As I mentioned a few days ago, I visited Luis Orozco’s lab during our trip to DC last week. I already talked about his cavity QED stuff, but that’s only one of the projects under development. He’s also working on a next-generation apparatus for the laser cooling and trapping of francium, to be done at the TRIUMF accelerator in Vancouver— francium is an element with no stable isotopes, and at most a few grams of it exist on the earth at any given moment. Luis and his students demonstrated the laser cooling of francium a few years back, using atoms made in an accelerator at Stony Brook out on Long Island.
Why would anyone care about francium? The reasons are laid out in “Measurement method for the nuclear anapole moment of laser-trapped alkali-metal atoms” (link to arXiv preprint, because it’s free; the published version is this Physical Review A paper). Francium is of interest precisely because it’s a heavy element with no stable isotopes. The very large nucleus of francium means that weak interactions can produce an anapole moment in the nucleus, which would be a signature of parity non-conservation (PNC), and a possible indicator of new physics. They have an idea for a way to measure this using precision spectroscopy.
First and foremost, what is an “anapole moment?” In physical terms, an anapole moment would correspond to something like the figure shown at right (taken from this PDF of slides from a talk Luis gave): a magnetic field that is confined to the nucleus of the atom, corresponding to a magnetization that goes around in a loop, or a current that wraps around the surface of a torus. It doesn’t produce any field at large distances from the nucleus (“large distances” here means “one ten-thousandth of a nanometer”), but does produce a field component inside the nucleus of the atom.
If the field is confined to the nucleus, though, how do they propose to measure it spectroscopically? Well, there’s a phenomenon in atomic physics called the “hyperfine interaction” that causes the energy levels of certain atoms to split into two very closely separated states– the “21-cm line” in hydrogen, beloved of radio astronomers, is a transition between two hyperfine states, and the current definition of the second is in terms of the energy difference between two hyperfine states in cesium. This energy splitting is caused by an interaction between the spin of the electron and the magnetic field of the nucleus. For the right states in the right atoms, this is extremely sensitive to the field right at the nucleus– contrary to the “solar system” model of the atom that you learn in grade school, in certain atomic states, the electrons spend a good deal of time very close to the nucleus, and even inside it.
What Luis and his colleagues propose to do is to measure this splitting very precisely. By making a very accurate measurement of the hyperfine splitting for several different isotopes of francium, they can determine the size of the anapole moment, and from that, extract the strength of the PNC interactions that they’re interested in. There’s a good deal of theory involved in getting the answer out, but the cool thing (from my point of view) is that the essential measurement is just good old atomic spectroscopy. Other than the need for an accelerator to make the francium (a big “other than,” I admit), this is another of those table-top searches for new physics that I’m so enamoured of.
The basic idea is really simple. They trap a bunch of francium atoms in a magneto-optical trap (MOT) at the end of an accelerator beam line. When they accumulate enough atoms in the MOT, they transfer the atoms to a second trap, in another chamber with a much lower backgground pressure (allowing a longer lifetime for atoms in the trap, and more precise measurements in general.
This second trap is located inside a microwave cavity (two mirrors facing one another, with their spacing adjusted to fit a standing wave at the frequency corresponding to the energy splitting between the hyperfine ground states in francium). Then they do basically what you do for an atomic clock: they prepare the atoms in a particular superposition state, hit them with microwaves, and see how many end up making a transition. They do this for various arrangements of the laser, microwave, and magnetic fields in the cavity, and see how things change.
In the ansence of an anapole moment, the microwaves should make basically no difference. The hyperfine transition is a forbidden transition, and can’t ordinarily be driven directly using a microwave electric field. The interactions that produce the anapole moment loosen that ban, somewhat, and allow a very small probability of driving the transition. This transition will depend on the polarization of the light and the direction of the magnetic field in the cavity in a particular way, and the anapole contribution can be extracted by looking at what happens when you reverse things– for one orientation of the field, the transition probability is slightly higher, say, but if you reverse the field direction, the probability is slightly lower. If you measure both of those, and take the difference, you can extract the anapole contribution.
Of course, there are a lot of tricky things to deal with here. The most important is the question of whether there’s another way to cause atoms to make the transition between levels, which might create a false signal. It turns out that there is– the electric field from the microwaves can’t drive the transition, but the magnetic field can, and it’s about a billion times more likely than the expected anapole-enabled transition probability.
To get around this, they rely on three clever tricks. One is to arrange the polarization of the microwave field so that the magnetic component can only drive a particular transition (the same idea as in the cavity QED experiment), and apply a magnetic field to shift the energy levels so that transition is at a different frequency than the transition driven by the electric component (which is oriented 90 degrees away). The other two rely on holding the atoms in a “dipole trap,” which is essentially the same thing as optical tweezers: first, they can position the trap in a spot where the electric field component of the microwaves is large but the magnetic field component is nearly zero, and second, they use the fact that the magnetic field changes direction as you go through the zero point. The atoms will experience one field direction on one side of the trap, and the other direction on the other side, which will tend to average out the effects. Putting these three effects together, they hope to make the magnetic transition probability about 500 times smaller than the probability created by the anapole moment.
So, where does this all stand? The paper linked above is a proposal, and the experiments haven’t been done yet. At the moment, they have built the chambers required for the experiments, and are testing them by trapping rubidium atoms and doing similar spectroscopic measurements in rubidium. So far, they have managed to produce a very good value for the hyperfine splitting of rubidium, and demonstrate that their apparatus is sensitive enough to detect the position of the CSS building’s elevator– the shaft is on the other side of the lab wall, and the presence or absence of the elevator changes the magnetic field by enough to shift the energy levels. That should give you an idea of the kind of sensitivity they’re dealing with.
They hope to have the testing finished soon, and move the whole apparatus up to Vancouver in the relatively near future. There are still some safety issues to be worked out with the beam line that will produce the francium (and a ton of other nasty radioactive isotopes), but once those are resolved, they’ve been promised a good chunk of beam time, and should be able to do all the measurements they need.
This is a pretty technical paper, but I wrote it up because I think it gives a good sense of what’s cool about this stuff. Not only do they have a possible method for detecting new physics through spectroscopy, but this also ties together ideas from a wide range of different fields of physics, from nuclear theory to hard-core atomic theory to modern experimental AMO.
Gomez, E., Aubin, S., Sprouse, G.D., Orozco, L.A., DeMille, D.P. (2007). Measurement method for the nuclear anapole moment of laser-trapped alkali-metal atoms. Physical Review A, 75(3) DOI: 10.1103/PhysRevA.75.033418