PRESENTER: Glad to see the
usual audience at our physics colloquium. [AUDIENCE LAUGHS] You may think that I'm going
to introduce Wolfgang Ketterle, but you'd be wrong. I'm introducing Dan Kleppner. Dan Kleppner came to MIT
after getting his PhD, and serving for some years
on the faculty at Harvard. And he's really famous
for many of the things he's done in atomic physics. For Rydberg atoms, making atoms
as large as 1,000 angstroms in diameter. For many other things,
and for his work with Tom Greytak on
Bose condensation, and spin-aligned hydrogen. Dan, when he came to MIT,
began the establishment of what is now a truly great
school of atomic physics. And the test of a great
school is the students, and several of those students
have won Nobel Prizes. Bill Phillips in 1997 was
a student of Dan's, and one of this year's
prizewinners, Carl Wyman, did his undergraduate
thesis research with Dan. Dan is a great physicist,
a great educator. Those of you who've taken 8012
know his wonderful textbook. He's a great writer. He writes columns in physics
today that are wonderful. And he's a great colleague. And it's my great
pleasure to introduce Dan. [APPLAUSE] KLEPPNER: So success has many
fathers, and some grandfathers. And today I am a very
happy grandfather. And I suspect that
Norman Ramsey over here is a very happy
great-grandfather. [AUDIENCE CHUCKLES] [APPLAUSE] I had the great good fortune
to have Dave Pritchard as one of my first graduate
students, and to have the experience of him
as a colleague at MIT for many years. Dave has many accomplishments. This year he has
mentored, directly, two of the Nobel Prize winners. Wolfgang Ketterle,
and Eric Cornell was a graduate student of his. He has made many seminal
contributions to physics. He invented the
field of atom optics. And he made seminal
contributions to the work, which garnered
the Nobel Prize in 1997 for atom cooling, and trapping. He invented the
Ioffe-Pritchard trap. He did the seminal
work on what's called the MOT, which is
the workhorse in that area. And in 1990 he invited
Wolfgang Ketterle to come as a post-doc to work
with him on the atom cooling, even though Wolfgang had
no background in that area. He spotted Wolfgang's
qualities, and the two of them together did such fabulous work
that it was clear that Wolfgang was headed for great things. But not at MIT, because we could
not appoint a young faculty member who would
be in competition with a senior faculty
member, or who would really be in close collaboration. Because we expect our faculty to
be able to work independently. Dave overcame that obstacle
by withdrawing from the field, and turning over his
equipment to Wolfgang. So Wolfgang could start out his
work towards Bose condensation at top speed. And the rest of that is history,
and you'll hear more about it. But to me this
example of mentoring, which is to leave a
field in which you're very interested in doing
outstanding work to make room for young talent, is in the
very best tradition of science and of teaching. And I hope in the very
best tradition of MIT. So with this those
thoughts, I would like now to introduce to
you, Dave Pritchard. [APPLAUSE] PRITCHARD: Well, thank
you very much Dan. By my account, this
is the fourth time that Wolfgang has given an
MIT physics colloquium here in the six years since
he's been on the faculty, and up to the seventh now. This is a sign that, well, it's
actually in the past six years. This is a sign that MIT
recognized his worth somewhat before the Nobel
Prize committee. Since I've had the honor of
introducing him three times, I thought I might just summarize
those introductions to give you an idea of his growth. Now I know some of you
were saying, wait a minute, three introducers, and
now three introductions. But it's okay because
we discussed it. And we realized that
if each one of us talked a factor of three
longer than the first, then the previous speaker, and
Wolfgang followed suit, he'd still talk for more
than 2/3 of the time. You can work that out. So in the first introduction I
said, Wolfgang was my post-doc, and we were fortunate
to get him here to take over running
my cold atom apparatus because he can run it so
much better than I could. As expected, he has
made fantastic progress. Now having made an
observation of BEC, but with orders of magnitude
more atoms than the condensate that Eric Cornell made at JILA. And the second introduction
not too much, but a year later I said, Wolfgang is
absolutely courageous. With the best BEC, the
biggest BEC in the world, and with his phone ringing
off the hook with invitations. Neither of these is
an irrelevant concern for a professor who
doesn't have tenure. He decided to scrap
his magnetic trap. He felt that a new cloverleaf
magnetic trap that he designed would allow for better
scientific studies of the condensate. As usual, he was
right, and so on. So you want to hear
something like that. That's the second one. The third time I said, Wolfgang
Ketterle needs no introduction. He-- [AUDIENCE LAUGHS] [CHUCKLES] He was just promoted
from assistant professor to a full professor
in a single jump, and he justifies this by doing
spectacular research, and so on, and so forth. Now when I hear the trite
phrase, needs no introduction, I often know the speaker myself. And I then agree, of
course, with the introducer. But it always raises in
my mind the question, well who's this guy who's
doing the introduction? Doesn't he need an introduction? And at this fourth
introduction I see that the relative
stature of Wolfgang has grown so disproportionately
that many other people are now concerned with this issue. And that obviously
explains today's format where there were two
guys who introduced me. [AUDIENCE LAUGHS] Occasionally I
feel that I should try to retain some
of my mentorship. A vestige of that, and
so I try to challenge Wolfgang to get even better. This is really difficult to
do, because of all the people that I've worked with,
which so far includes six Nobel laureates. He's the only one who was
A-plus at absolutely everything, scientific tastes,
design, organization, group management,
writing, scientific talks, public lectures, A-plus. So challenging them
is like saying, well very good about jumping
over all those tall buildings with a single bound,
but if you could learn to jump over
whole cities you wouldn't crush so
many automobiles. Now previously
I've challenged him by guaranteeing that he would
give a spectacular talk. And actually today I
can raise the bar simply by challenging him
to do that again, because I know that
this is a week where he hasn't had much sleep. And he's had numerous
distractions from the press, and from the many interviews. But I'm sure that he's up to it. One final note, just from
my personal perspective. I want to say that your most
