Wigner lecturer: William D. Phillips

William D. Phillips is co-recipient of the 1997 Nobel Prize in physics, given for the development of methods to cool and trap atoms with laser light.

He works at the National Institute of Standards and Technology as leader of the Laser Cooling and Trapping Group within the institute’s Quantum Measurement Division. The group pursues research in a variety of areas including laser cooling and trapping, Bose-Einstein condensation, atom optics, collisions of cold atoms and quantum information processing.

Phillips received a bachelor’s degree in physics from Juniata College in Pennsylvania and a Ph.D. from the Massachusetts Institute of Technology. He is a member of the American Physical Society and the National Academy of Sciences.

Phillips delivered the Eugene P. Wigner Distinguished Lecture August 9, 2017, on the topic “At the Crossroads of Atomic and Solid-State Physics: Ultracold Atoms as a New Condensed Matter System.” This is an edited transcript of our conversation following his lecture.

You shared a Nobel Prize in 1997 for developing the means to cool gases to a millionth of a degree above absolute zero using laser light. Why was this breakthrough important?

There are a number of reasons why it has become important to laser-cool atoms, but at the beginning we wanted to make better measurements on the atoms. I’m at an institute—the National Institute of Standards and Technology—that’s all about measurement. Temperature is about motion. When something is hotter, it means the atoms and molecules making up that thing are moving around fast. When something is colder, it means those atoms and molecules are moving more slowly. If you want to make measurements on those atoms and molecules, it’s a whole lot better if they’re moving more slowly. The big motivation for us was to make better atomic clocks. The best clocks that there are, are clocks whose tickers are atoms because all atoms of the same kind tick at exactly the same rate, as opposed to manufactured tickers like pendula or quartz crystals, which all tick at slightly different rates. But when atoms are moving really fast, it’s not so easy to measure their ticking rate, so our idea was to cool them down, make them more slow.

What new basic science discoveries/phenomena have been enabled by the study of ultracold atoms and materials?

One thing we learned in the process of cooling atoms was that the cooling process was not what we had thought. It was remarkable because it represented a violation of Murphy’s law, which tells you that anything that can go wrong, will go wrong. Well, in this case we got lucky; laser cooling works better than it was supposed to, and as a result we were able to get our atoms much, much colder than the original theory told us was possible. So our original idea about how laser cooling worked was wrong; it was both more complicated and worked better as a result, which is another violation of Murphy’s law because usually when something gets more complicated, it’s not as good. Since then we have used laser-cooled atoms to do all kinds of new physics that we hadn’t anticipated would be important. One thing we didn’t anticipate was that we could use our laser-cooled atoms as a way of making a kind of toy model of solids. In this toy model we use interfering laser beams—laser beams that overlap. They form what we call a standing wave, where the distance between bright regions and dark regions is a fraction of the wavelength of light. Atoms can be trapped in those—what we call nodes and antinodes of the standing wave of the laser beam—and we make a kind of a toy model of a solid where you have a crystal lattice with electrons that can move from one site in the crystal lattice to another. We can use it to study the behavior in a simplified form of electrons moving in a solid.

What applications are made available by the study of ultracold atoms?

As a result of having ultracold atoms, we have been able to, just as we had hoped, revolutionize the way in which atomic clocks work. Today essentially every big industrialized country keeps time using laser-cooled atomic clocks. In the United States the time is kept for civilian purposes by NIST, my organization, and for military purposes by the U.S. Naval Observatory. Both of those institutions use laser-cooled atomic clocks as the basis for their time standards. In fact, at the Navy the people running the atomic clocks were postdocs at my laboratory and took the skills they learned about laser-cooling atoms to the Navy and made clocks. But if you go to places like England or France or Germany or China or Japan, in all of these places laser-cooled clocks are keeping time for their country. And all of those places report their information to a central organization in France and keep time for the entire world.

Why was it important to visit ORNL, meet with researchers here, and participate in the Wigner Lecture Series?

For me, coming to Oak Ridge has importance both from a personal and from a professional point of view. Oak Ridge has always been, from my youth, kind of a legendary place. The story of the Manhattan Project and Oak Ridge’s role in making fissionable material was part of the story of physics that I read about when I was a child. When I was doing science projects, I remember writing a letter and just addressing it to Oak Ridge National Lab, and somehow or other it got to some scientist who knew something about what I was asking and wrote back a very kind letter explaining some things to me. So my relationship with Oak Ridge goes back in a very positive way, a very warm way, to my childhood. When I was an undergraduate, I spent a semester at Argonne National Laboratory, which was one of the first places where I was able to engage in research essentially full time and work side by side with other researchers who were doing research as their full-time occupation. It was one of the things that cemented in my mind the idea that this is what I wanted to do with my life.