He Wrote the Book on Atom Trapping
Retired Bell Labs scientist Arthur Ashkin discusses his years as a
physicist and how he discovered that light could trap atoms -- the
discovery that led Steven Chu and two others to the Nobel Prize
Murray Hill (November, 1997) -- When the Swedish Academy of Sciences
awarded the 1997 Nobel Prize in Physics to Steven Chu, William Phillips
and Claude Cohen-Tannoudji for their development of methods to cool and
trap atoms with laser light, they honored a branch of physics -- optical
trapping -- that has long been associated with Bell Labs.
Arthur Ashkin invented optical trapping, or the process by which
atoms are trapped by laser light, at Bell Labs. He found that
radiation pressure -- the ability of light to exert pressure to move
small objects -- could be harnessed to constrain atoms. Many
consider him the father of the field. In fact, the work that led Chu
to the Nobel Prize was started by Ashkin and Chu at the labs in
Holmdel. Ashkin retired from Bell Labs in 1992 after a 40-year
career during which he contributed to many areas of experimental
physics. He authored many research papers over the years and holds
47 patents. Ashkin was elected to the National Academy of Sciences
Ashkin recently spoke to the Bell Labs News about his long and
distinguished career at Bell Labs and the work that led Steven Chu
to the Nobel Prize.
BLN: What led you to become a
physicist? How did your interest in physics develop?
Ashkin: As a little kid, I was interested in how
things worked. In high school, I was very interested in science,
especially in math and physics. I had an older brother who was a
physicist and he inspired me.
I went to Columbia as an undergraduate. I got caught in World War
II -- I was a sophomore at the time -- and, since I was a good
student, I became a technician in the Columbia Radiation Lab. There
were three Nobel laureates there at that time. It was tremendously
exciting. I worked there for a few years, got drafted, and they
assigned me back. That was my introduction to electromagnetic
After I finished my physics degree at Columbia, I went to Cornell
and studied nuclear physics. My brother was in nuclear physics. I
met all those famous men there -- Hans Bethe, Richard Feynman, and
so on. Feynman was my brother's very good friend. He used to call me
"Ashkin's-brother-Ashkin." I thought that amusing.
Arthur Ashkin at work in his Holmdel, N.J.
lab in 1988
BLN: How did you come to Bell
Labs? What did you work on initially?
Ashkin: During the War, I met Sid Millman at
Columbia Radiation Labs. Millman came to Bell Labs after the War and
ended up executive director of Research. After I got my degree at
Cornell, he somehow got wind of it and asked me if I wanted to come
to Bell Labs. I accepted.
I got started working on microwaves. I worked on microwaves until
about 1960-61, just after the invention of the laser. That was so
exciting. Everybody was studying lasers. I started laser work.
In 1963, I got promoted to department head under Chape Cutler. I
worked with Gary Boyd. We did a lot of very nice things in nonlinear
optics. We were pioneers in parametric amplification and parametric
oscillators. One of our papers is going to be in the Classics of
Non-Linear Optics, coming out soon.
We also discovered the photo-refractive effect. It has to do with
the fact that light changes the index of refraction of piezoelectric
crystals. We were trying desperately to make parametric oscillators,
and index effects were a problem. We were terribly disappointed. But
it turns out this is a very important effect. This was the first
BLN: Was this all in the late
Ashkin: We came to Holmdel from Murray Hill in
1967. This was done before that. It must have been in 1964-65. And
then I did a lot of work subsequently.
BLN: Tell me about how you
started work on optical trapping, leading up to when you and Steve
Chu and colleagues did the now-famous experiments?
Ashkin: I started in 1970 with my discovery that
I could optically trap small latex spheres. And I went on to propose
that we could do the same thing with atoms.
BLN: How did you start thinking
about it in the first place?
Ashkin: Well, I had been interested in radiation
pressure for a long time. Even at Columbia, when I used to make
megawatt-magnetrons for Millman, I used to think, "What can you
do with this power? Maybe you could push small things ..." So I
got myself a microphone and I put pulses on it. This was when I was
still a sophomore. I heard some noise and I said, "Aaah, I am
seeing the effects of radiation pressure." Turns out, you know,
I am not sure whether I was seeing the effects or not. Anyway, I was
alerted to this problem.
And I was reminded of it when I went to a conference and there
was a guy who did an experiment with lasers and little particles in
a laser cavity. He saw the particles staying in the cavity and
moving back and forth and doing crazy things. He called them runners
and bouncers. People were fascinated by this. I heard this talk and
he said at the time, "We think it might be radiation
When I came home, I did a calculation and realized, given the
size of the beam and the particles, it couldn't be radiation
pressure. More likely, I thought, is that the particles were heated
and that led to the crazy behavior. This made me think of radiation
I decided to try to see radiation pressure. I made a calculation
of how much it would be on a small transparent spheres. That started
the whole business for me.
What I did was focus a beam down on little spheres in water and
watched as they were pushed along and mysteriously collected at the
chamber wall. I tried to understand this and figured it out using
simple ray diagrams.
Then, I replaced the glass wall with another opposite beam to
hold the particles in place with just light. I tried it and it
worked. This was the first optical trap. It turned out to be a
pretty important discovery. It led to Steve's Nobel Prize and, I
believe, it will lead to two more Nobel Prizes.
BLN: What do you think they
Ashkin: Well, I think, the Bose-Einstein
condensation and the atom laser will get a prize. And I hope that
the great things that biologists are doing with optical tweezers
will be rewarded with another one.
BLN: How did you and Steve Chu
get involved in doing the experiments that would ultimately get him
the Nobel Prize?
Ashkin: Steve Chu was at Murray Hill, doing his
great experiments on positronium. He was promoted and came to
Holmdel in 1984 or so as a fellow department head in electronics
research. He started talking to me, and to the other guys. We had
been working on trapping for almost 15 years. It was a very loose
group. It was basically me, working with Joe Dziedzic, playing
around with these little spheres, and John Bjorkholm and Jim Gordon
helping with atoms. At the same time, I was trying to understand
what were the forces were necessary to trap things. And I looked for
the same forces with the atoms.
We had hired a new guy, Rick Freeman, in our laboratory and, with
Bjorkholm, we did our first experiment with atoms and atomic beams.
We showed that we could focus atoms, defocus atoms. We could focus
them to small spots and see how the spots were limited by quantum
fluctuations. We learned a heck of a lot and got a lot of credit for
After that experiment, the field began to heat up. People on the
outside began to realize that something important was happening.
When Steve Chu came here, he said, "You know, people are very
interested in this." He wanted to understand it and we
discussed it. He wanted to make an atom trap. We were delighted. And
we went and made it.
BLN: So that was the famous
experiment with "optical molasses" and traps that you and
Chu and colleagues did in 1985?
Ashkin: The initial plan was to combine slowing
of atoms, cooling them and trapping them in a single experiment.
Steve argued for a simpler first step. Steve was very enthusiastic.
He wanted to first study the three-dimensional cooling scheme, using
a technique proposed by T.W. Hansch and A. L. Schawlow in 1975. This
is now referred to as "optical molasses." This was wise
because molasses cooling succeeded so well that it affected our
subsequent choice of traps. We used the now-famous optical tweezers
trap. Steve was a very hands-on person, a great experimentalist. He
did some absolutely brilliant experiments. I always thought in those
days that he would one day get the Nobel Prize. I am extremely happy