important, A-plus ability is as a colleague and friend. My world has been
tremendously brightened since you walked into my lab. And I'm glad to see that
MIT is the same way. [APPLAUSE] [SIGHS] KETTERLE: What a place,
and what an introduction. I think you heard from the
previous people who introduced me what nice a place this is. What great mentors I've
had, and sometimes I feel I was privileged to finish
what those people have started. And you will see
that in my talk. But you, I think you've also
heard from those introduction that this place, and
the atomic physics community, or the
physics department, is an inspiring and
friendly environment. And it is this environment
which brings the best out of us. Now Dave was raising
the bar by saying, by sort of announcing
an A-plus talk. I think this talk for
me has two superlatives. Number one, it is the
best-attended lecture I've ever given. [AUDIENCE LAUGHS] [HUFFS] But now I'm embarrassed
to say number two. It's a worse prepared one. Because I was given very
short notice, and I thought, so well, I'll just put a
few note cards together. But the closer this
talk came, the more I felt it's a very
emotional talk for me. Because this is my family. This is my place. And I wanted to tell a
story, and not just give a talk on our recent results. So I try to prepare
something special, but I couldn't really polish it. So let me tell this story of
Bose-Einstein condensates, which is the coldest
matter in the universe. And since I know here
is a part audience, I want to be explicit
in one of the concepts. So the ball's condensate,
you can regard it as metal made of matter waves. It's a form of matter where
the quantum mechanical nature, the wave nature of
matter, manifests itself at a microscopic scale. So you're all familiar
with the, or most of you I hope, with a particle-wave
duality for light, for photons. But we have the same
particle-wave duality for all particles. We know from the early
days of quantum mechanics that a particle, which is
classically characterized by position and
velocity, should either be described as a quantum
mechanical wave, a de Broglie wave, and the wavelengths
of the de Broglie wave of the quantum
mechanical wave follows the famous
de Broglie relation. It's inversely proportional to
the velocity of the particle. And I know when
we learn physics, we all struggled
with the concept that particles are all objects. You, and me, and
microscopic objects, and macroscopic objects
are waves and particles at the same time. Particles propagate as waves,
but when we detect them it makes click in a detector. So well, if you and
me feel all waves, why don't we perceive
the wave nature of matter in everyday life? So let me raise you, let
me raise the question, how do we perceive if
something is a wave or not? So if I would
challenge you and say, explain me why sound is a wave. A very reasonable
answer would be, if there are two people
who talk to each other, they can hear each other even
if they don't see each other. Because the wave propagates,
deflects around corners. If I would ask you, what
happens in the case of light? You all know that light is
an electromagnetic wave. It's not so obvious, because
the wavelengths of light is so short that light is
just deflecting a little bit. And you can't really perceive
the wave nature of light very easily. But now in the case
of matter waves, the wavelengths
are even shorter. The wavelengths is given
by the de Broglie relation, and the velocity
of a particle is determined by its temperature. So from that we know that
the colder the temperature, the longer is the de
Broglie wavelengths. Now talking about atoms,
at room temperature, the matter wavelengths,
the de Broglie wavelengths, is smaller than the
size of an atom. So therefore the wave nature
of atoms is not manifest. But now we can cool down,
and the title of my talk is, The Coldest Matter
in the Universe. We cool down to micro and
nano Kelvin temperatures. More than a million times
colder than interstellar space. And the de Broglie wavelengths
becomes longer and longer. And this value is
30 micrometer is a fraction of the
diameter of a human hair. So it's really
getting microscopic. And this long de
Broglie wavelengths, they are at the heart
of the phenomenon which I want to describe. Bose-Einstein condensation. What happens if you take
a gas in a container, and cool it down? The particles slow down. As I just mentioned,
we shouldn't describe the particles as
sort of little billiard balls. They are wave packets. They are quantum
mechanical waves. But as long as the
wavelengths is very short, those wave packets
are very localized. And we can follow the
motion of those wave packets as if they were particles. But now we can cool down. The de Broglie wavelengths
becomes longer and longer. And when we reach the point
where the wave packets, where the de Broglie
waves overlap, then we cannot follow
individual particles anymore. They become sort of a
quantum soup of wave packets, and it's exactly at this point
when the wavelengths, the de Broglie wavelengths is
comparable to the spacing of particles that a new
form of matter forms. And this is the
Bose-Einstein condensate. Or to say it again, if the
de Broglie wavelengths become longer and longer, and those
matter waves overlap, then all the particles
in the gas, they start to oscillate in concert. And what they form is
one giant matter wave. And this is the
Bose-Einstein condensates. Just use an analogy. The difference between
atoms in random motion, and the Bose condensate is
exactly the same difference as the light from a
light bulb, and the light coming out of a laser. So what Bose-Einstein
condensate is about, it's about the creation of atoms
with laser light properties. And this is a nice
artist's conception of how you can imagine the
Bose-Einstein condensate. First of all, these are
indistinguishable atoms. You see they have all the
same facial expression, and they march in lockstep,
which means they are in phase. It's a single wave function. And this is how you can
imagine what happens in a Bose-Einstein condensate. You also see sort
of those other guys. They walk in random direction. They're not Bose condensed. They are in different
quantum states, and are therefore
distinguishable from the atoms in the condensate. Now of course, what I just said,
that all the atoms go into one quantum state, and
fall in lockstep is only possible for atoms
which we'll refer to as bosons. Particles with
have integer spin. I don't want to go
into details here, but there is another class
of particles called fermions. The have half integer
spin, and their behavior at low temperature is
completely different. So the phenomenon of
Bose-Einstein condensation can be regarded as the
most striking manifestation of quantum statistics, of
the difference between bosons and fermions. And the name
