Carl Wieman: from theory to reality

09 June 2020

Interview with 

Carl Wieman, Stanford University

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Bose and Einstein first predicted their ‘quantum state’ in 1925, but physicists spent the next seventy years trying create it. Then, in 1995, two groups independently got there - an achievement that netted three men a joint Nobel Prize. Their technique was to cool down atoms of Rubidium using lasers - because rather than heating, lasers can make the atoms absorb photons and so, on average, slow down and become extremely cold.  Carl Wieman is one of the three Nobel laureates - and he told Phil Sansom the story...

Carl - It actually begins with us playing around with lasers for other purposes, and realising that we had the capability to do laser cooling - a few millions of a degree above absolute zero. We realised that people who had been trying for many years to make Bose-Einstein condensation, they weren't really thinking of the problem in the right way. Okay. So I have to take a quick step back here.

Phil - Sure.

Carl - Bose-Einstein condensation happens when you have the right coldness and denseness of atoms in a gas. Our approach, I like to call it ‘the low road to BEC’, was rather than getting atoms pretty cool with sort of traditional refrigerator things, and then squeezing them together as much as you could, we went this route of lasers; and that meant much, much lower temperatures, but not nearly as high a density of atoms. It took about five years of kind of figuring out how to get all the conditions right, and all the obscure collision processes.

Phil - Did it seem achievable when you were first playing around with getting these atoms ultra cold?

Carl - No. In fact it was explicitly uncertain as to whether it was achievable. Right up until we produced the condensate and saw that it could sit there for a fraction of a second.

Phil - So how did you know when you finally achieved it?

Carl - It was spectacularly clear. And in fact, I made sketches five years earlier to show people what ideally it would look like. And it looked just like that. You have this little brown cloud of atoms that we held in this magnetic trap. And as we got colder, it goes through a transition. Just like - not just like, but somewhat like - water vapour freezing into ice. You see this much higher density little peak. Our first reaction was "gosh, too good to be true! Turn all the knobs, make sure, test it in all the different ways we can just to make really sure that we're right on this".

Professor Cornell, Professor Ketterle, Professor Wieman, your groundbreaking work on Bose-Einstein condensation has opened up a very fruitful area of research and potential applications. I now ask you to step forward to receive your Nobel prizes from the hands .....

Carl - The point at which we got the prize - I guess this sounds awfully arrogant, but we'd been assured by enough prominent people that was going to be happening, you know, so it wasn't nearly the great shock and delight it can be for some people. It was kind of like, "okay, great. Now I move on to other things."

Phil - What do people often miss, do you think, about Bose-Einstein condensates?

Carl - The interesting part about Bose-Einstein condensates, and it's sort of all the things Einstein never thought about. This is a unique new quantum system, and it's a quantum system you can control in ways that nobody ever could do. In some ways it's a marvellous sandbox.

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