I hope so. At least I'm trying. I'm not sure if I have the same
balance and wisdom as Dan Kleppner, but I'm trying
to give him good advice whenever I can. INTERVIEWER: What's the
excitement that he brings to the department that you see in
his work that maybe reminds you of experiences
in the past? KETTERLE: Well Martin is just
a bundle of energy and joy. He can be excited about physics,
he's understanding physics, he conveys physics. You just realize that everybody
who's working with him is pulled along by him or
even more positively, he brings the best out of people. But you really realize that
after he came back to MIT and started his own lab, the sort
of level of liveliness and energy and excitement in our
hallway has increased. INTERVIEWER: Does that mean
different music being played on the CD players in the
hallways in addition to the great physics? KETTERLE: If you want to use
that as a metaphor, yes. It's like a new, an additional
dance with a lot of intensity. Intensity is good, where
he brings sort of intensity to it. That's also what MIT is about. MIT is a high intensity place. INTERVIEWER: How important is
teaching and interacting with students in your experience
here, in addition to being able to have access to
fundamental research? KETTERLE: I love to teach. Even if somebody would offer
me to opt out of teaching I would not take this offer. I want to teach because it keeps
me grounded, it keeps me connected to the basics, but
also it forces me to, again and again, present the logical
structure of physics and the basics of physics to
learning students. INTERVIEWER: What groups
of students do you teach at this point? All graduates? KETTERLE: I actually alternate
between undergraduate and graduate students. The past semester I was
recitation instructor in quantum physics for
undergraduate students, for juniors. In the spring semester I will
be teaching the graduate course in atomic physics. INTERVIEWER: For undergraduates,
what can you can convey to them that sort
of maybe reminds you of yourself or is important
about teaching? When you're presenting to them
material that is well worked out in your mind but is a
discovery in their mind? KETTERLE: I think it is this
deep understanding. I-- when I teach, I think it
is even more important than teaching the material to convey
your own personality. To convey your personality
as a working physicist. What young people need are role
models, people they can identify with in terms of the
passion they have for their subject or the intensity or they
way they think, the way they analyze. That's what young people
have to learn. When I'm in the classroom, I
like it if a question puts me on the spot. I may not know the answer
but I have to derive it. I'm struggling, I'm
reiterating. But I show my thought process
to the students. Tell them what I check out. How I may construct the answer
of something I know and then add something I have
to think about it. I think to present yourself as
a working physicist to the physics student is important. INTERVIEWER: To convey that
Legos are important over the long haul. KETTERLE: Yes. I think what is maybe
what I think is-- I think showcase here for MIT
is these kind of mentorship. I mean there is something
about-- I know that in the physics
department we rarely hire senior people. We want to nurture ourselves
young talent. So almost all the hires in our
department are done at the assistant professorship. So we want to identify talent
early on and then nurture and develop it. A lot of the people
have become the stars in their fields. Many of them have stayed at MIT
for many more years or for their whole lifetime. This focus on young people and
introducing young people or bringing young people to MIT
and make sure they succeed, this is really a
MIT tradition. I may have experienced it in an
even more unusual form when Dave Pritchard was stepping
back, was handing me over his lab. But in a way, what I experienced
in that form reflects the culture which
we have in the physics department. INTERVIEWER: That's great. Well said. KETTERLE: I don't know how much
it really applies to all the departments. I don't know the hiring kind
of policies of other departments. INTERVIEWER: Well, I mean I
think this mentorship idea is a consistent theme throughout
all the interviews that we've done across many disciplines
at the Institute. So it's not-- I don't think we've necessarily
heard as dramatic story as you-- KETTERLE: This may be one
element which also fits into tradition and mentorship, and
that is when I received the Nobel Prize, I shared it with
two of my colleagues and competitors in Boulder, Eric
Cornell and Carl Wieman. But it's interesting that Eric
Cornell was a graduate student of Dave Pritchard. He left shortly before
I arrived. Carl Wieman was an
undergraduate student with Dan Kleppner. INTERVIEWER: Well
there you go. KETTERLE: So it's clear that
there are other very famous people in the field who
have really been educated by Dan or Dave. INTERVIEWER: It's all
in the family. KETTERLE: Dan or Dave have
really created a family of atomic physicists. Their impact on the field
cannot be overestimated.
He's very good indeed
Fell asleep to it so I guess it worked