Bose-Einstein condensation is still alive from those
two well-known people who predicted it in the 1920s. However, Bose-Einstein
condensation in a gas was not realized until
'95 for reasons which will become obvious in a moment. So the prediction of
Bose-Einstein's statistics was 1925. And for many, many years,
the only manifestation of Bose-Einstein condensation
was in liquid helium. The superfluity of
liquid helium is a consequence of
Bose-Einstein condensation, but it's a liquid. It's not a gas. And there are high
density effects, which modify this phenomenon greatly. But then there was
an attempt to realize Bose-Einstein
condensation dilute gases, and a lot of credit for that
should go to Tom Greytak, and Dan Kleppner, who
are here on the faculty. They really put
Bose-Einstein condensation, and dilute atomic gases on the
agenda, and made it a goal. And it's, I'm very
pleased to mention that after 20 years
of efforts they realized Bose-Einstein
condensation in hydrogen, and achieved a
longstanding goal. However, a few
years earlier there were new cooling
techniques developed. Laser cooling, I would
briefly comment on them. And new systems, alkali
atoms, could be cooled to ultra low temperatures. And it's really the
alkali atoms which have created this
whole new field of Bose-Einstein condensation
with many, many activities. And the success
here came in '95. And I will tell you
the story today. So well, I think from
most of the concepts I've presented, if
you want to achieve Bose-Einstein condensation,
quote, unquote. All you have to do is
you have to cool down the gas to micro and
nano Kelvin temperatures. You have to cool it down until
the de Broglie wavelengths is comparable to the
spacing between atoms, and the matter waves
overlap, and start to oscillate in concert. So, I mention that the
de Broglie wavelengths depends on temperature. The distance between
atoms depends on density. So we haven't, so the transition
to Bose-Einstein condensation is characterized by a
combination of temperature and density. So let's just look at this
relation for something of the density of water. It would predict that the
transition temperature is 1 Kelvin, which is easy to reach. However, tough luck at 1 Kelvin
everything is liquid or frozen, and you can't observe the
phenomenon Bose-Einstein condensation. So, in order to
prevent that, and also to prevent formation
of molecules out of the atomic gas, we had to
work at extremely low density. So the system I'm
referring to is at a density which is a billion
times more dilute than water. It's 100,000 times
thinner than air. And in those gases,
we can observe Bose-Einstein condensation
without self-destruction by molecule cluster
formation, and solidification. But there is a price to be paid. If you work at
ultra low density, you need ultra long
de Broglie wavelengths to make those matter waves
oscillate in concept. So we needed the lowest
temperatures ever achieved. So we needed cooling methods. And what I'm actually
talking about is a combination of
two different cooling methods, which we had developed
in two different subareas. One is laser cooling. the pioneers of laser cooling. Some of them were recognized
with a Nobel Prize in '97. And major developments
to laser cooling. Were also done by
Dave Pritchard at MIT. The principle of laser
cooling is fairly simple. You shine laser light on
atoms, the atom skate on light. And if you play some tricks,
which I don't want to you explain, that the light, which
is scattered, which is emitted, has a shorter
wavelength, is more energetic than the
absorbed light, than the scattering of light
removes energy from the system, and the system cools down. It works great. You just shine laser
light on atoms, and you create
micro-Kelvin samples. And a very important
configuration to do that is the one co-invented
by Dave Pritchard. And I think this is probably one
of the most important singular techniques which
were ever introduced to the field of atomic physics,
the magneto-optical trap. It is the workhorse of the whole
field of ultra cold atoms now. So well, and if you have this
configuration of laser light, and shine light into a vacuum
chamber, and add some atoms. You see, this is in one
of our lab upstairs. You see a small ball, a few
millimeters in diameter, yellow glow. So you can see with the naked
eye, ultra cold sodium atoms. But at the fairly
hot temperature of a fraction of a milli-Kelvin. So this is the starting
point for the next stage of cooling towards
Bose-Einstein condensation. Laser cooling has
so far not been able to carry atoms down to
Bose-Einstein condensation, because it has limitations. And some of them are
very fundamental. If you scatter light from
atoms, each photon is the, photons are grainy. It's like sort of
sand corns which transfer a finite of equal
momentum to the atoms. So therefore, the temperature
of the standard laser cooling methods is limited
to micro-Kelvin. So well, if you
can't get very cold you should get very
dense, because it's a density-temperature
combination which makes Bose condensation happens. But unfortunately, there's
also a density limitation in laser cooling, which,
without going into details, is simply set by the process. If you have a very dense
sample, all the laser light is absorbed, and laser
cooling doesn't work. So laser cooling, at least
in the beginning of the 90s, fell short by four orders
of magnitude in density, or three orders of
magnitude in temperature, to reach Bose-Einstein
condensation. So well, this is something
which Dave Pritchard developed. And I was a post-doc with him. But next door is
Dan Kleppner, who together with Tom
Greytak, and Harold Hess, developed evaporative cooling. And this is a different
cooling method, which is not suffering from
all the limitations of laser cooling. So let me explain what
evaporative cooling is. Actually, before I
explain the slide, evaporative cooling is what
happens in everyday life. If you sit in a bathtub,
the water cools down. And it cools down because
the hottest particles escape as steam. And what remains behind are
the less energetic particles. This is called evaporation. So well, if you do
it in an atom trap, if you can find atoms
with magnetic forces, we create such a potential. It's like an invisible
container, the walls of which are magnetic fields. I don't want to say
more about it right now. But so if particles
in this container, and if two such
particles collide, one can lower its
energy, come to rest. These are the particles
we want to keep. They're really cold. And the more energetic
one can escape. And this was suggested
by Harold Hess, and realized in the group of
Dan Kleppner and Tom Greytak. So if we have a method to
kind of eliminate the hottest particles in many, many steps,
and the remaining particles we thermalize. We create a thermal
