The Nobel Prize in Physics 1997

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Nobel Prize Award Ceremony

• Award Ceremony Speech

Steven Chu

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Claude Cohen-Tannoudji

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William D. Phillips

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Steven Chu

Born: 28 February 1948, St. Louis, MO, USA

Affiliation at the time of the award: Stanford University, Stanford, CA, USA

Prize motivation: "for development of methods to cool and trap atoms with laser light"

Field: Atomic physics

Autobiography

My father, Ju Chin Chu, came to the United States in 1943 to continue his education at the Massachusetts Institute of Technology in chemical engineering, and two years later, my mother, Ching Chen Li, joined him to study economics. A generation earlier, my mother's father earned his advanced degrees in civil engineering at Cornell while his brother studied physics under Perrin at the Sorbonne before they returned to China. However, when my parents married in 1945, China was in turmoil and the possibility of returning grew increasingly remote, and they decided to begin their family in the United States. My brothers and I were born as part of a typical nomadic academic career: my older brother was born in 1946 while my father was finishing at MIT, I was born in St. Louis in 1948 while my father taught at Washington University, and my younger brother completed the family in Queens shortly after my father took a position as a professor at the Brooklyn Polytechnic Institute.

In 1950, we settled in Garden City, New York, a bedroom community within commuting distance of Brooklyn Polytechnic. There were only two other Chinese families in this town of 25,000, but to our parents, the determining factor was the quality of the public school system. Education in my family was not merely emphasized, it was our raison d'être. Virtually all of our aunts and uncles had Ph.D.'s in science or engineering, and it was taken for granted that the next generation of Chu's were to follow the family tradition. When the dust had settled, my two brothers and four cousins collected three MDs, four Ph.D.s and a law degree. I could manage only a single advanced degree.

In this family of accomplished scholars, I was to become the academic black sheep. I performed adequately at school, but in comparison to my older brother, who set the record for the highest cumulative average for our high school, my performance was decidedly mediocre. I studied, but not in a particularly efficient manner. Occasionally, I would focus on a particular school project and become obsessed with, what seemed to my mother, to be trivial details instead of apportioning the time I spent on school work in a more efficient way.

I approached the bulk of my schoolwork as a chore rather than an intellectual adventure. The tedium was relieved by a few courses that seem to be qualitatively different. Geometry was the first exciting course I remember. Instead of memorizing facts, we were asked to think in clear, logical steps. Beginning from a few intuitive postulates, far reaching consequences could be derived, and I took immediately to the sport of proving theorems. I also fondly remember several of my English courses where the assigned reading often led to binges where I read many books by the same author.

Despite the importance of education in our family, my life was not completely centered around school work or recreational reading. In the summer after kindergarten, a friend introduced me to the joys of building plastic model airplanes and warships. By the fourth grade, I graduated to an erector set and spent many happy hours constructing devices of unknown purpose where the main design criterion was to maximize the number of moving parts and overall size. The living room rug was frequently littered with hundreds of metal "girders" and tiny nuts and bolts surrounding half-finished structures. An understanding mother allowed me to keep the projects going for days on end. As I grew older, my interests expanded to playing with chemistry: a friend and I experimented with homemade rockets, in part funded by money my parents gave me for lunch at school. One summer, we turned our hobby into a business as we tested our neighbors' soil for acidity and missing nutrients.

I also developed an interest in sports, and played in informal games at a nearby school yard where the neighborhood children met to play touch football, baseball, basketball and occasionally, ice hockey. In the eighth grade, I taught myself tennis by reading a book, and in the following year, I joined the school team as a "second string" substitute, a position I held for the next three years. I also taught myself how to pole vault using bamboo poles obtained from the local carpet store. I was soon able to clear 8 feet, but was not good enough to make the track team.

In my senior year, I took advanced placement physics and calculus. These two courses were taught with the same spirit as my earlier geometry course. Instead of a long list of formulas to memorize, we were presented with a few basic ideas or a set of very natural assumptions. I was also blessed by two talented and dedicated teachers.

