Paul Corkum’s journey from experiment to theory and back again

While Paul Corkum considers himself a “natural experimentalist,” he admits that others likely think of him as a theorist. It even came up during his interview with the National Research Council of Canada, where he was hoping to land a postdoctoral role in an experimental plasma physics lab after earning a Ph.D. in theoretical physics.

“In graduate school, my wife used to get embarrassed when our old car would spew smoke, [so] I got a book out — like a good theorist — opened it up and started taking the car apart,” he recalls. “I replaced the pistons, bearings, and other parts, put it all back together again, and it started right up. So when I was being interviewed for the postdoc position, they asked me, ‘What makes you think you can do experiments?’ and I told them, ‘I can take my car apart, put it back together, and it works,’ and they gave me the job.”

This expertise in both theory and experiment has shaped Corkum’s decorated research career. As one of the pioneers of attosecond science, Corkum helped shape our current understanding of how intense light pulses, lasting just one-quintillionth of a second, interact with matter. Corkum and his fellow attosecond pioneers Anne L’Huillier and Ferenc Krausz were recognized for their contributions with the 2022 Wolf Prize in Physics. L’Huillier and Krausz also received the 2023 Nobel Prize in Physics for their experimental contributions.

Now, Corkum — who splits his time between the University of Ottawa and the National Research Council of Canada — is the recipient of the 2025 APS Medal for Exceptional Achievement in Research, the largest prize awarded by APS. He is recognized “for the synthesis of plasma physics, strong-field spectroscopy, and electron scattering concepts to form a new science of strong-field physics ranging from atomic to solid state physics, as well as for pioneering of attosecond science.”

Corkum spoke with APS News about his career path, how attosecond science has evolvedsince his seminal papers, and his advice for early-career researchers.

This interview has been edited for length and clarity.

Tell us about your academic journey.

I had a superb physics teacher in high school who really ignited an interest in physics. He began his lectures by talking about equations and how, if they were to be real, they had to balance. A bit like apples to apples, except in physics it’s mass to mass, length to length, and time to time. I liked that idea.

I went to Acadia University, which was close to where I grew up. I signed up for engineering under the influence of my mother, who thought I should be an engineer. In high school, I had the chance to take a first-year college physics course, so I came straight into the second-year course.

All of a sudden, I was doing physics at what seemed like a rapid rate and doing engineering at what seemed like a glacial rate, so I changed my major and said I was going to be a physicist from now on. A professor hired me during my first summer, so I got involved in research — mainly fluid dynamics, looking at wakes behind bubbles and things like that. My first paper was in Nature.

I went to Lehigh University in Pennsylvania for graduate school. I was initially intimidated, coming from the peripheral part of Canada out to the East Coast. I thought all the students would know way more than I knew, but they didn’t. Lehigh was a good place for me, and they taught physics well.

I thought I would do fluids research, and I even did a Master’s project with a professor who was quite famous in fluid experiments. However, I was keen on doing theory and wanted to do something more esoteric, so I did my Ph.D. in statistical mechanics.

What were the origins of your work on attosecond laser pulses?

Before I joined the NRC, I had been thinking about how you transport materials in plasma and how this was related to the scattering of light from a plasma. In the 1960s, a Russian physicist named Leonid Keldysh was thinking about laser electric fields and how the electron would get out of an atom’s field. The plasma physicists accepted his ideas, but the atomic physicists less so. He went on to talk about what the electrons would look like, but nobody knew it because it was couched so deep in theory that you couldn’t really see it. 

I was influenced by Keldysh, and I began to look at this as a plasma problem. I rediscovered his idea but in a way that was very simple. I did that in 1989 with the first part of my most famous work, and in 1993, the second part, where I put it all together. Both papers described the atomic physics problem like a plasma physics problem.

Then it hit me on the head. I had a way of making attosecond pulses. We started doing experiments on what is now called polarization gating very quickly and the theory of how to measure.

What are some of the most significant findings from this field so far?

Maybe the most interesting experimental finding is that when an atom ionizes, the electron interferes with its former self, because quantum mechanics allows any quantum particle to be in many places at once. That interference allows you to do tomography. Like medical tomography, you can determine what an object looks like.

There is a second, related point that’s important. I'm not sure yet, but I think producing high harmonics and attosecond pulses is a universal response of all matter. In our day, we don't often get a chance to discover a universal response.

That ability to create an interferometer from a material’s own electrons with light is amazing. One might argue that an interferometer is the most important tool in physics, and attosecond science has found a way to put an electron interferometer inside any material and to control the interferometer.

That's important. We might exhaust what can be measured with attosecond pulses, but we will never get tired of putting interferometers inside materials.

What do you see as the potential of attosecond science moving forward?

I think going small will be important. Each cell in your body is subdivided into organelles on the nanometers scale. These organelles do all kinds of important things, and now we have a laser-based tool to see and manipulate those organelles.

Also, because you've got x-rays, you can excite electrons from core states. In a molecule, the core state is fixed to an atom. So now you have a position inside materials, or molecules, from which to probe the material. We know almost nothing about these core electrons, except they move really fast, so attosecond techniques are required if we are to exploit the non-valence electrons for material science or chemistry.

Given your academic background in theory and your experience working as an experimentalist, how do you think about these two approaches in physics?

I don’t see theory and experiment as two different things. Experimentalists and theorists use the tools available to them, and they are not necessarily the same tools — just two alternate ways of looking at the same thing.

I consider myself an experimentalist — I don’t look at an equation and see the beauty and simplicity of it, but I more see how I go about doing it. However, I think my study of theory for my Ph.D. encouraged me to look at the concept behind any theory or experiment. At the concept level, experiment and theory merge.

What is your advice for students and early-career physicists?

Do something creative, something new, and trust your instincts. Don't try to follow what somebody else has done. You spend your whole education learning about what everybody else has done, and now the test is to do it yourself. That’s tough, and many people struggle with it, so I think you just have to have faith in yourself and give it a try.

How do you feel about receiving the APS Medal?

The APS Medal means my colleagues, the people who really know, think I've made a major contribution. It’s humbling, and I can't think of a more meaningful award that I can get in my whole career.