Laser-Cooled Atoms: Rubidium

Element: Rubidium (Rb)

Atomic Number: 37

Mass: two "stable" isotopes, 85 and 87 amu (rubidium-87 is technically radioactive, but it's half-life is 48 billion years, so it might as well be stable for atomic physics purposes.

Laser cooling wavelength: 780 nm

Doppler cooling limit: 140 μK

Chemical classification: Alkali metal, column I of the periodic table. Like the majority of elements, it’s a greyish metal at room temperature. Like the other alkalis, it’s highly reactive, and bursts into flame on contact with water, even more so than sodium (in general, the alkalis get more violently reactive as you move down the column). For this reason, all physicists working with rubidium have True Lab Stories about accidentally blowing stuff up with it, and if you buy me a beer, I'll tell you some.

Other properties of interest: Scattering length of around 100 a₀; nearly identical scattering lengths for two trappable states in 87Rb; a whole slew of Feshbach resonances-- this paper claims 40 in 87Rb; negative scattering length for 85Rb, but this can be made positive using a Feshbach resonance at around 230G.

History: Rubidium was a later arrival on the laser-cooling scene, but took off in popularity when it was realized that the diode lasers used for CD players worked in the near-infrared, at a wavelength very close to that needed to laser cool rubidium. That brought laser cooling within reach of a huge range of labs-- rather than dropping a couple hundred thousand dollars on a dye laser system, you could spend maybe ten grand on the complete laser system, with money left over for vacuum hardware. When Eric Cornell and Carl Wieman got BEC in 1995, a lot of the press releases talked about how the whole apparatus only cost around $50K (which is a bit of a cheat, since that didn't include the salaries of the many very smart people who put the stuff together, but those get left out of everybody's calculations, so it wasn't a major lie).

In addition to being enabled by cheap lasers, rubidium turns out to have a bunch of nice properties, particularly for people doing BEC experiments. It's got hyperfine structure, meaning you need a repumping laser, but the frequency difference is relatively easy to manage, and that allows you to do "dark spot" traps to bump up the atom number. The number of atoms you can get in a condensate, and the energy of those atoms, depend on the collisional properties of the atoms in question, and those work out very nicely for rubidium-- the "scattering length" that characterizes the collision for rubidium-87 is moderately large and positive, and by happy coincidence is nearly the same for two different magnetically trappable states, and between those states, so they've been able to do all sorts of fun two-species experiments. The collisional properties are also vastly better than those of cesium, so laser-cooled rubidium samples have found lots of applications in "clocks," and for comparisons of time standards of different types.

Rubidium was the system for the first dilute-vapor BEC, earning Cornell and Wieman their share of the 2001 Nobel Prize. They've kind of gone back and forth with the Ketterle group at MIT ever since for "coolest recent BEC experiment," with the JILA team being the first to see vortices in a condensate (one of the signatures of superfluid behavior), and all sorts of two-species stuff because there are two trappable states that will happily coexist. The Bose condensation of rubidium-85 was also pretty impressive, because 85Rb by itself has a negative scattering length which prevents the formation of a stable condensate, but they were able to change the collisional properties using a Feshbach resonance, opening the door for a lot of other experiments using those resonances.

These days the laser situation is less rosy than it was in the early 1990's-- the electronics inductry has moved on to different laser technologies, so the only people who still buy 780nm lasers are atomic physicists, and there aren't enough of us to support a robust market for cheap lasers. But there are more commercial suppliers of complete laser systems these days, so it's not all that bad. If you were going to set up a generic ultracold-atom system from scratch to allow a wide range of possible experiments, rubidium is still probably the atom of choice.

Random fun things: I've heard rubidium half-jokingly called "God's atom," a line that almost certainly originates with Eric Cornell. The properties are really that nice for doing BEC.

In college, I worked on making a rubidium MOT, and made a mix tape of the songs I listened to a lot that year which I labeled with the spectroscopic notation for the hyperfine transition for laser cooling in rubidium. That used to be good for some double takes whenever I'd give other physicists a ride someplace.

(It was a great tape, too, but I don't seem to have transcribed the track list on the blog, alas... this one from the same era was also pretty awesome, though.)

Art: The cartoon version of rubidium is a scarlet femme fatale. It only rates a single mention in the Comic Book Periodic Table, despite being kind of fun to say.