distribution which gets colder, and colder, and colder. So the question is
how to implement it? And Dave Pritchard
had a brilliant idea. We sort of X which,
so well, it's even dripping with blood here. Okay, well what is a
suitable X for atoms? So well, if you're in a magnetic
trap, there's a very simple X. And this is just switch the
magnetic moment of the atoms so that the attractive
force, the confinement force turns into a repulsive one. You just have to shine a radio
frequency onto the atoms, and you have your X, and
you can do the cooling. So this was Dave Pritchard's
suggestion in '89. So well, here are
two cooling methods developed by the people
who had their office next to me, laser cooling
and evaporative cooling. Laser cooling works
great initially, has certain limitations. But evaporative cooling
does not suffer from them. So it probably doesn't
require a rocket scientist to realize it would be
a good idea to combine the best of two worlds. So this was sort of the
idea in the early '90s. But there was a problem,
which some people regarded as fundamental. And the problem is
that, I mentioned that laser calling doesn't work
when the sample becomes opaque, when the light gets absorbed. On the other hand,
evaporative cooling requires collisions
between particles. One particle gets colder,
and one particle gets hotter. And now, if you look at the
collision cross-section, you face a dilemma. We don't want light
to be absorbed, but we want
collisions to happen. And the difference, the
ratio of those collision cross-sections effect of 1,000. So it seemed absolutely
impossible to have many collisions for evaporative
cooling, and at the same time avoid the complete
absorption of laser light. A solution, which Dave Pritchard
developed together with me was, so well, we can modify. We can push optical
cooling to higher densities by just playing a simple trick. If we hide the cold atoms,
in a different quantum state so that the light
is not absorbed, we can push laser cooling
to higher densities. I don't want to go into
details, but by modifying the standard knot,
which Dave co-invented, we realized something which
is called the dark spot And it was a key technique
to achieve Bose condensation both at MIT, and at Boulder. And I just showed you
the result, which at that point in '92, published in
'93 was fairly impressive. We were shining light
at a cloud of atoms, and the shadow was really black. We could infer that
the transmitted light was 10 to the minus-80. This was absolutely black. And so in '92 what
we had in our hands was an unprecedented
combination of high density and large number of atoms. And then, you told
stories about me, Dave. Now I have to tell
a story about you. Namely I still, I mean I
remember those dramatic days. This was the summer of
'92, and Dave Pritchard, he had funded work,
funded proposals, and he had great ideas to
study cold collisions, to study cold molecule formation,
and eventually many people got a lot of credit for that. But we had, so this
was the agenda, and this was the proposed work,
and this is what the funding agencies had supported. But then we sat together,
and decided maybe we can do something else with
those high density sample. Maybe we can go for the very
speculative, and challenging goal of Bose-Einstein
condensation. And it was for me
just this signature of a great scientist
that Dave said, okay, I give up all my ideas,
which were really great. And which other people got
enormous credit for that. Let's do what you suggest. Let's go for the long haul. Let's try to go
further, and use this as a starting
point for the quest for Bose-Einstein condensation. And it was very dramatic. Within a few weeks, even
before publishing the paper, we placed all the orders
for additional equipment necessary to do the next step. To combine laser cooling
with evaporative cooling. So eventually we
wanted to embark on the agenda to use laser
cooling as the first step. And then lower the temperature
by evaporative cooling, and get to Bose-Einstein
condensation. So well, we had to build a
complete machinery for that. And it took one or two
years to assemble it. And then we were sort of ready. And tried to see
evaporative cooling. And this, actually May of
'94 was an important step. I went to a meeting, and
announced for the first time that laser cooling,
and evaporative cooling had been combined. By using this high
density sample of atoms, we could bridge the gap between
this factor of thousands difference in cross section
of light absorption, elastic collision. We had seen evaporative
cooling, and now we were hoping to push further
to lower temperature. But still, we were five orders
of magnitude away from BEC. At the same meeting,
the Boulder group announced that they
accomplish something similar. So the race went on, and it was
one of the most exciting races of my lifetime. And Eric Cornell, Carl
Wieman, and myself, we remained friends. And we talked to each other
on the phone on Tuesday. And people would
say to each other, you got the best out of me
because if you have competition you work harder, you think
harder, and you do better work. Actually, at that
point I thought it could take the
rest of my life to push further
incrementally, and explore the new physics, which would
come about at higher densities, and lower temperatures. And I knew that the quest for
Bose-Einstein condensation in Tom Greytaks, and
Dan Kleppner's lab had already lasted 15 years. And I was expecting,
so well we trying on something alternative. And let's see which
method works better. And to the big, big
surprise, and it came completely
unexpected, just one year later Bose-Einstein
condensation was achieved. It's one of the rare
examples, I think, where you think you do
an incremental step, and you step forward by
a factor of a million. So these were very
dramatic moments. Let me mention a little bit
about the last obstacles on the finish line to
Bose-Einstein condensation. So the combination of
evaporative cooling, and laser cooling was accomplished. But in order to have enough
collisions between the atoms to have good evaporation,
we had to keep the gas tightly confined. So we needed tightly confining
containers, or magnetic traps. Now this may be a
little bit technical, but some people in the
audience who probably are interested in that. There are two types
of magnetic traps. Some have a pointy potentially,
a linear re-shape potential. And others have a round
potential at the bottom. Those traps are more
tightly confining, and that's what both the
group in Boulder and at MIT used to demonstrate initial
evaporative cooling because it makes it easier to
get many collisions, and see evaporation. But they have a problem. There is a point of zero
magnetic field in the center. And if the field
