Calcium ions are produced by multi photon ionization of gaseous calcium atoms using uv laser light at 355 nm. The Ca+ ions can be laser cooled on the 397 nm transition.
The laser cooling takes place within a linear Paul trap (left) – an electric quadrupole trap to which a combination of radiofrequency and static fields are applied.
The Ca+ ions are fluorescing continuously and can be observed individually using a microscope located above the ion trap and a CCD camera.
Under the influence of the laser cooling, and with the ion trap fields operating, a diffuse cloud of ions (right) undergoes a phase transition into a Coulomb crystal, (as shown below).
The image below shows a 2-dimensional projection of a spheroidal crystal: each red spot is an individual ion.
Above: False-colour fluorescence image of a Coulomb crystal of about 50 laser-cooled CA+ ions in an ion trap
Properties of Coulomb Crystals
Size: ranging from a few ions up to >105 ions
Distance between the ions in the crystal ~ 20 mm
Storage time: hours
Translational temperature: down to mK near the trap axis
Typical ions used: Mg+, Ca+, Be+, Ba+
The size and shape of the crystal can be controlled, as shown in the three figures below, by manipulating the fields applied to the ion trap
Sympathetic cooling of molecular ions down to milli-Kelvin temperatures
As shown originally in the work of Molhave and Drewsen, molecular ions produced within the ion trap can be sympathetically cooled into the crystal. These ions are dark and form a layer around the fluorescing atomic ions.