What’s So Interesting About AMO Physics?

That’s the title of my slightly insane talk at the DAMOP (Division of Atomic, Molecular, and Optical Physics of the American Physical Society) conference a couple of weeks ago, summarizing current topics of interest in Atomic, Molecular, and Optical Physics. I’ll re-embed the slides at the end of this post, for anyone who missed my earlier discussion.

I put a ton of work into that talk, and had a huge amount of material that I didn’t have time to include. I’d hate for that to go to waste, so I’m going to repurpose it for blog content over the next week or so. It’ll probably be about a half-dozen posts all told, which should give you some idea of how crazy it was to try to pack all this material into a single talk.

As I note in the slides, the reason for the summary talk in the first place was that the DAMOP meeting has gotten much larger over the last ten or fifteen years. I happened to have a copy of the 2001 program among the junk in my office, and this year’s meeting was almost twice as big as ten years ago: the number of talks increased from 270 to 477, and the number of posters from 293 to 548. It’s still a long way from the March Meeting, but it’s getting difficult to keep track of all the different things going on.

To impose a little structure on the vast amount of material I was trying to summarize, I identified five broad areas of current research interest, and highlighted one invited-talk session from each area. The selection of areas and topics to highlight was a little idiosyncratic– this was definitely my opinion of what’s most interesting in AMO physics, and a different speaker might’ve broken things down differently. The categories I used are:

—Ultracold Matter: This was the largest single category in terms of the number of invited-talk sessions, and is probably the largest overall, including all the contributed sessions as well. It’s also my “home” in the field, as my professional background is in laser-cooled atomic collisions and BEC. Thus, it gets to go first.

The study of ultracold atomic systems typically begins with laser cooling, using light-scattering forces to slow the motion of atoms, and accumulate large numbers of atoms in a vapor at microkelvin temperatures (that is, 0.000001 K, or one one-millionth of a degree above absolute zero). The 1997 Nobel Prize in Physics went to Chu, Cohen-Tannoudji, and Phillips (my Ph.D. advisor) for the development of laser cooling back in the mid-1980’s.

While there are still interesting things being done just with laser-cooled samples, microkelvin temperatures really only count as “cold” these days. “Ultra-cold” generally requires a further step, evaporative cooling, where the highest-energy atoms are selectively removed from the sample, lowering the average energy of the remaining atoms. After some collisions to redistribute the energy, you’re left with a sample that is colder, and also typically denser. Through a process of repeatedly removing “hot” atoms and letting the rest rethermalize, a laser-cooled vapor at microkelvin temperatures densities a billion times lower than room-temperature air can be cooled and compressed until it forms a Bose-Einstein Condensate (which I explained to Emmy in this Seed article), with typical temperatures in the nanokelvin range, and densities a million times lower than air. the 2001 Nobel Prize in Physics went to Cornell, Ketterle, and Wieman for making the first BEC’s in dilute atomic vapors.

There are a whole slew of things people do with BEC, studying fundamental atomic properties, making precision measurements, looking at condensed matter physics and thermodynamics. It’s an incredibly versatile tool for basic research, and that’s why it’s one of the largest areas of current research in DAMOP.

—Extreme Lasers: In the last twenty-odd years, there has been a huge amount of work on pushing the limits of laser technology. As a result, it is now relatively easy to make lasers that produce pulses only a few femtoseconds in duration (1fs = 0.000000000000001 s). These pulses also tend to pack a great deal of energy into the short pulse, so the electric field associated with the laser pulse can be strong enough to highly ionize any material it encounters, which opens a huge range of possibilities. One of the student speakers in the undergraduate session was even looking at situations where the field strength becomes great enough to produce electron-positron pairs from the vacuum (theoretically, not experimentally. Not yet.)

The availability of ultra-fast and ultra-intense pulses opens a lot of possibilities for exciting physics. Short pulses allow you to follow atomic and molecular dynamics on the time scale of the motion of the electrons, which is pretty amazing. High intensities allow the creation of strongly ionized systems, which have interesting properties of their own, and can allow things like laser wakefield acceleration. The dynamics of the electric field interacting with a sample of gas can lead to processes where extreme ultraviolet and even x-ray beams are produced in a coherent fashion, using “table-top” sources. And femtosecond lasers even find applications in precision metrology, through “frequency comb” sources (which won a share of the 2005 Nobel Prize in Physics).

