We are fabricating very small and optically open ion trap structures for integration with small-volume and medium-finesse optical cavities in order to efficiently extract single 369 nm photons from trapped Yb⁺ ions.

Ion trap and optical cavity-QED systems are ideally suited as benign environements for their respective quantum systems.  However, when these two systems are combined, extreme care must be taken to ensure that each system is sufficiently isolated from the other's environment.  Dielectric mirrors are usually insulators at rf frequencies typically associated with the trapping fields, which poses a serious threat to the stability of the ion trap as free charges on the dielectric can produce large offset electric fields.  Conversely, the ion trap acts as an aperture in the optical cavity volume and diffractive losses may degrade the performance of the cavity.

One approach is to combine the two systems with a "double endcap" quadrupole trap geometry consisting of two needlepoints mounted on a stage that allows the trap electrode gap to be continuously varied between < 20 - 1000 microns (0.02 - 1 mm).

Cavity optics can be added to this trap as well as to the variety of ion "chip" traps that have characteristic dimensions of tens of microns.

The smallest ion trap ever demonstrated: a single Cd⁺ ion trapped between two tungsten needle points

Another approach is being currently designed, in which a single Yb⁺ ion will reside in a microtrap positioned in one mode of a tight Fabry-Perot cavity. Coupling the ion to the cavity in this way enhances spontaneous emission into the mode via the Purcell effect.  This not only boosts the fidelity and speed of trapped ion qubit measurement, but it also can greatly improve probabilistic entangling schemes that rely on the collection and interference of single photons. The ion-electrode spacing will be much smaller than the ion-cavity distance, ideally mitigating the effect of stray fields from the dielectric mirror surfaces.

For an accessible treatment of the basics of cavity QED and a wealth of references on the subject, try Kevin Fortier's thesis.