My physics teacher, Thomas Miner was particularly gifted. To this day, I remember how he introduced the subject of physics. He told us we were going to learn how to deal with very simple questions such as how a body falls due to the acceleration of gravity. Through a combination of conjecture and observations, ideas could be cast into a theory that can be tested by experiments. The small set of questions that physics could address might seem trivial compared to humanistic concerns. Despite the modest goals of physics, knowledge gained in this way would become collected wisdom through the ultimate arbitrator - experiment.

In addition to an incredibly clear and precise introduction to the subject, Mr. Miner also encouraged ambitious laboratory projects. For the better part of my last semester at Garden City High, I constructed a physical pendulum and used it to make a "precision" measurement of gravity. The years of experience building things taught me skills that were directly applicable to the construction of the pendulum. Ironically, twenty five years later, I was to develop a refined version of this measurement using laser cooled atoms in an atomic fountain interferometer.

I applied to a number of colleges in the fall of my senior year, but because of my relatively lackluster A-average in high school, I was rejected by the Ivy League schools, but was accepted at Rochester. By comparison, my older brother was attending Princeton, two cousins were in Harvard and a third was at Bryn Mawr. My younger brother seemed to have escaped the family pressure to excel in school by going to college without earning a high school diploma and by avoiding a career in science. (He nevertheless got a Ph.D. at the age of 21 followed by a law degree from Harvard and is now a managing partner of a major law firm.) As I prepared to go to college, I consoled myself that I would be an anonymous student, out of the shadow of my illustrious family.

The Rochester and Berkeley Years
At Rochester, I came with the same emotions as many of the entering freshman: everything was new, exciting and a bit overwhelming, but at least nobody had heard of my brothers and cousins. I enrolled in a two-year, introductory physics sequence that used The Feynman Lectures in Physics as the textbook. The Lectures were mesmerizing and inspirational. Feynman made physics seem so beautiful and his love of the subject is shown through each page. Learning to do the problem sets was another matter, and it was only years later that I began to appreciate what a magician he was at getting answers.

In my sophomore year, I became increasingly interested in mathematics and declared a major in both mathematics and physics. My math professors were particularly good, especially relative to the physics instructor I had that year. If it were not for the Feynman Lectures, I would have almost assuredly left physics. The pull towards mathematics was partly social: as a lowly undergraduate student, several math professors adopted me and I was invited to several faculty parties.

The obvious compromise between mathematics and physics was to become a theoretical physicist. My heroes were Newton, Maxwell, Einstein, up to the contemporary giants such as Feynman, Gell-Mann, Yang and Lee. My courses did not stress the importance of the experimental contributions, and I was led to believe that the "smartest" students became theorists while the remainder were relegated to experimental grunts. Sadly, I had forgotten Mr. Miner's first important lesson in physics.

Hoping to become a theoretical physicist, I applied to Berkeley, Stanford, Stony Brook (Yang was there!) and Princeton. I chose to go to Berkeley and entered in the fall of 1970. At that time, the number of available jobs in physics was shrinking and prospects were especially difficult for budding young theorists. I recall the faculty admonishing us about the perils of theoretical physics: unless we were going to be as good as Feynman, we would be better off in experimental physics. To the best of my knowledge, this warning had no effect on either me or my fellow students.

After I passed the qualifying exam, I was recruited by Eugene Commins. I admired his breadth of knowledge and his teaching ability but did not yet learn of his uncanny ability to bring out the best in all of his students. He was ending a series of beta decay experiments and was casting around for a new direction of research. He was getting interested in astrophysics at the time and asked me to think about proto-star formation of a closely coupled binary pair. I had spent the summer between Rochester and Berkeley at the National Radio Astronomy Observatory trying to determine the deceleration of the universe with high red-shift radio source galaxies and was drawn to astrophysics. However, in the next two months, I avoided working on the theoretical problem he gave me and instead played in the lab.

One of my "play-experiments" was motivated by my interest in classical music. I noticed that one could hear out-of-tune notes played in a very fast run by a violinist. A simple estimate suggested that the frequency accuracy, times the duration of the note,did not satisfy the uncertainty relationship. In order to test the frequency sensitivity of the ear, I connected an audio oscillator to a linear gate so that a tone burst of varying duration could be produced. I then asked my fellow graduate students to match the frequency of an arbitrarily chosen tone by adjusting the knob of another audio oscillator until the notes sounded the same. Students with the best musical ears could identify the center frequency of a tone burst that eventually sounded like a "click" with an accuracy of .

