Using Laser Cooling to Mark Time

Because of William Phillips’ work in laser cooling, atomic clocks are almost a thousand times better than they used to be.
  • Transcript


Question: What is laser cooling? 

William Phillips: Laser cooling means shining light on stuff and making it cold. Now, that in itself sounds like it’s completely backwards because, after all, you typically think that if you shine light on something, it’s going to get warm. So how is it even possible to shine light on something and make it cold? 

To cool the air down means to make those molecules, atoms in the air, move more slowly. That’s the difference between hot and cold. So, how do you make something cold with a laser? Well, lasers, all light, pushes on stuff. There’s a thing called radiation pressure... light pushes on things. But what we’ve figured out what to do over the years is how to push on atoms in such as way as to make them slow down. 

Question: What did your team discover about laser cooling? 

William Phillips: The thing that was perhaps the crowning achievement of the early days of laser cooling was the discovery that we made in our laboratory that it was possible to get these atoms colder than everyone had thought was the limit to how cold you could get something. 

The prediction said that we could get down to temperatures of 240 millionths of a degree. In other words, one-quarter of one-thousandth of a degree above absolutely zero. Pretty cold, right? Well, in fact, we got a whole lot colder than that. And that was the big breakthrough discovery that made a whole lot of other things possible. 

Question: How has laser cooling contributed to the development of atomic clocks? 

William Phillips: Well, all clocks have tickers. All of these tickers have imperfections. Every quartz crystal is made a little bit differently, the length of the pendulum can change a little bit and that changes how fast it will swing back and forth. So, all these clocks have imperfections. 

And so one has throughout history been trying to make these clocks better by making the tickers be more reliable. That is always had the same ticking frequency. Well, it turns out that atoms are the best choice for making tickers that always tick at the same frequency. Even atoms have their imperfections. Temperature means that the atoms are moving with a certain velocity having a certain kinetic energy. The hotter the gas is, the faster the atoms are moving. It’s not so easy to measure the ticking frequency of something that whizzing around at the speed of sound. And that’s the problem that everybody was facing with atomic clocks was that atoms were moving at approximately the speed of sound and it wasn’t so easy to measure the ticking frequency. 

So, we said, "let’s cool them down using lasers so they’re going more slowly and that’ll make it easier to measure the ticking frequency and you can make better clocks.” 

When I started doing laser cooling, the very best clocks were accurate to a part in ten to the 13, so that’s one part divided by 1, with 13 zeroes after it. That’s the fractional error in how good that clock was. Sounds incredible right? But today, those clocks are a couple and ten to the 16th. Almost three orders of magnitude better and that has been made possible because of laser cooling. 

Question: How did you first get interested in science? 

William Phillips: I suppose that young children are curious about everything and very early my curiosity tended toward a curiosity about science. My parents got me a microscope when I was very young, maybe six years old, and I remember looking at all kids of things around the house with this microscope. I remember collecting various household chemicals and fluids to mix together, which was my homemade chemistry set. And in addition, I was doing all the other things that kids do, climb trees and scurry up and down cliffs and collect huckleberries in the woods. But there was always a lot of physical activity and a lot of that physical activity for me involved doing things that related to science; looking at stuff, being curious about the natural world.

Recorded on June 4, 2010
Interviewed by Jessica Liebman