The third installment of my Physics World series on the history of laser cooling is now live on the web, so I can give you the full thing:
—Part One: Cold: The first experiments in laser manipulation of particles, and cooling of ions and atoms
—Part Two: Colder: The development of the magneto-optical trap and sub-Doppler cooling
—Part Three: Coldest: The race to make Bose-Einstein Condensation and a Degenerate Fermi Gas
The series spans about 30 years worth of experiments, starting from Ashkin’s earliest experiments in the late 60’s up through degenerate Fermi gases in the late 90’s. It also covers at least a factor of a million in temperature: from millikelvin-ish temperatures in the earliest experiments to microkelvin in the optical molasses era, to nanokelvin in the BEC/DFG regime. It’s also gloriously counterintuitive—the idea of hitting things with lasers and making them cold thereby is unexpected even to a physicist, as illustrated by a great Hal Metcalf anecdote in Part One.
The whole series was a lot of fun to write, though a bit stressful because the deadlines fell in times when I was busy with other stuff, too. The third installment was particularly good, because I was a young grad student when the BEC stuff went down, and remember hearing about the results “in real time,” as it were. I remember being in the room for debates about whether the collisional properties of the relevant atoms would ever allow BEC to form, and also for a teleconference from Boulder in which Cornell and Wieman announced their first detection of BEC.
This personal history also made for some weird moments in the Fermi gas part of the story, because when I was looking up the chronology there, I was really surprised to see the first DFG by Jin and DeMarco cited as being in 1999. I thought that was more in the 2000-2001 range, but the literature is pretty clear: the first potassium DFG experiments are in the late 90’s, and it’s a few years before the lithium groups catch up and there’s a real burst of excitement in the (sub)field in 2001-ish.
I think what’s going on is that the early experiments in Boulder were based on imaging/modeling that was (necessarily) nowhere near as compelling as what you see with BEC, where a dramatic spike appears in the density profile. The DFG signature is a failure to produce a big spike, plus some kind of subtle changes in the shape of the cloud that’s there, so it didn’t make a big splash— the initial paper doesn’t have any raw images in it at all, only processed data. It’s not until the lithium experiments using the bosonic isotope to do sympathetic cooling that you can really see the transition by comparing the two side-by-side: the bosons form a condensate, where the fermions stop shrinking (there are images from the paper in the part 3 article).
I was sufficiently wrapped up in my own stuff in the 1997-2001 period that I wasn’t paying super close attention to the DFG work until the dramatic images started to appear. And the competing groups obviously didn’t have an incentive to really trumpet the accomplishment when Jin’s group was the only game in town, so it was really only talked about by folks from Boulder, who I didn’t see that often. Once everybody got there, though, it was generally accepted that the 1999 results were valid, and it’s retroactively become more definitive than it seemed at the time.
I was also interviewed (remotely) for a podcast about the story, which I think came out pretty well, though there’s a striking difference between the host’s deliberate pace and my very American fast-talking. I think it came out reasonably well; if you’re a podcast person, give it a listen.
This was, as I said, fun to write, and I have a ton of additional material— hours of interviews that I couldn’t fit into the word count I was allotted. So my plan for the Next Book Project is to see what I can make of that that might sell to a publisher. There hasn’t been a great deal of progress in that area, though, because Winter term at Union started yesterday, and class prep has been consuming a lot of my time and energy.
Anyway, I hope you click on the links, and I hope you enjoy the stories (and podcast).
That’s where things stand in terms of my science writing. If you’d like further updates on this sort of thing, here’s a button:
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