DonorsChoose Payoff: Why AMO Physics?

Months ago, during the DonorsChoose fundraiser, I offered to answer questions from people who donated to the Challenge. I then promptly forgot to respond to the questions sent in. Mea maxima culpa. Here's a way-too-late response to a good question from "tcmJOE":

I've spent the past few years trying to explore physics and figure out what I would be interested in doing--I've settled more towards energy research, somewhere between CM and MatSci, but I've tried at a variety of different things in the along the way. So my question for you is: How did you end up in AMO? Were there any other fields you might have gone into instead?

It's a good question, especially since high-energy physics dominates the public perception to the point where book reviewers identify me as a "particle physicist," out of pure reflex. Why did I choose Atomic, Molecular, and Optical physics as a sub-field, anyway?

It's largely a matter of chance, actually. As anybody who has followed this blog for a while knows, I attended Williams College (Cult of the Purple Cow, baby!), and the Physics Department at Williams had (and still has) a strong concentration of people doing research in AMO physics. At the time I was there, there were four professors (Kevin Jones, Jeff Strait, Stuart Crampton, and Fielding Brown) doing experimental work with lasers, and a fifth (Bill Wootters) doing quantum information theory, which tends to have a lot of overlap with AMO stuff.

This is one of the two methods small colleges use to deal with being, well, small. If you're a large research university with dozens of faculty, you can have multiple faculty working in all the different fields of physics, and offer a wide range of courses covering everything, and have them run every year. If you're a small college, you can really only have one person per major field, and sometimes not even that, which makes it difficult to cover everything, though many schools do that (Union, for example...).

Williams back in my day took the other approach, which is to concentrate your faculty on a smaller number of fields, and build your program around that strength. Williams had a bunch of AMO types, so there was a strong emphasis on lasers and optics throughout the curriculum-- there was an intro-level course on light and waves, the sophomore level "math methods in disguise" course was on waves and optics, the junior-level intro quantum course used lots of optics, etc.

The cost of this, obviously, is that it becomes difficult to cover other areas of physics, because you don't have faculty with experience in those areas. So, for example, I never took a course on nuclear or particle physics while I was there. There wasn't really anybody to teach it.

On the other hand, though, the concentrated approach does allow you to really prepare students to excel in your areas of strength. Which is why there were 13 Williams alumni at DAMOP a couple of years ago (from a school that graduates roughly ten physics majors per year): students who go through the heavily optics-based curriculum there are particularly well prepared to go on to graduate study in AMO and related fields.

Of course, undergrad education isn't destiny-- it's perfectly possible to go through an AMO-centric undergrad curriculum and then go become a nuclear physicist. I have a classmate who did just that. So what made me stick with AMO?

Credit for that probably belongs to Claude Cohen-Tannoudji. He gave a colloquium talk on Sisyphus cooling my junior year (this was long enough ago that the Sisyphus cooling mechanism was relatively new), and it blew my mind. Anybody who has heard Claude speak knows that he gives wonderfully clear presentations, and the idea of laser cooling, let alone Sisyphus cooling, was so wonderfully counter-intuitive that I wanted to do something with that. When I learned that there was a project on campus trying to build a rubidium MOT, I signed up right away, and did my thesis on that.

That really locked me into AMO physics. When it came time to apply to grad school, I applied to places where there were interesting laser-cooling based experiments going on. A couple of my professors had done sabbaticals at NIST, which helped connect me with the Laser Cooling Group there, after I was accepted to the Chemical Physics Program at Maryland, and the rest is history.

So, in a lot of ways, my association with AMO physics is historically contingent. There are a number of aspects of the field that make it particularly appealing to me, though. For one thing, it's not excessively mathematical. Contrary to what many of my students from this past term thing, I'm not all that fond of mathematical formalism-- I can deal with it, but my mathematical background is not that strong relative to other physicists, so it's a hard slog. I've never really gotten my head around group theory, which makes it kind of difficult to follow a lot of conversations about particle physics.

AMO physics is also at a very comfortable scale-- a typical AMO experiment fits in a single good-sized room. The characteristic scale of the apparatus is on the order of a meter or so, and one person can usually understand every piece of it. Most experiments are self-contained, and do not require beam time at accelerators or other facilities, so there's no need to worry about anybody else's schedule. There are no externally-imposed deadlines, nor external constraints on what you can study.

AMO experiments are also generally fairly simple conceptually. I can almost always break them down into a mental picture of what is happening to individual atoms, without much additional baggage. The conceptual tools needed to understand AMO experiments are usually just energy-level diagrams and things like the Bloch sphere picture of a two-level system. This kind of goes back to the mathematical formalism thing, and is why I prefer AMO to condensed matter physics-- I could never really get a handle on the whole "reciprocal lattice vector" thing when I took solid state physics, and as a result, I'm not that comfortable with interpreting condensed matter experiments.

Finally, AMO physics connects very nicely to some of the absolute coolest phenomena in science. Bose-Einstein condensation, quantum information, precision spectroscopy, quantum statistics-- these are all subjects that reveal absolutely mind-blowing things about the universe we live in, and all of them can be studied realtively easily using AMO physics.

So, given those elements, there's a decent chance I would've ended up in AMO physics anyway, even if I had gone to an undergraduate school with a less AMO-centric department. Or maybe not. As it is, though, I'm very happy to be in the field of AMO physics, and look forward to DAMOP every year to see what amazing new things my colleagues are up to.


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Maybe someday you can explain how to make a sphere out of Blochs. ;-)

What you say about the clarity of AMO problems also applies to scattering problems in nuclear physics, while the complications of condensed matter show up in similar many-body problems in nuclear physics structure or heavy ion reactions. There is also a parallel to high energy physics, where clean experiments like SLAC finding the psi and psi' (and the use of QED and semi-classical QCD to interpret them) give way to many body problems except when doing the lowest energy experiment - usually at the highest energy you can reach - that exposes some new particle. Going to ever higher energy is both a form of exploration and a form of difficult-problem avoidance. Example: fifty to sixty years ago, high energy physicists were scattering protons to determine the phenomenological properties of the NN interaction. Now that they have a theory that is supposed to explain those data, they avoid the messy problem of using QCD to do just that.

PS -
Thanks for the blog about exploring new physics in AMO. That exposes why low energy physics can also play a role in particle physics, and why I was careful to say "high energy physics" in my comments above.

I didn't do any undergraduate research, and my choice of discipline didn't have any historical inevitability to it. I picked my grad school partly on the basis that they did have lots of different fields to choose from.

I picked AMO for almost exactly the reasons you cite, especially the lack of love for solid state or for the increasingly impersonal group theory / group experiments of particle physics.

But I also picked it over astronomy and complex dynamics (chaos theory etc) because I liked the fact that it had practical applications. Lasers and optics play a role in almost every industry these days -- biology/biochem/biophysics is full of spectrosopy and optical tweezers and so on, and then there's the whole world of fiber optics, of DVDs and other optical storage media, of LCD and displays... And then there's MRIs, clocks, and other applications of the actual atomic physics.

I found this practical applicability attractive both in terms of feeling useful in the world and in increasing the chances of getting a job -- and I did end up going into industry, trying to make optical and atomic sensors.