Laser-Cooled Atoms: Helium

Element: Helium (He)

Atomic Number: 2

Mass: two stable isotopes, 3 and 4 amu.

Laser cooling wavelength: 1083 nm

Doppler cooling limit: 38 μK

(It should be noted, though, that despite the low temperature, laser-cooled helium has a relatively high velocity– that Doppler limit corresponds to an average velocity that’s just about the same as for sodium at 240 μK. This is because temperature is a measure of kinetic energy, and helium is much, much lighter than any of the other laser-cooled elements.)

Chemical classification: Noble gas, part of column VIII of the periodic table. Doesn’t react with anything, so poses much less danger to scientists than any of the alkalis. And, of course, it’s a colorless gas, so you get a cartoon of balloons rather than a picture of a sample of helium.

Other properties of interest: Helium is not cooled in its true ground state, because the laser wavelength required is beyond current technology. Instead, it’s cooled starting from its lowest excited state, which sits about 20eV above the ground state. This state has a lifetime of essentially forever, and can be treated as an effective ground state for laser cooling, using a transition to the next state up. The huge internal energy means that each atom is like a little bomb, and will release that 20eV in a collision with another atom, a metal surface, or pretty much anything else. This produces an electron and an ion, either of which can be detected with very high efficiency, and is one of the principal diagnostics for laser cooling of He*.

History: Helium was laser cooled and trapped for the first time in 1992, part of a sort of second wave of laser cooled systems after a decade of playing around with alkali metals. Metastable trapping in general has always been kind of a niche thing, and only a handful of groups ever did He*– one in France, one in Japan, one in the Netherlands, and maybe another one or two I’m forgetting.

The French group did some really neat experiments in helium with VSCPT– “Velocity Selective Coherent Population Trapping.” This is a clever trick using lasers of carefully chosen frequency to create a “dark” state where atoms won’t absorb light, provided they’re moving at a very particular velocity. The atoms interact with the lasers for a while, until they reach the appropriate velocity, then get “trapped” in the dark state, producing a sample of atoms at a particular velocity with an extremely narrow width in velocity. It was a big Thing for a while in the mid-90’s, but I haven’t heard much about it since BEC came around.

Amazingly, given its propensity for blasting electrons out of things, Bose-Einstein condensates of metastable helium were made by both the French and Dutch groups around 2001, and since then by a different technique in John Doyle’s group at Harvard. This is possible because of a quantum effect: the principal ionizing collision mechanism between two helium atoms in the same spin state requires one of them to flip an electron spin, which can’t happen easily– this reduces the collision rate by a factor of something like 100,000, making BEC possible.

The metastability offers some nice properties for quantum optics experiments, chiefly that you can do position-sensitive detection by just dropping them onto a charged surface. The laser cooling wavelength is a little inconvenient, though, and making the metastables is kind of a hassle, so it’s never been a really popular system.

Random fun things: Someone from one of the Dutch groups once told me that the had a local in-joke about how metastable helium could knock an electron loose from a dead cow, if you could get one in the vacuum chamber. It might be funnier in Dutch.

Art: The cartoon version of helium is a levitating hipster. Its entry in the Comic Book Periodic Table is two utterly daft pages from a Thor comic. And, of course, it figures prominently in that Pixar movie…