is zero, the atoms don't know where to
spin up, and spin down. They lose their orientation, and
virtually flip out of the trap. So we lost the atoms. It's like a hole
where atoms leak out. And the months before
BEC was achieved, both groups had
to invent methods. And this, how it was dubbed,
how to plug the hole. Eric Cornell's group in Boulder
used an ingenious method with rotating magnetic fields. And we decided to use
a different method. We wanted to plug the hole
with a laser beam, which was keeping the atoms away
from this dangerous region where they would just fall
out of the magnetic trap. So what we created
was a potential, which has a hole in the middle. And you see here the
shape of our clouds. And it worked great. It was really dramatic. We could push forward
orders of magnitude towards Bose-Einstein
condensation. And then, this was
in June of '95, we heard about the
Boulder results. They had succeeded to get
Bose-Einstein condensation. And that's maybe
something, I don't know if I've shared that publicly. If you're an
assistant professor, you put all your money,
all your resources, all your efforts in one
goal, and then you're notified that somebody
else has accomplished it. This is a difficult situation. [AUDIENCE LAUGHS] You wonder about promotion. You wonder about your
scientific career, and of course I know at
MIT the bar is very high. MIT is not keeping losers. So I was really
scratching my head. I woke up early in the morning. What to do? And people ask me, why
don't you just copy the design in Boulder? And I mean this seems to be
a successful way to do it. And then a whole
new field opens up. And we had fierce
discussion in my group. And what we did is actually
a two-fold strategy. We designed a new magnetic trap. I will talk about it. And this turned into the
workhorse of the field. And this has given us
an enormous productivity over the last few years. But, and I was sort of
willing to say let's just put all our eggs
into this basket. Let's pursue that, because
this new magnetic trap is a really good idea. And indeed, it turned
out to be a good idea. But I work with a good group
of people, and they told me, so well, this trap with the
plug, maybe it will work. Or even if it doesn't work,
because maybe the laser beam is jittering, and
heating up the atoms. Let's just give it
one, or two more tries. And that's really what,
let's just figure out how far we've come,
because the Boulder group has a big success. And we should at least
document how close we came. So well, I try to listen to
my collaborators, and said, okay, but just a few times. We can't waste time. We don't want to miss the boat. And it was one of the
very last attempts to see how far we could
push with our original idea of this optically
plugged trap that we saw Bose-Einstein condensation. This is a picture,
which doesn't look nice, but I want to show it
to sort of the experts. This was our first signal of
Bose-Einstein condensation on September 30th in '95. We were not really
ready to observe. We just wanted to see where are
the limits of the plugged trap. And for the experts, we couldn't
switch off the laser beam. So in time of flight,
the atoms were just pushed away by the laser beam. So it's not a pretty
time of flight picture. It doesn't look as
pretty as the other ones. But we saw something black. Sharp, black spots
on the screen. And if a cloud
expands ballistically. And stays compact, that means
there's very little velocity. There is very little motion. It's very, very cold. And we got enormously excited. And within a few
hours, we worked throughout the whole night. We improved the set up,
and we got clear evidence that we had observed
Bose-Einstein condensation. We were running the
experiment only one more time to get final data. And this is actually,
for me, still amazing. This is the quality of the data. Excuse me. Which was obtained
the second time we were ever running the machine
for Bose-Einstein condensation. Let me just explain
what it means. We let the gas go,
and it expands out. And then we take a
snapshot, and the larger the cloud is, the
faster is the velocity, the hotter are the atoms. And now you see what
happens when we cool down. You see how this cloud
is sort of shrinking because the atom slowed down. And this is sort of
the last factor of two. The last fraction of
a second to achieve Bose-Einstein condensation, and
then this new form of matter. This ultra cold gas, which is a
Bose-Einstein condensate forms. It's very visual. It's very dramatic. And we obtained that without
really optimizing our machine. Okay, so I mentioned that this
was a night on September 30th. And so well, at MIT
you have to teach. And October 1st I
had to teach 8012. [AUDIENCE LAUGHS] I received an email message
from one of my students now, six years later. As a freshman in the fall of
'95, I took your 8012 class. I remember one day when
you were unusually late for your lecture,
and you came rushing in very excited, yet very
tired from the previous night's experiment. I think I told the students I
didn't get any sleep at all. And please, if I fall
asleep during my class, ask me a question. [AUDIENCE LAUGHS] So well, a few weeks later
there was some short report in Tick Tock, but
this was the night that Bose-Einstein condensation
was realized at MIT. Okay, but immediately
with our approach, we had 100 times more atoms. And we had a system,
which was more versatile to pursue
the new physics of Bose-Einstein condensation. This is a picture of the
group a few months earlier. It doesn't show
Ken Davis, but it shows the people, the other
people who were on board. Dylan Durfee, Mark
Mavis, Michael Andrews, Ben Stamper-Kurn, Klaas-Jan
van Druten as a post-doc, and Chris Townsend joined a
few months after we saw BEC. Anyway, we were
playing, and I'll tell you a story of sometimes
you do the right thing for the wrong reason. And this is when we changed our
magnetic trap, which is really the core of the apparatus. We were one of the
two or three groups that was a little bit
ubiquitous observation by a third group, who had seen
Bose-Einstein condensation. And everybody was
brimming with excitement, and I made a wrong assessment. I said, hey it wasn't really
so complicated, what we do. In the next few months,
many groups around the world will repeat it. And now we have to
be really careful. Maybe with our jittery laser
beam, we're not competitive. We really have to go forward,
and make an improvement. And we thought, let's
build a new magnetic trap. What happened, actually,
is it took us five months to build it. We couldn't get any
Bose condensation for the next five months. But when we got it, we were
really going like gangbusters. But my assessment was
wrong, because nobody else got Bose condensation
in the meantime. Indeed, to build such
a complex apparatus it took two years for
