Toward Direct Laser Cooling of Molecular Ions

Laser cooling of trapped ions allows for highly controlled atomic-scale experiments—as few as one atom may be held for hours or longer, removed from nearly all environmental disturbance, at temperatures approaching absolute zero.

Unprecedentedly precise measurements are possible: an ion-trap-based atomic clock is currently the world’s most exact with accuracy to within one second over ten billion years. Among the currently developing research applications are fundamental tests of the Standard Model, coherent control experiments, and the development of a quantum computer.

These applications are based almost entirely on laser cooling of only a few, particularly suitable, atomic species. Expanding the class of laser coolable ions promises to likewise expand the range of applications. In particular, cooling of molecules, whose internal structure is more complicated than that of atoms, is highly desired and thus far accomplished only by indirect means: the molecules are brought into thermal contact with either a cold background gas or directly coolable atomic ions.

My aim is direct laser cooling of a trapped molecular ion. Here I present research I conducted in my group’s search for a molecular ion suitable for direct cooling. Working from spectroscopic data I calculated transition probabilities between molecular states, and I show some representative results, highlighting the features required for direct cooling. SiO+ was the most promising candidate found in our search. I explain the desirable features it possesses and present results from my currently ongoing attempts to trap and directly cool this molecule.