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.

Motivation

A promising scheme for a quantum computing network – which has been experimentally demonstrated by our group – exploits the phenomenon of heralded entanglement to entangle two remote qubits. The qubits are manifested in the quantum state of two trapped ions in separate vacuum chambers.

Enhanced light collection: Needle-like electrodes will position a trapped ion at the focus of a mirror with 5 mm radius of curvature

By collecting and interfering two photons emitted from the ions, the two ions can be entangled without destroying their quantum state. This process can in principle be extended to form a network of entangled qubits separated by arbitrary distances.

One drawback to this scheme is that it is essentially probabilistic. Although it still scales in a feasible way, the entanglement event rate is currently extremely low.

Surrounding a single ion with an optical cavity should boost the fidelity and speed of the trapped ion qubit measurement, and therefore the “network speed”, via the well-known Purcell effect [1,2] for lossy cavities from quantum electrodynamics (QED).

Another method of enhancing the amount of detected light from a single ion is to place a reflective optic nearby to increase the collection solid angle. In the limit of an ideal parabolic mirror, the entire solid angle of a point source could be collected [3]. One of our experiments explores this concept with a spherical mirror of small radius of curvature.

Design

A single ion will be trapped and coupled to the fundamental mode of a small Fabry-Perot cavity. There are considerable challenges to overcome, including charge build-up on the dielectric surfaces.

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

Ion trap and optical cavity QED systems are ideally suited as benign environments 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 radio 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.

Cavity assembly with inserted ion trap electrodes

Previously, our group developed a "double endcap" quadrupole trap geometry consisting of two needlepoints mounted on a stage that allowed the trap electrode gap to be continuously varied between 0.02 - 1 mm.

Our current approach, now under construction, will use the positioning technology of the needle trap experiment to precisely control the position of a novel microtrap.

This new trap is extremely thin (0.127 mm) and is designed to fit between the cavity mirrors. The ion-electrode spacing will thus be much smaller than the ion-cavity distance, ideally mitigating the effect of stray fields from the dielectric mirror surfaces. In addition, the cavity mirrors will be encased in conducting sheaths, which will be voltage controlled to offset any errant axial fields.