the other groups. So if we had known that, we
would have probably just stuck to our original set up. But we went forward,
and created what turned out to be a
good trap, and which gave us enormous productivity. So we realized this
trapping configuration, which is not suffering from
this leak, it's sort of a round bottom trap. It actually carries the
name Ioffe-Pritchard because Dave Pritchard
suggested that in the mid '80s. And we found a new
way to wind the trap. It's now referred to
as a cloverleaf trap. And so well, this is how we
have produced Bose condensates ever since. This shows you the
actual winding path. And now I show
you a few examples of what we were able
to do in this trap. It was an enormously
productive time, and one of my
graduate students, who was with me between '95 and
'99, he graduated with 20 papers in [INAUDIBLE],,
Nature, or Science. So these were one of
the first experiments we did in this novel trap. We could now visualize the
condensate inside the trap using a novel light
scattering technique, and now we could really
observe what was going on inside the condensate. And here you see the cool down. Our magnetic container
was elongated, so it's an elongated cloud,
and it shrinks as we cool. And when we reach
the phase transition in the middle of that
trap we see this formation of a dense core that
Bose-Einstein condensate. And if you just take a
cut through this cloud you can see, you know, this. You remember the blue guys
who march in lockstep? This is this peak, and then
you have a thermal component. These were the colorful guys
which were moving and whizzing around in all directions. And we did a series
of experiments. Here you see a condensate
set in oscillation. People call it the
study of sound, and some of those
results were just the frequency of oscillation. What is its damping was really
causing major challenges to many body theory, and led to
hundreds of theoretical papers. On something which I
felt was very dramatic, those nano-Kelvin atoms, you
think they are so fragile. It's colder than anything
else, but by using this light scattering
technique and just stroboscopic illumination
we could really see how, color-coded in red,
how condensate formed. It was for me like watching
how nature is giving birth to something, which
is very fragile, but nevertheless we
were able to observe it in its natural environment. And actually, the way how
the condensate forms was challenging theory, and
over many, many years now, quantum kinetics theory has
been developed, to some extent, to explain the dynamics of the
formation of the condensate. But let me now talk about two
examples of our research, which really demonstrates what makes
Bose condensate so special. Now, if I talk to
a lay audience, so if I want to help you. If you meet your friends,
and you want to tell them, what is really special about
Bose-Einstein condensates? I want to now give
you two examples how you can convince people
that this is something that has unusual properties. And it's really related
to what I call it here, the magic of matter waves. Now I want to show
you two examples. The interference of
condensates, which allow you to directly
photograph matter waves. And I want to show
you the example of rotating
condensates, which have a Swiss cheese-like appearance. This is very different from how
any ordinary matter behaves. Okay. What I want to
explain to you now, the interference
between condensates was actually a key
experiment which we did. And it was also actually one
of our key contributions, which was emphasized
by the Swedish Academy. Because what we could do in
'96, and publish in '97 was we could show for the first time
that the condensate is not really cold. So well, coldness is
relative, but that it has these special properties
that all the atoms march in lockstep. That they're just one quantum
mechanical matter wave. Okay, if you want to
show that something, if you want to show, for
instance, that this laser beam is an electromagnetic wave,
the best way to do it is you take two laser beams,
and interfere them. And then you see an
interference pattern. The interference is the clear
evidence for the wave nature. So we needed to condensates, and
so well by using a laser knife, and cutting a condensate
into two pieces, foregoing all the details. We created two condensates. And so now we had
two condensates, and we switched off our magnetic
container, and the condensates, so well, if you switch
it off, the condensates fall down due to gravity. But they also expand because
of zero point motion, and atomic repulsion. So what we are hoping
for is as the condensates expand into each other,
and overlap to see the wave nature of those atoms. Certainly showed you
what we expected to see. So let me just give you an
example for the interference between two sources. So assume that this is the
antenna of our favorite radio station in Boston, and
it radiates radio waves. And now we bring in a
second radio station, and now the waves of the two
radio stations interfere. And now you see the
interference pattern. And of course, the
interference pattern becomes more microscopic
the closer the distance is. And at least those
who took 803, they should realize that this
interference pattern has the form of parabolas,
because a parabola is a mathematical curve for which
the path length difference to the two origins is constant. So well, we're not
interfering radio waves, we're interfering matter waves. So we have our container,
the atoms spread out, and I have to
remind you probably of pretty much the simple
equation I showed you, the de Broglie relation. The faster the atoms
are, the shorter is the de Broglie wavelengths. So if you have a puff
of gas, and let it go, and then take a
snapshot, the atoms which are further out they
have moved there faster. And the atoms which are
closer in, they are slower. So the wave pattern
of such a pulsed matter wave looks like this. The matter wavelengths
is longer where the atoms haven't traveled so far, and
it is shorter where they have travelled further. So now we interfere our two
Bose-Einstein condensates. And you again see now, as we
bring them closer together, that there is a distinct
interference pattern. But in contrast to
the two radio station, this interference
pattern consists of perfectly straight lines. And when we observe the
overlap of the two condensates, and we saw that, we were really
jumping up from our chairs. And I think I remember the
first night we saw that. Again, we had worked into
the wee hours in the morning. I think it was you, Dan,
you came in at 8:00 or 9:00, at 8 or 9 o'clock and
I was almost drunk, and I came to Dan's office. Dan, I have to
show you something. And Dan came to the office,
and Dan came to the lab and we had this
pattern on the screen. So we knew the Bose condensate