By this time it was becoming obvious (even to me) that I would be much happier as an experimentalist and I told my advisor. He agreed and started me on a beta-decay experiment looking for "second-class currents", but after a year of building, we abandoned it to measure the Lamb shift in high-Z hydrogen-like ions. In 1974, Claude and Marie Bouchiat published their proposal to look for parity non-conserving effects in atomic transitions. The unified theory of weak and electromagnetic interactions suggested by Weinberg, Salam and Glashow postulated a neutral mediator of the weak force in addition to the known charged forces. Such an interaction would manifest itself as a very slight asymmetry in the absorption of left and right circularly polarized light in a magnetic dipole transition. Gene was always drawn to work that probed the most fundamental aspects of physics, and we were excited by the prospect that a table-top experiment could say something decisive about high energy physics. The experiment needed a state-of-the-art laser and my advisor knew nothing about lasers. I brashly told him not to worry; I would build it and we would be up and running in no time.

This work was tremendously exciting and the world was definitely watching us. Steven Weinberg would call my advisor every few months, hoping to hear news of a parity violating effect. Dave Jackson, a high energy theorist, and I would sometimes meet at the university swimming pool. During several of these encounters, he squinted at me and tersely asked, "Got a number yet?" The unspoken message was, "How dare you swim when there is important work to be done!"

Midway into the experiment, I told my advisor that I had suffered enough as a graduate student so he elevated me to post-doc status. Two years later, we and three graduate students published our first results. Unfortunately, we were scooped: a few months earlier, a beautiful high energy experiment at the Stanford Linear Collider had seen convincing evidence of neutral weak interactions between electrons and quarks. Nevertheless, I was offered a job as assistant professor at Berkeley in the spring of 1978.

I had spent all of my graduate and postdoctoral days at Berkeley and the faculty was concerned about inbreeding. As a solution, they hired me but also would permit me to take an immediate leave of absence before starting my own group at Berkeley. I loved Berkeley, but realized that I had a narrow view of science and saw this as a wonderful opportunity to broaden myself.

A Random Walk in Science at Bell Labs
I joined Bell Laboratories in the fall of 1978. I was one of roughly two dozen brash, young scientists that were hired within a two year period. We felt like the "Chosen Ones", with no obligation to do anything except the research we loved best. The joy and excitement of doing science permeated the halls. The cramped labs and office cubicles forced us to interact with each other and follow each others' progress. The animated discussions were common during and after seminars and at lunch and continued on the tennis courts and at parties. The atmosphere was too electric to abandon, and I never returned to Berkeley. To this day I feel guilty about it, but I think that the faculty understood my decision and have forgiven me.

Bell Labs management supplied us with funding, shielded us from extraneous bureaucracy, and urged us not to be satisfied with doing merely "good science." My department head, Peter Eisenberger, told me to spend my first six months in the library and talk to people before deciding what to do. A year later during a performance review, he chided me not to be content with anything less than "starting a new field". I responded that I would be more than happy to do that, but needed a hint as to what new field he had in mind.

I spent the first year at Bell writing a paper reviewing the current status of x-ray microscopy and started an experiment on energy transfer in ruby with Hyatt Gibbs and Sam McCall. I also began planning the experiment on the optical spectroscopy of positronium. Positronium, an atom made up of an electron and its anti-particle, was considered the most basic of all atoms, and a precise measurement of its energy levels was a long standing goal ever since the atom was discovered in 1950. The problem was that the atoms would annihilate into gamma rays after only 140x10^-9 seconds, and it was impossible to produce enough of them at any given time. When I started the experiment, there were 12 published attempts to observe the optical fluorescence of the atom. People only publish failures if they have spent enough time and money so their funding agencies demand something in return.

My management thought I was ruining my career by trying an impossible experiment. After two years of no results, they strongly suggested that I abandon my quest. But I was stubborn and I had a secret weapon: his name is Allen Mills. Our strengths complemented each other beautifully, but in the end, he helped me solve the laser and metrology problems while I helped him with his positrons. We finally managed to observe a signal working with only ~4 atoms per laser pulse! Two years later and with 20 atoms per pulse, we refined our methods and obtained one of the most accurate measurements of quantum electrodynamic corrections to an atomic system.

