Established laser cooling techniques do not work for helium so Stephan Schiller and colleagues at Heinrich-Heine University in Düsseldorf had to use a technique called "sympathetic cooling" instead. This involved cooling the ions with a gas of beryllium ions that had already been cooled with lasers (B Roth et al. 2004 arXiv.org/abs/physics/0412053).

Schiller and co-workers began by loading large numbers of beryllium-9 ions, produced by electron impact ionisation, into a linear radio-frequency trap. An ultraviolet laser was then used to cool these ions until they underwent a phase transition from the fluid state to an ordered phase known as a "Coulomb crystal".

Next, they loaded the helium-4 ions -- which had also been generated by electron impact ionisation -- into the trap. A dark core of ultracold helium ions was observed in the centre of the beryllium ion crystal. Schiller and co-workers were able to cool as many as 150 helium ions to a temperature of about 20 millikelvin.

The team has also demonstrated sympathetic cooling with helium-3 ions and is now applying the technique to other species, including diatomic molecules made of hydrogen and deuterium.

"Helium ions are important for measurements of the fundamental constants and tests of QED because they are complementary to hydrogen," team member Bernhard Roth told PhysicsWeb. "Using cold ensembles of HD+ ions we hope to measure the ratio of the electron to the proton mass, and test if it remains constant over time."