What’s So Interesting About Precision Measurement?

The final content area from my DAMOP overview is Precision Measurement. This is also the smallest area, with only one invited session on the topic on Fundamental Symmetry Tests, though two of the “Hot Topics” talks (by Zheng-Tian Lu and Ed Hinds) were precision measurement talks. You might be able to make an argument that this doesn’t really deserve its own category, but I was the one giving the talk, and I love this stuff (though I absolutely do not have the temperament necessary to do it), so it gets its own category.

I also think there is a fairly distinct culture to precision measurement experiments, which have a common set of concerns and techniques even though they work on very different sorts of problems. People who do real precision measurement work have to have an intense focus on detail and systematic effects that is really qualitatively different than a lot of the other areas of AMO physics. If you’re measuring tunneling rates through an optical lattice for the very first time, a 10% experimental uncertainty is perfectly acceptable, and agreement with theory can be primarily qualitative. If you’re doing precision measurement work, though, the limits have to be incredibly tight, and that leads to a different ethos that really sets groups in this area apart.

So, precision measurement. The goal here, as the name suggests, is to measure something really, really well, where “really well” usually starts at the part-per-million kind of level. Pretty much all of these experiments end up measuring a frequency, because we have fantastically well-developed techniques for measuring frequencies in regions of the spectrum from the radio frequency well into the ultraviolet– laser spectroscopy, Ramsey interferometry, etc.. As I try to impress on my research students, with laser spectroscopy it’s almost trivial to measure atomic level splittings with frequencies that are about a millionth of the absolute laser frequency. People in the AMO field take that kind of thing for granted, but looked at from outside, it’s kind of amazing.

If you want to split the subfield further, the next major division is between those experiments designed to do very precise measurements of a known quantity or effect, and those experiments designed to look for effects that are probably not detectable. You might call these the “digit-adding” and “limit-placing” categories of precision measurements.

The latter of those two categories sounds kind of funny, but it’s an incredibly important subject within the field, exemplified by the search for a permanent electric dipole moment (edm) of the electron by Ed Hinds that made news recently. The goal here is to put an upper limit on the size of an effect that might exist, but might also be smaller than the current apparatus can detect. In the case of the electron edm, finding a non-zero edm result would be a huge breakthrough and tell us a lot about physics beyond the Standard Model. There are at least as many theories predicting an edm below the sensitivity of the current experiments as ones predicting a real and detectable edm, though, so the most likely result is zero.

In a limit-placing experiment, you will most likely end up with a measured quantity that is smaller than the uncertainty in that measurement. The goal is to make that uncertainty as small as possible, so as to put the tightest possible limits on new physics that might cause non-zero results for the measured quantity. Examples beyond the edm experiment include things like last year’s confirmation of the bosonic nature of photons, a variety of searches for changes in the fine structure constant, and searches for deviations from the inverse-square law of gravity.

A digit adding experiment seeks to, well, extend the precision of a quantity that is known to be non-zero. Sometimes, the extra digits aren’t terribly numerous, if you’re demonstrating effects that haven’t previously been seen, like the optical clocks and relativity experiment from last year. At other times, the number of digits is kind of amazing, like the magnetic moment measurements that make QED the most precisely tested theory in science.

The real goal of a digit-adding experiment is to get to a place where your measurement no longer agrees with theory, because at that point, you’re exploring new physics, and that’s where the fun happens. Of course, there’s a huge amount of systematic trouble to deal with, which is why you try to use multiple different techniques to measure the same thing, and compare the results. When you get disagreements, as in the measurements of the gravitational constant G or the size of the proton, then things get really interesting as you sort out which measurements are right, and which have some undetected systematic effect skewing their results. Randolf Pohl gave a terrific talk about the proton size measurement at DAMOP, in which he devoted the last third or so of his talk to pointing out ways that the other measurements might be brought into agreement with his group’s measurement. That kind of thing is not uncommon in precision measurement, and can produce some spirited (if highly arcane) debate.

Names to Conjure With: The precision measurement field is kind of diffuse, with lots of different people doing very different things. If you’d like to try to keep abreast of it, or just sound like you know a little something, here are a few specific researchers to follow:

Gerald Gabrielse at Harvard is one of the masters of this stuff, and the guy behind the g-factor measurements. Ed Hinds at Imperial College in London was mentioned above and the Eöt-Wash Group of Eric Adelberger at the University of Washington does spectacular gravitational measurements. Dmitry Budker at Berkeley and Dave DeMille at Yale do really cool things with atoms, molecules, and lasers, and Ron Walsworth’s group at Harvard is involved in a bunch of cool projects. Dave Wineland’s group at NIST makes unbelievably sensitive clocks, and Jun Ye at JILA does astonishing things as well. Jan Hall at JILA and Ted Haensch at the Max Planck Institute in Munich shared a Nobel for precision measurement work, so you’d be crazy not to follow them.

This is by no means a complete list of groups doing precision measurement, but those are some of the biggest of the names in the field. If you keep an eye on what they’re doing, you’ll have a decent sense of what’s exciting in the world of precision measurement.