—Quantum Phenomena: Single atoms and molecules are inherently quantum systems, and thus provide a very nice test bed for lots of quantum phenomena. In recent years, it’s also become relatively easy to make “non-classical” states of light, that have to be described in terms of discrete numbers of photons, and these, too are rich sources for studying quantum phenomena.

Roughly speaking, there are two major areas of interest in this subject: quantum information processing, and quantum communications. Quantum information processing, as the name suggests, is about building quantum computers, replacing classical “bits” with “qubits” that can be in a superposition of “0” and “1” at the same time. The internal states of atoms and molecules can readily serve as the logical states for information processing, and the interatomic interactions that AMO physicists have been studying for decades provide a variety of clever ways to couple these qubits together to do computations.

Quantum communication is about moving quantum information from one place to another, or between different forms. This involves things like quantum cryptography, where pairs of photons are used to generate “unbreakable” codes; quantum teleportation, where arbitrary states are moved from one place to another without measuring them in the process; and the exchange of quantum information between light and matter, for example, by taking the state of an atomic qubit and converting it to a superposition of polarization states of a single photon.

This is a large and very active field at the moment. Nobody has won any Nobel Prizes for quantum information science, but it’s probably only a matter of time. (Aspect, Clauser, and Zeilinger would be a great set of laureates, or Aspect, Wineland, and Zeilinger).

—“Traditional” AMO Physics: This is a slightly vague “what I’m pointing at when I say ‘traditional'” sort of category, which basically boils down to the study of atomic and molecular properties by bouncing things off atoms and molecules. Sometimes the things bounced off the atoms and molecules are photons, sometimes electrons or positrons, and sometimes other atoms and molecules. The ultimate result of the process is always information about the internal structure and interactions of the target atoms and molecules, though.

Given that this kind of science has been going on for decades, now, you might wonder why anybody would still be interested in it. The measurements made in this area are of continued interest to astrophysics, though– lots of mysteries remain regarding how large clouds of atoms and molecules come together to form stars and planets, for example. There’s also a lot of interest in this are for atmospheric science, looking at the interactions of molecules and light or charged particles in the upper atmosphere. The numbers that are produced in traditional AMO contexts serve as crucial inputs for models of astrophysical and atmospheric properties, which help us better understand the planet we live on and the universe we live in.

—Precision Measurement: This almost doesn’t deserve to have its own category, as there was only one invited-talk session devoted to precision measurements (though two of the “Hot Topics” talks on the last day were precision measurement talks). There’s a distinct enough culture around these topics, though, that I think it’s useful to break it out as a separate category.

The goal of precision measurement research is, as the name suggests, to measure things as precisely as possible. Sometimes, this takes the form of standards work– making ever better atomic clocks, for example– other times, it’s a search for exotic physics. In the prize session on the first day, Gerry Gabrielse described what he does as looking for new physics through high precision, not high energy. In all cases, the cetnral concerns are the measurement of extremely tiny effects, and also all the associated systematic effects that might confuse such a measurement.

These five areas cover most of what’s currently interesting in DAMOP. There’s considerable overlap between them, of course– lots of people use ultrafast lasers to study traditional AMO properties, for example, or ultracold atoms as a source for quantum phenomena– but it makes a useful system for thinking about the various sessions within the meeting.

Over the next week or two, I’ll do a series of posts, at least one per topic area, giving a little more detail about what goes on in that sub-sub-field, and highlighting some of the dozens of papers I read in preparation for this talk. If you’d like a tiny taste of what’s going on, though, here are my talk slides again:

(Before you take me to task for having too much text on these, these were quite deliberately designed to be text-heavy so they’ll be readable on the Internet. I put the SlideShare URL up during the talk, and directed people to look at the slides online for things like the session lists and references.)