is a coherent form of matter. Actually, just to talk in terms
of layperson, what you actually see here is the-- well, two things. One is, this is a
direct observation of the wave nature of matter. We simply take two puffs
of atoms, they overlap, and then we illuminate the
atoms with a laser beam. And we take a shadow image. There is, it's the
most direct observation of the nature of matter. But secondly, this
is also demonstrating in a fairly dramatic way, this
equation that atoms plus atoms, gives vacuum. Because where there is no
shadow, there are no atoms. So the two matter waves
interfere each other away. Atoms plus atoms gives vacuum. I mean, I sometimes get
fan mail from people with their own theories
of physics, and. [AUDIENCE LAUGHS] And I got some
questions about, yeah, what happens if you annihilate
atoms by interference, but I think you all know. The atoms which
are missing here, they appear in the dark
fringes because atom plus atom equals four atoms. If you have coherent
interference. Okay, so this is just
a different, more fancy representation of that. At about the same time, we found
a trick to release atoms out of the magnetic trap. We were just sort of
spin-flipping atoms. They were accelerated
down by gravity, and so within a few months
we had done two things. We had shown that the
atoms are laser-like, and secondly we were able to
create pulse beams of atoms. And this together
is now recognized as the first realization
of an atom laser. An atom laser, which
emits atomic matter with laser light properties. And other groups have
followed, and improved on that. So this is sort of an
atom laser gallery. Continues atom lasers
and in different ways its a rich field, and
there's a lot to be done. Let me now talk about
this other example, and where the wave nature of
matter really becomes manifest. And this is actually related
to a very special property of the Bose condensate. The Bose condensate is
sort of one giant matter wave, and its possible
for wave-like matter to move, to propagate
without any friction. This one similar
system which has enormous technical importance,
and these are super conductors. In superconductors
electricity can flow without any dissipation. You can switch on
a ring current, and the ring-- you can go
home, and months later the ring current still flows
without any active power. Those phenomena are
usually dubbed, "super." Super conductivity for
electrons, super fluidity for liquid helium. And here we have
a system which was predicted to be super
fluid, but its a gas. Its eight orders of magnitude
more dilute the liquid helium. So we have the most
fundamental system in our hands to
understand details of super fluidity or
macroscopic random phenomena. I don't want to go into
details, and rather emphasize one aspect,
which is fairly directly connected to superfluity. But I'm not really
explaining it, and this is the
physics of vortices. Now, I think you are
familiar with vortices. This as a vortex on
a very large scale. You see how they can,
off the coast of Florida. I think we also encounter
vortices in our everyday life. At least, I could
used one today. [AUDIENCE LAUGHS] You simply push the button,
and you create a vortex. So what is special
about vortex physics? Okay, there's something
important difference. And that's what I
want to explain. Let's consider this
rotating bucket experiment. And I want to show
you that there's a dramatic difference between
rotating, and ordinary fluid. Let's say a bucket
full of water, and rotating what I
call a quantum fluid. And this is the system where
all the particles are just one big wave. First of all, if you
rotate a normal fluid, you know that upon the rotation
it forms this parabolic surface because of centrifugal forces. If you take, for instance,
superfluid helium, and you do the same. I just told you there's
a big difference, but the same happens. It's a parabolic surface. And it has to be like this,
because on large scales there is a correspondence
equivalence between a classical system,
and the quantum system. But if you now look a little bit
closer onto the quantum system, you will find that
the system is littered with tiny little holes. Tiny little vortices, so
there's a big difference, and I want to
explain that to you. The reason is the
magic of matter waves. We're not asking particles
to go around, and go around. We are asking
waves to go around. And due to the laws
of quantum mechanics, we have a snake of
waves, and the snake has to bite into its tail. It has to it form a closed wave. So therefore, we have the
quantization condition that we need an integer number
of de Broglie wavelengths around the circumference. So that means now if
you rotate the bucket, the classical system
just rigid body rotation has a velocity which increases
linearly with distance. But the quantum fluid,
the quantum gas, cannot do it because it has,
sort of, to make a decision. Single matter waves around the
circumference, one, two, three, so something has to go in jumps. One possibility would be
that we have a region where the system is not moving. This means single matter waves. Here we have one
de Broglie wave. Here two, and here three. But a little bit more
detailed consideration tells us that instead
of forming this kind of ring-shaped
geometry, it rather the system rather breaks
up into little whirlpools, little vortices. And Around each vortices
we have one full de Broglie wavelengths. Now this is a very
universal consequence. If you have matter,
which is a single wave. And indeed, there is a
very important experiment done 20 years ago
at Berkeley where this phenomenon was
demonstrated with liquid helium. And also, with Bose condensates,
recently a group in Paris showed this effect. I'm done talking
about recent work, so I want to talk
now about things which went on in our laboratory
less than a year ago. So we try to sort of
go further, and study further aspects of it. So what we did is,
and this gives you sort of a taste what
happens in Building 26. Is this tiny little cloud
in a big vacuum chamber, and then we use a laser beam,
and we rotate the laser beam around the cloud. And with the laser beam,
we are spinning up the gas, and we're setting
the gas in rotation. And then, if you let the gas go. If you let the cloud
ballistically expand, we can now compare the
cloud without rotation. It's just a blob, which expands,
as I've shown you before. But with rotation you
observe those whirlpools. So this is, again a
very clear manifestation of the wave nature of matter. Ordinary, standard substances
would not behave like this. And again, it's the
most direct observation. It's a cloud which rotates. You let it expand,
you take a picture, and this is the shadow. The shadow of this cloud. Actually, this is a simulation
by one of my colleagues shows that this