In the fall of 1983, I became head of the Quantum Electronics Research Department and moved to another branch of Bell Labs at Holmdel, New Jersey. By then my research interests had broadened, and I was using picosecond laser techniques to look at excitons as a potential system for observing metal-insulator transitions and Anderson localization. With this apparatus, I accidentally discovered a counter-intuitive pulse-propagation effect. I was also planning to enter surface science by constructing a novel electron spectrometer based on threshold ionization of atoms that could potentially increase the energy resolution by more than an order of magnitude.

While designing the electron spectrometer, I began talking informally with Art Ashkin, a colleague at Holmdel. Art had a dream to trap atoms with light, but the management stopped the work four years ago. An important experiment had demonstrated the dipole force, but the experimenters had reached an impasse. Over the next few months, I began to realize the way to hold onto atoms with light was to first get them very cold. Laser cooling was going to make possible all of Art Ashkin's dreams plus a lot more. I promptly dropped most of my other experiments and with Leo Holberg, my new post-doc, and my technician, Alex Cable, began our laser cooling experiment. This brings me to the beginning of our work in laser cooling and trapping of atoms and the subject of my Nobel Lecture.

Stanford and the future
Life at Bell Labs, like Mary Poppins, was "practically perfect in every way". However, in 1987, I decided to leave my cozy ivory tower. Ted Hänsch had left Stanford to become co-director of the Max Planck Institute for Quantum Optics and I was recruited to replace him. Within a few months, I also received offers from Berkeley and Harvard, and I thought the offers were as good as they were ever going to be. My management at Bell Labs was successful in keeping me at Bell Labs for 9 years, but I wanted to be like my mentor, Gene Commins, and the urge to spawn scientific progeny was growing stronger.

Ted Geballe, a distinguished colleague of mine at Stanford who also went from Berkeley to Bell to Stanford years earlier, described our motives: "The best part of working at a university is the students. They come in fresh, enthusiastic, open to ideas, unscarred by the battles of life. They don't realize it, but they're the recipients of the best our society can offer. If a mind is ever free to be creative, that's the time. They come in believing textbooks are authoritative but eventually they figure out that textbooks and professors don't know everything, and then they start to think on their own. Then, I begin learning from them."

My students at Stanford have been extraordinary, and I have learned much from them. Much of my most important work such as fleshing out the details of polarization gradient cooling, the demonstration of the atomic fountain clock, and the development of atom interferometers and a new method of laser cooling based on Raman pulses was done at Stanford with my students as collaborators.

While still continuing in laser cooling and trapping of atoms, I have recently ventured into polymer physics and biology. In 1986, Ashkin showed that the first optical atom trap demonstrated at Bell Labs also worked on tiny glass spheres embedded in water. A year after I came to Stanford, I set about to manipulate individual DNA molecules with the so-called "optical tweezers" by attaching micron-sized polystyrene spheres to the ends of the molecule. My idea was to use two optical tweezers introduced into an optical microscope to grab the plastic handles glued to the ends of the molecule. Steve Kron, an M.D./Ph.D. student in the medical school, introduced me to molecular biology in the evenings. By 1990, we could see an image of a single, fluorescently labeled DNA molecule in real time as we stretched it out in water. My students improved upon our first attempts after they discovered our initial protocol demanded luck as a major ingredient. Using our new ability to simultaneously visualize and manipulate individual molecules of DNA, my group began to answer polymer dynamics questions that have persisted for decades. Even more thrilling, we discovered something new in the last year: identical molecules in the same initial state will choose several distinct pathways to a new equilibrium state. This "molecular individualism" was never anticipated in previous polymer dynamics theories or simulations.

I have been at Stanford for ten and a half years. The constant demands of my department and university and the ever increasing work needed to obtain funding have stolen much of my precious thinking time, and I sometimes yearn for the halcyon days of Bell Labs. Then, I think of the work my students and post-docs have done with me at Stanford and how we have grown together during this time.

From Les Prix Nobel. The Nobel Prizes 1997, Editor Tore Frängsmyr, [Nobel Foundation], Stockholm, 1998

This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/Nobel Lectures. The information is sometimes updated with an addendum submitted by the Laureate.