Precision spectroscopy of polar molecular ions

Cold, trapped polar molecular ions are interesting to work with for a variety of reasons. Like polar molecules, they provide energy scales spanning the RF, microwave, and optical frequencies. These include very narrow transitions at ~10 GHz, as well as (sometimes) nearly-closed optical transitions reminiscent of traditional alkali atoms. Such rich, narrow, spectra potentially allow the molecules to be coupled simultaneously to both electrical and optical cavities, providing a means to efficiently convert between circuit and cavity QED quantum states. The collective modes of polar molecules may also provide unique access to the quantum physics of collective spin states.

Fig. 1: CCD image of strontium ion chain: the spacing between ions is about 15 micrometers.

One great advantage of molecular ions, over neutral molecules, is the ease with which they can be trapped and cooled. Uncharged polar molecules may be trapped using stark shift traps, but the trapping potential significantly interferes with internal states of the molecule. They may also be trapped using hexapole RF fields, but at the cost of low trap depth. In contrast, molecular ions may be trapped using standard Paul traps, without interfering with their internal states. Also, a variety of experiments have already demonstrated trapping of molecular ions such as MgH+, O+2, CO+2, HD+, and others. These molecular ions may also be sympathetically cooled with simultaneously trapped atomic ions, such as Be+, Ba+, and Mg+; this approach has been used to cool HD+ to 20 mK.

Little work has been done, however, with the precision spectroscopy of polar molecular ions, compared with polar molecules. In particular, experiments to-date with molecular ions have only provided coarse information about their spectroscopic properties. They are usually detected indirectly, through their impact on the motion of the simultaneously trapped atomic ions, or through agreement of atomic ion crystal structures with molecular dynamics models. The microwave and RF transitions of polar molecular ions have remained inaccessible.

We are constructing an experiment to trap and cool polar molecular ions, using a novel surfaceelectrode ion trap configuration which will allow the ions to be coupled to a microwave stripline resonator. This will allow the microwave transitions of the molecules to be explored, and eventually coupled to a high-Q resonator.