structure of whirlpools is preserved upon the
expansion of the gas. We can directly photograph
the cloud while it is rotating because the structures are too
small, but by simply switching off the magnetic trap,
and everything expands. We get almost, we get
the magnification factor almost for free. And the faster we rotated
the cloud, the more vortices we observed. So this is an example
of a vortex lattice, which is perfectly regular. This is just a more
fancy representation. But let's just look at this
very regular vortex lattice. With those lines
which guide the eye, we see how perfect nature is. But it's not always
perfect, and if you'll look carefully on
the left-hand side, you'll see a lattice defect. You see a dislocation,
and extra row of vortices. So we have now a system. It's a very dilute gas. And we can now study the
properties of vortices, study their
dynamics, and there's a lot of interest in doing that. But finally, let me
talk about something which is even more recent. And some work, which is
currently in progress. This is-- okay, I will
tell you in a moment. This is related to
optical trapping. I told you that our workhorse
is a magnetic container. But so well, magnetic
containers are nice. They have allowed us
to do all this work, but they've also
their limitations. Because if you want
to move magnets, you may have to move
all the magnets around. You can only keep atoms
in spin up states, and not spin down states. So there are many reasons to
put those Bose condensates into other containers. And this is a
container, which is not using magnetic field,
but electric fields. And its electric field
over focused laser beam. We call it optical tweezers
because it's really like a laser pointer. We bring the light to
a focus, and this focus sucks atoms into it
just by electric forces. So, couple of
years ago we showed that we can transfer condensates
from the magnetic trap into an optical trap. But our piece of pride, and
our latest accomplishment is that we are now able
to use this optical trap as a transport mechanism
for Bose condensates. So by taking, not really
the laser, but the lens, and translating it, we can move
the focus by 40 centimeters. So our group has now
the unique ability to form a condensate
in one vacuum chamber, then focus laser light on
it, and translate the focus by about 40
centimeters, and carry the atoms in a new chamber. So we are now able to deliver
condensates with precision through pin holes,
and put condensates into whole new environments. Close to surfaces, into
micro traps, into resonators, and there's enormous excitement. And let me just show
you one of the results. This is 40 centimeters
away from where we produced the condensate. And here it is, and it is
less than a millimeter away from the surface. And we think there's a lot of
future of atomic physics in it. Some people dream that we
can take atoms-guided matter waves on atom chips,
tiny patterns of wires on the surface. And we guide the atoms around
like in an ordinary chip, electrons are guided. And this may allow us
to have new geometries, to find new properties
of quantum systems. But it may also allow us
to build precision sensors for rotation. But I think this
example will also, or the last two
examples demonstrates some of my surprises. When we were hunting
for the Bose condensate we thought we would be so
happy if we would just see it. If you would get there. But now, we can take
condensates, transport them over 40 centimeters. We can keep them for a
minute, and do experiments. We can spin them
at high rotation, and see those vortices. So the system is
enormously robust, and allows us to branch out
in a large variety of studies. So for me, the Bose condensate
is now a new laboratory to do all sorts of physics. To do condensed metaphysics,
vortex nucleation, sound at densities which
are extremely low, that it's very easy for theorists
to calculate the effects. And what I've done here is
it's more for the experts. I've just listed
major developments in the field, which have
happened just this year. And the other
exciting things, and I think there is more to come. So, it's a privilege
for scientists to be in the middle of
something like this, to be able to contribute
to the basic concepts, and see the scope of the field,
and see how it's branching out. Actually, I don't
have a slide for that. But one question, which
almost everybody asks me, what is the application? What will come out of it? Now, my take on that is twofold. One is it's
fundamental research, and we do fundamental research
to learn about properties of nature, to find
properties of matter at ultra low temperatures, to
understand macroscopic quantum mechanics. On the other hand,
what we're doing here is we are learning how to
manipulate atoms, the building blocks of nature with
unprecedented precision. We have quantum
control over the atoms. We control their
wave properties, and this unprecedented
control over atoms may lead to precision
measurements, may contribute to nanotechnology. Or let me speculate
even further, may find uses in
quantum computation. But this is at the horizon, and
collaborators and other groups are exploring that. And future will tell
what is possible. But if I look backwards,
usually if something basic is done our imagination is
not sufficient to predict what it is good for. So finally, let me
acknowledge the people I was really privileged to work with. I think the best
team of students, and post-docs in
the whole world. This lists the people
who have left the group, and contributed to the work. But here is also a
long list of people who are currently carrying
out this exciting work. Usually I show a
group photo, but I realize we haven't taken
a group photo recently. So may I just ask the people
who are working with me just to stand up? And it's a live group photo. And they are carrying
out this work, and they deserve the applause. [APPLAUSE] Thank you. [AUDIENCE LAUGHS] [APPLAUSE CONTINUES] PRESENTER: Pretty good, huh? Enjoy it. KETTERLE: Huh? PRESENTER: Enjoy it. KETTERLE: Thanks. [APPLAUSE] PRESENTER: Well that
was fantastic, Wolfgang. You lived up to expectations. Are there any questions? [LAUGHTER FROM THE AUDIENCE] Oh, okay. I just, fine. I don't think there
have to be any question. [CHUCKLES] Mark, would you like
to say something? PRESENTER: We have a reception
in the Marlar Lounge. I just want to add, on behalf
of the physics department, of course. This is an unbelievably
exciting occasion. We've had five Nobel Prize
winners in the last 30 years. So I told my colleagues,
I'd like the rate to go a little higher. But I cannot imagine a more
wonderful person to receive the prize than someone who
not only has this passion for science, but can
convey it so beautifully. So let's end just by thanking
Wolfgang one more time. [APPLAUSE]
